v3.25.1
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Risk Return Abstract rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName First Trust Exchange-Traded Fund
Prospectus Date rr_ProspectusDate May 01, 2025
First Trust Natural Gas ETF  
Risk Return Abstract rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading <span style="font-family:Arial;font-size:12.60pt;font-weight:bold;">First Trust Natural Gas ETF (FCG)</span>
Objective [Heading] rr_ObjectiveHeading <span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">Investment Objective</span>
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The First Trust Natural Gas ETF (the “Fund”) seeks investment results that correspond generally to the price and yield (before the Fund’s fees and expenses) of an equity index called the ISE-Revere Natural GasTM Index (the “Index”).
Expense [Heading] rr_ExpenseHeading <span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">Fees and Expenses of the Fund</span>
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock The following table describes the fees and expenses you may pay if you buy, hold and sell shares of the Fund. Investors may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the table and example below.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption <span style="font-family:Arial;font-size:9.00pt;font-weight:bold;">Shareholder Fees</span> <br/><span style="font-family:Arial;font-size:9.00pt;">(fees paid directly from your investment)</span>
Operating Expenses Caption [Text] rr_OperatingExpensesCaption <span style="font-family:Arial;font-size:9.00pt;font-weight:bold;">Annual Fund Operating Expenses</span> <br/><span style="font-family:Arial;font-size:9.00pt;">(expenses that you pay each year as a percentage of the value of your investment)</span>
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination <span style="font-family:Arial;font-size:8.10pt;font-style:italic;">April 30, </span><span style="font-family:Arial;font-size:8.10pt;font-style:italic;">2026</span>
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading <span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">Portfolio Turnover</span>
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 38% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 38.00%
Expense Example [Heading] rr_ExpenseExampleHeading <span style="font-family:Arial;font-size:9.00pt;font-style:italic;font-weight:bold;">Example</span>
Expense Example, No Redemption Narrative [Text Block] rr_ExpenseExampleNoRedemptionNarrativeTextBlock The example below is intended to help you compare the cost of investing in the Fund with the cost of investing in other funds.The example assumes that you invest $10,000 in the Fund for the time periods indicated and then hold or sell all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain at current levels. The example assumes that the Fund’s investment advisor’s agreement to waive fees and/or pay the Fund’s expenses to the extent necessary to prevent the operating expenses of the Fund (excluding interest expense, brokerage commissions and other trading expenses, acquired fund fees and expenses, taxes, and extraordinary expenses) from exceeding 0.60% of average daily net assets will be terminated following April 30, 2026. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading <span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">Principal Investment Strategies</span>
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The Fund will normally invest at least 90% of its net assets (including investment borrowings) in the securities that comprise the Index. The Fund, using an indexing investment approach, attempts to replicate, before fees and expenses, the performance of the Index. The Index is developed and owned by Nasdaq, Inc. (the “Index Provider”), and is calculated and maintained by Nasdaq. The Index Provider may, from time to time, exercise reasonable discretion as it deems appropriate in order to ensure Index integrity. The Index is designed to track public companies that derive a substantial portion of their revenues from midstream activities and/or the exploration and production of natural gas. According to the Index Provider, to be eligible for inclusion in the Index, a security must be listed on a qualifying U.S. securities exchange and be issued by a company involved in the exploration and production of crude oil and natural gas, according to FactSet Revere, whereby its natural gas proved reserves accounts for 30% or greater of its total proved reserves. Natural gas shall be converted to Barrels of Oil Equivalent (“BOE”) using the industry standard conversion of 1 BOE equals 6,000 cubic feet. In addition, according to the Index Provider, the Index removes all duplicate (multiple share classes) securities and securities that do not meet the size, float, seasoning, and liquidity requirements of the Index. Security types eligible for the Index generally include common stocks, depositary receipts and master limited partnership ("MLP") interests. All eligible operating companies and the 10 eligible MLPs with the highest market capitalizations are included in the Index. The Index uses modified linear-weighted methodology where Index securities are weighted based on their market capitalization and liquidity ranks. The Index allocates 15% of its weight to securities issued by MLPs and 85% of its weight to securities issued by operating companies. The initial weights are then adjusted within each of these two segments so that no security has a weight that exceeds 50% of the ratio between that security’s three-month average daily dollar trading volume and $100 million or that exceeds 4% of the ratio between that security’s free float market capitalization and $1 billion. The maximum weight of any security is 4.5%. The Fund may invest in companies with various market capitalizations. The Index is rebalanced and reconstituted quarterly and the Fund will make corresponding changes to its portfolio shortly after the Index changes are made public. The Index’s quarterly rebalance and reconstitution schedule may cause the Fund to experience a higher rate of portfolio turnover. The Fund will be concentrated in an industry or a group of industries to the extent that the Index is so concentrated. As of March 31, 2025, the Index was composed of 41 securities and the Fund had significant investments in energy companies, although this may change from time to time. The Fund’s investments will change as the Index changes and, as a result, the Fund may have significant investments in jurisdictions or investment sectors that it may not have had as of March 31, 2025. To the extent the Fund invests a significant portion of its assets in a given jurisdiction or investment sector, the Fund may be exposed to the risks associated with that jurisdiction or investment sector.
