Label | Element | Value | ||
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William Blair Small-Mid Cap Core Fund | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
Risk/Return [Heading] | rr_RiskReturnHeading | WILLIAM BLAIR SMALL‑MID CAP CORE FUND SUMMARY | ||
Objective [Heading] | rr_ObjectiveHeading | INVESTMENT OBJECTIVE: | ||
Objective, Primary [Text Block] | rr_ObjectivePrimaryTextBlock | The William Blair Small‑Mid Cap Core Fund seeks long-term capital appreciation. | ||
Expense [Heading] | rr_ExpenseHeading | FEES AND EXPENSES: | ||
Expense Narrative [Text Block] | rr_ExpenseNarrativeTextBlock | This table describes the fees and expenses that you may pay if you buy, hold and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below. | ||
Shareholder Fees Caption [Text] | rr_ShareholderFeesCaption | Shareholder Fees (fees paid directly from your investment): | ||
Operating Expenses Caption [Text] | rr_OperatingExpensesCaption | Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment): | ||
Fee Waiver or Reimbursement over Assets, Date of Termination | rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination | April 30, 2026 | ||
Portfolio Turnover [Heading] | rr_PortfolioTurnoverHeading | Portfolio Turnover: | ||
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 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 59% of the average value of its portfolio. | ||
Portfolio Turnover, Rate | rr_PortfolioTurnoverRate | 59.00% | ||
Expense Example [Heading] | rr_ExpenseExampleHeading | Example: | ||
Expense Example Narrative [Text Block] | rr_ExpenseExampleNarrativeTextBlock | This example is intended to help you compare the cost of investing in shares of the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and the Fund’s operating expenses remain the same. The figures reflect the expense limitation for the first year. Although your actual costs may be higher or lower, based on these assumptions your costs would be: | ||
Strategy [Heading] | rr_StrategyHeading | PRINCIPAL INVESTMENT STRATEGIES: | ||
Strategy Narrative [Text Block] | rr_StrategyNarrativeTextBlock | Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in equity securities of small capitalized (“small cap”) and medium capitalized (“mid cap”) companies. The Fund invests in a diversified portfolio of equity securities, primarily common stocks, of small cap and mid cap U.S. companies that the Adviser deems to be of high quality but undervalued by the marketplace. For purposes of the Fund, the Adviser considers a company to be a small cap or a mid cap company if it has a market capitalization no larger than the largest capitalized company included in the Russell Midcap® Index at the time of the Fund’s investment. Securities of companies whose market capitalizations no longer meet this definition after purchase may continue to be held in the Fund. To a limited extent, the Fund may also purchase stocks of companies with business characteristics similar to small cap and mid cap companies, but that may have market capitalizations above the market capitalization of the largest member of the Russell Midcap® Index. The Russell Midcap® Index measures the performance of the 800 companies with the lowest market capitalizations in the Russell 1000® Index. The size of companies in the Russell Midcap® Index may change with market conditions. In addition, changes to the composition of the Russell Midcap® Index can change the market capitalization range of companies included in the index. As of March 31, 2025, the Russell Midcap® Index included securities issued by companies that ranged in size between $272.6 million and $197.9 billion. The Russell 2500TM Index, the Fund’s performance benchmark, measures the performance of the 2,500 smallest companies in the Russell 3000® Index with a weighted average market capitalization of approximately $7.8 billion, median capitalization of $1.3 billion and market capitalization of the largest company at $42.8 billion as of March 31, 2025. In choosing investments, the Adviser performs fundamental company analysis and focuses on stock selection. The Adviser evaluates the extent to which a company meets the criteria set forth below. All of the criteria are evaluated relative to the valuation of the security. The weight given to a particular investment criterion will depend upon the circumstances, and Fund holdings may not meet all of the following criteria: (a) the company should be, or should have the expectation of becoming, a significant provider in the primary markets it serves; (b) the company should have some distinctive attribute relative to present or potential competitors (this may, for example, take the form of proprietary products or processes, a unique distribution system, an entrenched brand name or an especially strong financial position relative to its competition); and (c) the company should have a strong management team.