Strategy Portfolio Concentration [Text] rr_StrategyPortfolioConcentration <span style="font-family:Arial;font-size:9.00pt;">The Fund will be concentrated in an industry or a group of industries to the </span><span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">extent that the Index is so concentrated. As of </span><span style="font-family:Arial;font-size:9.00pt;">March 31, 2025, the Index was composed of 41 securities and the Fund had </span><span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">significant investments in energy companies, although this may change from time to time.</span>
Risk [Heading] rr_RiskHeading <span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">Principal Risks</span>
Risk Caption rr_RiskCaption <span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">You could lose money by investing in the Fund.</span><span style="font-family:Arial;font-size:9.00pt;">An investment in the Fund is not a deposit of a bank and is not insured or </span><span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.</span><span style="font-family:Arial;font-size:9.00pt;"> There can be no assurance that </span><span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">the Fund’s investment objective will be achieved. The order of the below risk factors does not indicate the significance of any </span><span style="font-family:Arial;font-size:9.00pt;">particular risk factor.</span>
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading <span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">Annual Total Return</span>
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The bar chart and table below illustrate the annual calendar year returns of the Fund based on net asset value as well as the average annual Fund and Index returns. The bar chart and table provide an indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year-to-year and by showing how the Fund’s average annual total returns based on net asset value compared to those of the Index, a broad-based securities market index and a market index. The Fund’s performance information is accessible on the Fund’s website at http://www.ftportfolios.com.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns <span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">The bar chart and table below illustrate the annual calendar year returns of the Fund based on net asset value as well as the average annual Fund and Index returns. The bar chart and table provide an indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year-to-year and by showing how the Fund’s average annual total returns based on net asset value compared to those of the Index, a broad-based securities market index and a market index. </span>
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress <span style="font-family:Arial;font-size:9.00pt;">http://www.ftportfolios.com</span>
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture <span style="font-family:Arial;font-size:9.00pt;">The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.</span>
Bar Chart [Heading] rr_BarChartHeading <span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">First Trust Natural Gas ETF</span> <br/><span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">Calendar Year Total Returns as of 12/31</span>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock During the periods shown in the chart above:ReturnPeriod EndedBest Quarter81.18%June 30, 2020Worst Quarter-65.00%March 31, 2020
Performance Table Heading rr_PerformanceTableHeading <span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">Average Annual Total Returns for the Periods Ended December 31, 2024</span>
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate <span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">All after-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of any state or local tax.</span>
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred <span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">Your own actual after-tax returns will depend on your specific tax situation and may differ from what is shown here. After-tax returns are not relevant to investors who hold Fund shares in tax-deferred accounts such as individual retirement accounts </span><span style="font-family:Arial;font-size:9.00pt;">(</span><span style="font-family:Arial;font-size:9.00pt;font-style:italic;">IRAs</span><span style="font-family:Arial;font-size:9.00pt;">) or employee-sponsored retirement plans.</span>
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher <span style="font-family:Arial;font-size:9.00pt;">Returns after taxes on distributions reflect the taxed return on the payment of dividends </span><span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">and capital gains. Returns after taxes on distributions and sale of shares assume you sold your shares at period end, and, therefore, are also adjusted for any capital gains or losses incurred. Returns for an index do not include expenses, which are </span><span style="font-family:Arial;font-size:9.00pt;">deducted from Fund returns, or taxes.</span>
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.All after-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of any state or local tax.Returns after taxes on distributions reflect the taxed return on the payment of dividends and capital gains. Returns after taxes on distributions and sale of shares assume you sold your shares at period end, and, therefore, are also adjusted for any capital gains or losses incurred. Returns for an index do not include expenses, which are deducted from Fund returns, or taxes.Your own actual after-tax returns will depend on your specific tax situation and may differ from what is shown here. After-tax returns are not relevant to investors who hold Fund shares in tax-deferred accounts such as individual retirement accounts (IRAs) or employee-sponsored retirement plans.
First Trust Natural Gas ETF | Risk Lose Money [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock You could lose money by investing in the Fund.
First Trust Natural Gas ETF | Risk Not Insured Depository Institution [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.
First Trust Natural Gas ETF | Authorized Participant Concentration Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock AUTHORIZED PARTICIPANT CONCENTRATION RISK. Only an authorized participant may engage in creation or redemption transactions directly with the Fund. A limited number of institutions act as authorized participants for the Fund. To the extent that these institutions exit the business or are unable to proceed with creation and/or redemption orders and no other authorized participant steps forward to create or redeem, the Fund’s shares may trade at a premium or discount (the difference between the market price of the Fund's shares and the Fund's net asset value) and possibly face delisting and the bid/ask spread (the difference between the price that someone is willing to pay for shares of the Fund at a specific point in time versus the price at which someone is willing to sell) on the Fund’s shares may widen.
First Trust Natural Gas ETF | Current Market Conditions Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock CURRENT MARKET CONDITIONS RISK. Current market conditions risk is the risk that a particular investment, or shares of the Fund in general, may fall in value due to current market conditions. As a means to fight inflation, which remains at elevated levels, the Federal Reserve and certain foreign central banks have raised interest rates; however, the Federal Reserve has recently lowered interest rates and may continue to do so. U.S. regulators have proposed several changes to market and issuer regulations which would directly impact the Fund, and any regulatory changes could adversely impact the Fund’s ability to achieve its investment strategies or make certain investments. Recent and potential future bank failures could result in disruption to the broader banking industry or markets generally and reduce confidence in financial institutions and the economy as a whole, which may also heighten market volatility and reduce liquidity. Additionally, challenges in commercial real estate markets, including rising interest rates, declining valuations and increasing vacancies, could have a broader impact on financial markets. The ongoing adversarial political climate in the United States, as well as political and diplomatic events both domestic and abroad, have and may continue to have an adverse impact the U.S. regulatory landscape, markets and investor behavior, which could have a negative impact on the Fund’s investments and operations. The change in administration resulting from the 2024 United States national elections could result in significant impacts to international trade relations, tax and immigration policies, and other aspects of the national and international political and financial landscape, which could affect, among other things, inflation and the securities markets generally. Other unexpected political, regulatory and diplomatic events within the U.S. and abroad may affect investor and consumer confidence and may adversely impact financial markets and the broader economy. For example, ongoing armed conflicts between Russia and Ukraine in Europe and among Israel, Iran, Hamas and other militant groups in the Middle East, have caused and could continue to cause significant market disruptions and volatility within the markets in Russia, Europe, the Middle East and the United States. The hostilities and sanctions resulting from those hostilities have and could continue to have a significant impact on certain Fund investments as well as Fund performance and liquidity. The economies of the United States and its trading partners, as well as the financial markets generally, may be adversely impacted by trade disputes, including the imposition of tariffs, and other matters. For example, the United States has imposed trade barriers and restrictions on China. In addition, the Chinese government is engaged in a longstanding dispute with Taiwan, continually threatening an invasion. If the political climate between the United States and China does not improve or continues to deteriorate, if China were to attempt invading Taiwan, or if other geopolitical conflicts develop or worsen, economies, markets and individual securities may be adversely affected, and the value of the Fund’s assets may go down. A public health crisis and the ensuing policies enacted by governments and central banks may cause significant volatility and uncertainty in global financial markets, negatively impacting global growth prospects. As the COVID-19 global pandemic illustrated, such events may affect certain geographic regions, countries, sectors and industries more significantly than others. Advancements in technology may also adversely impact markets and the overall performance of the Fund. For instance, the economy may be significantly impacted by the advanced development and increased regulation of artificial intelligence. Additionally, cyber security breaches of both government and non-government entities could have negative impacts on infrastructure and the ability of such entities, including the Fund, to operate properly. These events, and any other future events, may adversely affect the prices and liquidity of the Fund’s portfolio investments and could result in disruptions in the trading markets.
First Trust Natural Gas ETF | Cyber Security Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock CYBER SECURITY RISK. The Fund is susceptible to operational risks through breaches in cyber security. A breach in cyber security refers to both intentional and unintentional events that may cause the Fund to lose proprietary information, suffer data corruption or lose operational capacity. Such events could cause the Fund to incur regulatory penalties, reputational damage, additional compliance costs associated with corrective measures and/or financial loss. Cyber security breaches may involve unauthorized access to the Fund’s digital information systems through “hacking” or malicious software coding but may also result from outside attacks such as denial-of-service attacks through efforts to make network services unavailable to intended users. In addition, cyber security breaches of the issuers of securities in which the Fund invests or the Fund’s third-party service providers, such as its administrator, transfer agent, custodian, or sub-advisor, as applicable, can also subject the Fund to many of the same risks associated with direct cyber security breaches. Although the Fund has established risk management systems designed to reduce the risks associated with cyber security, there is no guarantee that such efforts will succeed, especially because the Fund does not directly control the cyber security systems of issuers or third-party service providers.
First Trust Natural Gas ETF | Deferred Tax Liability Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock DEFERRED TAX LIABILITY RISK. Cash distributions from an MLP to the Fund that exceed the Fund’s allocable share of the MLP’s taxable income are considered return of capital that reduce the Fund’s adjusted tax basis in the Fund’s MLP equity securities. Such distributions are not ordinarily subject to tax at the time of distribution unless the distributions exceed the Fund’s adjusted tax basis in the Fund’s MLP equity securities. The reductions in the Fund’s adjusted tax basis in Fund’s interest in the MLP will increase the amount of gain (or decrease the amount of loss) recognized by the Fund on a subsequent sale of the MLP equity securities. The Fund will accrue deferred income taxes for any future tax liability associated with: (i) that portion of MLP distributions considered to be a tax-deferred return of capital; and (ii) capital appreciation of its investments. Upon the sale of an MLP security, the Fund may be liable for previously deferred taxes. The Fund’s accrued deferred tax liability will be reflected each day in the Fund’s net asset value (“NAV”). Increases in deferred tax liability will decrease NAV. Conversely, decreases in deferred liability will increase NAV. The Fund will rely to some extent on information provided by the MLPs, which is not necessarily timely, to estimate deferred tax liability for purposes of financial statement reporting and determining the NAV. From time to time, the Fund will modify the estimates or assumptions regarding the Fund’s deferred tax liability as new information becomes available. The Fund’s estimates regarding its deferred tax liability are made in good faith; however, the daily estimate of the Fund’s deferred tax liability used to calculate the Fund’s NAV could vary significantly from the Fund’s actual tax liability. The Fund will generally compute deferred income taxes based on the federal income tax rate applicable to corporations and an assumed rate attributable to state taxes.
First Trust Natural Gas ETF | Depositary Receipts Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock DEPOSITARY RECEIPTS RISK. Depositary receipts represent equity interests in a foreign company that trade on a local stock exchange. Depositary receipts may be less liquid than the underlying shares in their primary trading market. Any distributions paid to the holders of depositary receipts are usually subject to a fee charged by the depositary. Holders of depositary receipts may have limited voting rights, and investment restrictions in certain countries may adversely impact the value of depositary receipts because such restrictions may limit the ability to convert the equity shares into depositary receipts and vice versa. Such restrictions may cause the equity shares of the underlying issuer to trade at a discount or premium to the market price of the depositary receipts.
First Trust Natural Gas ETF | Energy Companies Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock ENERGY COMPANIES RISK. The success of energy companies may be cyclical and highly dependent on energy prices. The market value of securities issued by energy companies may decline for many reasons, including, among other things, changes in the levels and volatility of global energy prices, energy supply and demand, capital expenditures on exploration and production of energy sources, exchange rates, interest rates, economic conditions, tax treatment, energy conservation efforts, increased competition and technological advances. Energy companies may be subject to substantial government regulation and contractual fixed pricing, which may increase the cost of doing business and limit the earnings of these companies. A significant portion of the revenues of energy companies may depend on a relatively small number of customers, including governmental entities and utilities. As a result, governmental budget constraints may have a material adverse effect on the stock prices of energy companies. Energy companies may also operate in, or engage in transactions involving, countries with less developed regulatory regimes or a history of expropriation, nationalization or other adverse policies. Energy companies also face a significant risk of liability from accidents resulting in injury or loss of life or property, pollution or other environmental problems, equipment malfunctions or mishandling of materials and a risk of loss from terrorism, political strife or natural disasters.
First Trust Natural Gas ETF | Equity Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock EQUITY SECURITIES RISK. The value of the Fund's shares will fluctuate with changes in the value of the equity securities in which it invests. Equity securities prices fluctuate for several reasons, including changes in investors’ perceptions of the financial condition of an issuer or the general condition of the relevant equity market, such as market volatility, or when political or economic events affecting an issuer occur. Common stock prices may be particularly sensitive to rising interest rates, as the cost of capital rises and borrowing costs increase. Equity securities may decline significantly in price over short or extended periods of time, and such declines may occur in the equity market as a whole, or they may occur in only a particular country, company, industry or sector of the market.
First Trust Natural Gas ETF | Index Concentration Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock INDEX CONCENTRATION RISK. The Fund will be concentrated in an industry or a group of industries to the extent that the Index is so concentrated. To the extent that the Fund invests a significant percentage of its assets in a single asset class or the securities of issuers within the same country, state, region, industry or sector, an adverse economic, business or political development may affect the value of the Fund’s investments more than if the Fund were more broadly diversified. A significant exposure makes the Fund more susceptible to any single occurrence and may subject the Fund to greater market risk than a fund that is more broadly diversified. There may be instances in which the Index, for a variety of reasons including changes in the prices of individual securities held by the Fund, has a larger exposure to a small number of stocks or a single stock relative to the rest of the stocks in the Index. Under such circumstances, the Fund will not deviate from the Index except in rare circumstances or in an immaterial way and therefore the Fund’s returns would be more greatly influenced by the returns of the stock(s) with the larger exposure.
First Trust Natural Gas ETF | Index Or Model Constituent Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock INDEX OR MODEL CONSTITUENT RISK. The Fund may be a constituent of one or more indices or ETF models. As a result, the Fund may be included in one or more index-tracking exchange-traded funds or mutual funds. Being a component security of such a vehicle could greatly affect the trading activity involving the Fund’s shares, the size of the Fund and the market volatility of the Fund. Inclusion in an index could increase demand for the Fund and removal from an index could result in outsized selling activity in a relatively short period of time. As a result, the Fund’s net asset value could be negatively impacted and the Fund’s market price may be below the Fund’s net asset value during certain periods. In addition, index rebalances may potentially result in increased trading activity in the Fund's shares.
First Trust Natural Gas ETF | Index Provider Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock INDEX PROVIDER RISK. There is no assurance that the Index Provider, or any agents that act on its behalf, will compile the Index accurately, or that the Index will be determined, maintained, constructed, reconstituted, rebalanced, composed, calculated or disseminated accurately. The Index Provider and its agents do not provide any representation or warranty in relation to the quality, accuracy or completeness of data in the Index, and do not guarantee that the Index will be calculated in accordance with its stated methodology. The Advisor’s mandate as described in this prospectus is to manage the Fund consistently with the Index provided by the Index Provider. The Advisor relies upon the Index provider and its agents to accurately compile, maintain, construct, reconstitute, rebalance, compose, calculate and disseminate the Index accurately. Therefore, losses or costs associated with any Index Provider or agent errors generally will be borne by the Fund and its shareholders. To correct any such error, the Index Provider or its agents may carry out an unscheduled rebalance of the Index or other modification of Index constituents or weightings. When the Fund in turn rebalances its portfolio, any transaction costs and market exposure arising from such portfolio rebalancing will be borne by the Fund and its shareholders. Unscheduled rebalances also expose the Fund to additional tracking error risk. Errors in respect of the quality, accuracy and completeness of the data used to compile the Index may occur from time to time and may not be identified and corrected by the Index Provider for a period of time or at all, particularly where the Index is less commonly used as a benchmark by funds or advisors. For example, during a period where the Index contains incorrect constituents, the Fund tracking the Index would have market exposure to such constituents and would be underexposed to the Index’s other constituents. Such errors may negatively impact the Fund and its shareholders. The Index Provider and its agents rely on various sources of information to assess the criteria of issuers included in the Index, including information that may be based on assumptions and estimates. Neither the Fund nor the Advisor can offer assurances that the Index’s calculation methodology or sources of information will provide an accurate assessment of included issuers. Unusual market conditions or issuer-specific events may cause the Index Provider to postpone a scheduled rebalance, exclude or substitute a security in the Index or undertake other measures which could cause the Index to vary from its normal or expected composition. The postponement of a scheduled rebalance in a time of market volatility could mean that constituents that would otherwise be removed at rebalance due to changes in market capitalizations, issuer credit ratings, or other reasons may remain, causing the performance and constituents of the Index to vary from those expected under normal conditions. Apart from scheduled rebalances, the Index Provider or its agents may carry out additional ad hoc rebalances to the Index due to unusual market conditions or in order, for example, to correct an error in the selection of index constituents.
First Trust Natural Gas ETF | Inflation Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock INFLATION RISK. Inflation risk is the risk that the value of assets or income from investments will be less in the future as inflation decreases the value of money. As inflation increases, the present value of the Fund’s assets and distributions may decline.
First Trust Natural Gas ETF | Large Capitalization Companies Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock LARGE CAPITALIZATION COMPANIES RISK. Large capitalization companies may grow at a slower rate and be less able to adapt to changing market conditions than smaller capitalization companies. Thus, the return on investment in securities of large capitalization companies may be less than the return on investment in securities of small and/or mid capitalization companies. The performance of large capitalization companies also tends to trail the overall market during different market cycles.
First Trust Natural Gas ETF | Market Maker Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock MARKET MAKER RISK. The Fund faces numerous market trading risks, including the potential lack of an active market for Fund shares due to a limited number of market markers. Decisions by market makers or authorized participants to reduce their role or step away from these activities in times of market stress could inhibit the effectiveness of the arbitrage process in maintaining the relationship between the underlying values of the Fund’s portfolio securities and the Fund’s market price. The Fund may rely on a small number of third-party market makers to provide a market for the purchase and sale of shares. Any trading halt or other problem relating to the trading activity of these market makers could result in a dramatic change in the spread between the Fund’s net asset value and the price at which the Fund’s shares are trading on the Exchange, which could result in a decrease in value of the Fund’s shares. This reduced effectiveness could result in Fund shares trading at a discount to net asset value and also in greater than normal intraday bid-ask spreads for Fund shares.
First Trust Natural Gas ETF | Market Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock MARKET RISK. Market risk is the risk that a particular investment, or shares of the Fund in general, may fall in value. Securities are subject to market fluctuations caused by real or perceived adverse economic, political, and regulatory factors or market developments, changes in interest rates and perceived trends in securities prices. Shares of the Fund could decline in value or underperform other investments. In addition, local, regional or global events such as war, acts of terrorism, market manipulation, government defaults, government shutdowns, regulatory actions, political changes, diplomatic developments, the imposition of sanctions and other similar measures, spread of infectious diseases or other public health issues, recessions, natural disasters, or other events could have a significant negative impact on the Fund and its investments. Any of such circumstances could have a materially negative impact on the value of the Fund’s shares, the liquidity of an investment, and may result in increased market volatility. During any such events, the Fund’s shares may trade at increased premiums or discounts to their net asset value, the bid/ask spread on the Fund’s shares may widen and the returns on investment may fluctuate.
First Trust Natural Gas ETF | MLP Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock MLP RISK. Investments in securities of MLPs involve certain risks different from or in addition to the risks of investing in common stocks. MLP common units can be affected by macro-economic factors and other factors unique to the partnership or company and the industry or industries in which the MLP operates. Certain MLP securities may trade in relatively low volumes due to their smaller capitalizations or other factors, which may cause them to have a high degree of price volatility and illiquidity. The structures of MLPs create certain risks, including, for example, risks related to the limited ability of investors to control an MLP and to vote on matters affecting the MLP, risks related to potential conflicts of interest between an MLP and the MLP's general partner, the risk that an MLP will generate insufficient cash flow to meet its current operating requirements, the risk that an MLP will issue additional securities or engage in other transactions that will have the effect of diluting the interests of existing investors, and risks related to the general partner's right to require unit-holders to sell their common units at an undesirable time or price.
First Trust Natural Gas ETF | MLP Tax Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock MLP TAX RISK. The Fund’s ability to meet its investment objective relies in part upon the level of taxable income it receives from the MLPs in which it invests, a factor over which the Fund has no control. The benefit the Fund derives from its investment in MLPs is largely dependent on their being treated as partnerships for U.S. federal income tax purposes. Partnerships do not pay U.S. federal income tax at the partnership level. Rather, each partner is allocated a share of the partnership’s income, gains, losses, deductions and expenses. A change in current tax law or a change in the underlying business mix of a given MLP could result in an MLP being treated as a corporation for U.S. federal income tax purposes, which would result in the MLP being required to pay U.S. federal income tax (as well as state and local income taxes) on its taxable income at the applicable corporate tax rate. This would have the effect of reducing the amount of cash available for distribution by an MLP and could result in a significant reduction in the value of the Fund’s investment. The classification of an MLP as a corporation for U. S. federal income tax purposes would have the effect of reducing the amount of cash available for distribution by the MLP and causing any such distributions received by the Fund to be taxed as dividend income to the extent of the MLP’s current or accumulated earnings and profits. To the extent a distribution received by the Fund from an MLP is treated as a return of capital, the Fund’s adjusted tax basis in the interests of the MLP may be reduced, which will result in an increase in the amount of income or gain (or decrease in the amount of loss) that will be recognized by the Fund for tax purposes upon the sale of any such interests or upon subsequent distributions in respect of such interests. Furthermore, any return of capital distribution received from an MLP may require the Fund to restate the character of its distributions and amend any shareholder tax reporting previously issued. See "Deferred Tax Liability Risk."
First Trust Natural Gas ETF | Natural Gas Companies Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock NATURAL GAS COMPANIES RISK. One of natural gas companies’ primary risks is the competitive risk associated with the prices of alternative fuels, such as coal and oil. For example, major natural gas customers such as industrial users and electric power generators often have the ability to switch between the use of coal, oil or natural gas. During periods when competing fuels are less expensive, the revenues of gas utility companies may decline with a corresponding impact on earnings. After years of booming production, natural gas firms have recently begun scaling back after record low prices and huge surpluses. Weather is another risk that may affect natural gas companies. Mild weather contributes to a scaled back demand for natural gas and declining stock prices for natural gas companies. Additionally, natural gas companies are sensitive to increased interest rates because of the capital intensive nature of their business. Furthermore, there are additional risks and hazards that are inherent to natural gas companies that may cause the price of natural gas to widely fluctuate. The exploration for, and production of, natural gas is an uncertain process with many risks. The cost of drilling, completing and operating wells for natural gas is often uncertain, and a number of factors can delay or prevent drilling operations or production, including: unexpected drilling conditions; pressure or irregularities in formations; equipment failures or repairs; fires or other accidents; adverse weather conditions; pipeline ruptures or spills; and shortages or delays in the availability of drilling rigs and the delivery of equipment.
First Trust Natural Gas ETF | Non Correlation Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock NON-CORRELATION RISK. The Fund’s return may not match the return of the Index for a number of reasons. The Fund incurs operating expenses not applicable to the Index, and may incur costs in buying and selling securities, especially when rebalancing the Fund’s portfolio holdings to reflect changes in the composition of the Index. In addition, the Fund’s portfolio holdings may not exactly replicate the securities included in the Index or the ratios between the securities included in the Index. Additionally, in order to comply with its investment strategies and policies, the Fund portfolio may deviate from the composition of the Index. Accordingly, the Fund's return may underperform the return of the Index.
First Trust Natural Gas ETF | Non US Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock NON-U.S. SECURITIES RISK. Non-U.S. securities are subject to higher volatility than securities of domestic issuers due to possible adverse political, social or economic developments, restrictions on foreign investment or exchange of securities, capital controls, lack of liquidity, currency exchange rates, excessive taxation, government seizure of assets, the imposition of sanctions by foreign governments, different legal or accounting standards, and less government supervision and regulation of securities exchanges in foreign countries.
First Trust Natural Gas ETF | Operational Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock OPERATIONAL RISK. The Fund is subject to risks arising from various operational factors, including, but not limited to, human error, processing and communication errors, errors of the Fund’s service providers, counterparties or other third-parties, failed or inadequate processes and technology or systems failures. The Fund relies on third-parties for a range of services, including custody. Any delay or failure relating to engaging or maintaining such service providers may affect the Fund’s ability to meet its investment objective. Although the Fund and the Fund's investment advisor seek to reduce these operational risks through controls and procedures, there is no way to completely protect against such risks.
First Trust Natural Gas ETF | Passive Investment Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock PASSIVE INVESTMENT RISK. The Fund is not actively managed. The Fund invests in securities included in or representative of the Index regardless of investment merit. The Fund generally will not attempt to take defensive positions in declining markets. In the event that the Index is no longer calculated, the Index license is terminated or the identity or character of the Index is materially changed, the Fund will seek to engage a replacement index.
First Trust Natural Gas ETF | Portfolio Turnover Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock PORTFOLIO TURNOVER RISK. High portfolio turnover may result in the Fund paying higher levels of transaction costs and may generate greater tax liabilities for shareholders. Portfolio turnover risk may cause the Fund’s performance to be less than expected.
First Trust Natural Gas ETF | Premium Discount Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock PREMIUM/DISCOUNT RISK. The market price of the Fund’s shares will generally fluctuate in accordance with changes in the Fund’s net asset value as well as the relative supply of and demand for shares on the Exchange. The Fund’s investment advisor cannot predict whether shares will trade below, at or above their net asset value because the shares trade on the Exchange at market prices and not at net asset value. Price differences may be due, in large part, to the fact that supply and demand forces at work in the secondary trading market for shares will be closely related, but not identical, to the same forces influencing the prices of the holdings of the Fund trading individually or in the aggregate at any point in time. However, given that shares can only be purchased and redeemed in Creation Units, and only to and from broker-dealers and large institutional investors that have entered into participation agreements (unlike shares of closed-end funds, which frequently trade at appreciable discounts from, and sometimes at premiums to, their net asset value), the Fund’s investment advisor believes that large discounts or premiums to the net asset value of shares should not be sustained. During stressed market conditions, the market for the Fund’s shares may become less liquid in response to deteriorating liquidity in the market for the Fund’s underlying portfolio holdings, which could in turn lead to differences between the market price of the Fund’s shares and their net asset value and the bid/ask spread on the Fund’s shares may widen.
First Trust Natural Gas ETF | Smaller Companies Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock SMALLER COMPANIES RISK. Small and/or mid capitalization companies may be more vulnerable to adverse general market or economic developments, and their securities may be less liquid and may experience greater price volatility than larger, more established companies as a result of several factors, including limited trading volumes, fewer products or financial resources, management inexperience and less publicly available information. Accordingly, such companies are generally subject to greater market risk than larger, more established companies.
First Trust Natural Gas ETF | Trading Issues Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock TRADING ISSUES RISK. Trading in Fund shares on the Exchange may be halted due to market conditions or for reasons that, in the view of the Exchange, make trading in shares inadvisable. In addition, trading in Fund shares on the Exchange is subject to trading halts caused by extraordinary market volatility pursuant to the Exchange’s “circuit breaker” rules. There can be no assurance that the requirements of the Exchange necessary to maintain the listing of the Fund will continue to be met or will remain unchanged. The Fund may have difficulty maintaining its listing on the Exchange in the event the Fund’s assets are small, the Fund does not have enough shareholders, or if the Fund is unable to proceed with creation and/or redemption orders.
First Trust Natural Gas ETF | First Trust Natural Gas ETF  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)[ctag:span_t-rule1] rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Management Fees[ctag:span_t-indent1] rr_ManagementFeesOverAssets 0.40%
Distribution and Service (12b-1) Fees[ctag:span_t-indent1] rr_DistributionAndService12b1FeesOverAssets none
Other Expenses[ctag:span_t-indent1-rule] rr_OtherExpensesOverAssets 0.17%
Total Annual Fund Operating Expenses[ctag:span_t-indent1-rule3] rr_AcquiredFundFeesAndExpensesOverAssets 0.57%
Total Annual Fund Operating Expenses[ctag:span_t-indent1-rule3] rr_ExpensesOverAssets none [1]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursement[ctag:span_t-indent1-rule3] rr_FeeWaiverOrReimbursementOverAssets 0.57%
1 Year rr_ExpenseExampleNoRedemptionYear01 $ 58
3 Years rr_ExpenseExampleNoRedemptionYear03 183
5 Years rr_ExpenseExampleNoRedemptionYear05 318
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 714
2015 rr_AnnualReturn2015 (59.10%)
2016 rr_AnnualReturn2016 19.48%
2017 rr_AnnualReturn2017 (11.53%)
2018 rr_AnnualReturn2018 (34.77%)
2019 rr_AnnualReturn2019 (15.87%)
2020 rr_AnnualReturn2020 (23.22%)
2021 rr_AnnualReturn2021 98.69%
2022 rr_AnnualReturn2022 47.27%
2023 rr_AnnualReturn2023 2.55%
2024 rr_AnnualReturn2024 3.98%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel <span style="font-family:Arial;font-size:9.00pt;margin-left:0.0pt;">Best Quarter</span>
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 81.18%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel <span style="font-family:Arial;font-size:9.00pt;margin-left:0.0pt;">Worst Quarter</span>
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2020
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (65.00%)
1 Year rr_AverageAnnualReturnYear01 3.98%
5 Years rr_AverageAnnualReturnYear05 19.10%
10 Years rr_AverageAnnualReturnYear10 (5.49%)
Since Inception rr_AverageAnnualReturnSinceInception (6.06%)
Inception Date rr_AverageAnnualReturnInceptionDate May 08, 2007
First Trust Natural Gas ETF | After tax on distributions | First Trust Natural Gas ETF  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 2.86%
5 Years rr_AverageAnnualReturnYear05 17.56%
10 Years rr_AverageAnnualReturnYear10 (6.50%)
Since Inception rr_AverageAnnualReturnSinceInception (6.70%)
First Trust Natural Gas ETF | After tax on distributions and sale of fund shares | First Trust Natural Gas ETF  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 2.37%
5 Years rr_AverageAnnualReturnYear05 14.69%
10 Years rr_AverageAnnualReturnYear10 (4.35%)
Since Inception rr_AverageAnnualReturnSinceInception (4.13%)
First Trust Natural Gas ETF | Russell 3000® Index (reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 23.81%
5 Years rr_AverageAnnualReturnYear05 13.86%
10 Years rr_AverageAnnualReturnYear10 12.55%
Since Inception rr_AverageAnnualReturnSinceInception 9.93%
First Trust Natural Gas ETF | ISE-Revere Natural Gas(TM) Index (reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 4.68%
5 Years rr_AverageAnnualReturnYear05 18.68%
10 Years rr_AverageAnnualReturnYear10 (5.60%)
Since Inception rr_AverageAnnualReturnSinceInception (5.88%)
First Trust Natural Gas ETF | S&P Composite 1500® Energy Index (reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 5.98%
5 Years rr_AverageAnnualReturnYear05 12.11%
10 Years rr_AverageAnnualReturnYear10 4.34%
Since Inception rr_AverageAnnualReturnSinceInception 4.25%
[1] First Trust Advisors L.P., the Fund’s investment advisor, has agreed to waive fees and/or reimburse Fund expenses to the extent that the operating expenses of the Fund (excluding interest expense, brokerage commissions and other trading expenses, acquired fund fees and expenses, taxes and extraordinary expenses) exceed 0.60% of its average daily net assets (the “Expense Cap”) at least through April 30, 2026. The agreement may be terminated by the Trust, on behalf of the Fund, at any time and by the Fund’s investment advisor only after April 30, 2026 upon 60 days’ written notice.