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Risk [Heading] | rr_RiskHeading | PRINCIPAL RISKS: | ||
Bar Chart and Performance Table [Heading] | rr_BarChartAndPerformanceTableHeading | FUND PERFORMANCE HISTORY: | ||
Performance Narrative [Text Block] | rr_PerformanceNarrativeTextBlock | The information below provides some 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 for the periods indicated compare with those of a broad measure of market performance and an additional index. The Russell 3000® Index represents a broad measure of market performance, while the Russell 2500TM Index is generally representative of the market sectors or types of investments in which the Fund invests. The Fund’s past performance (before and after taxes) does not necessarily indicate how it will perform in the future. For more recent performance information, go to www.williamblairfunds.com or call 1‑800‑635‑2886. | ||
Performance Information Illustrates Variability of Returns [Text] | rr_PerformanceInformationIllustratesVariabilityOfReturns | The information below provides some 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 for the periods indicated compare with those of a broad measure of market performance and an additional index. | ||
Performance Additional Market Index [Text] | rr_PerformanceAdditionalMarketIndex | The information below provides some 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 for the periods indicated compare with those of a broad measure of market performance and an additional index. The Russell 3000® Index represents a broad measure of market performance, while the Russell 2500TM Index is generally representative of the market sectors or types of investments in which the Fund invests. | ||
Performance Availability Phone [Text] | rr_PerformanceAvailabilityPhone | 1‑800‑635‑2886 | ||
Performance Availability Website Address [Text] | rr_PerformanceAvailabilityWebSiteAddress | www.williamblairfunds.com | ||
Performance Past Does Not Indicate Future [Text] | rr_PerformancePastDoesNotIndicateFuture | The Fund’s past performance (before and after taxes) does not necessarily indicate how it will perform in the future. | ||
Bar Chart [Heading] | rr_BarChartHeading | Annual Total Returns. | ||
Bar Chart Narrative [Text Block] | rr_BarChartNarrativeTextBlock | The bar chart below provides an illustration of how the Fund’s Class I shares performance has varied in each of the calendar years since the Fund commenced operations. | ||
Bar Chart Closing [Text Block] | rr_BarChartClosingTextBlock | Highest QuarterlyReturn28.67% ( | )Lowest QuarterlyReturn(26.97)% ( )||
Performance Table Heading | rr_PerformanceTableHeading | Average Annual Total Returns (For the periods ended December 31, 2024). | ||
Performance Table Uses Highest Federal Rate | rr_PerformanceTableUsesHighestFederalRate | After‑tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. | ||
Performance Table Not Relevant to Tax Deferred | rr_PerformanceTableNotRelevantToTaxDeferred | After‑tax returns are not relevant to investors who hold their Fund shares through tax‑deferred arrangements, such as 401(k) plans or individual retirement accounts. | ||
Performance Table One Class of after Tax Shown [Text] | rr_PerformanceTableOneClassOfAfterTaxShown | The table below shows returns on a before‑tax and after‑tax basis for Class I shares and on a before‑tax basis for Class R6 shares. After‑tax returns for Class R6 shares will vary. | ||
Performance Table Explanation after Tax Higher | rr_PerformanceTableExplanationAfterTaxHigher | In some instances, the “Return After Taxes on Distributions and Sale of Fund Shares” may be greater than the “Return Before Taxes” because the investor is assumed to be able to use the capital loss on the sale of Fund shares to offset other taxable capital gains. | ||
Performance Table Narrative | rr_PerformanceTableNarrativeTextBlock | The table below shows returns on a before‑tax and after‑tax basis for Class I shares and on a before‑tax basis for Class R6 shares. After‑tax returns for Class R6 shares will vary. After‑tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. In some instances, the “Return After Taxes on Distributions and Sale of Fund Shares” may be greater than the “Return Before Taxes” because the investor is assumed to be able to use the capital loss on the sale of Fund shares to offset other taxable capital gains. Actual after‑tax returns depend on an investor’s tax situation and may differ from those shown. After‑tax returns are not relevant to investors who hold their Fund shares through tax‑deferred arrangements, such as 401(k) plans or individual retirement accounts. | ||
William Blair Small-Mid Cap Core Fund | Risk Lose Money [Member] | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
Risk [Text Block] | rr_RiskTextBlock | The Fund’s returns will vary, and you could lose money by investing in the Fund. | ||
William Blair Small-Mid Cap Core Fund | Equity Funds General Risk [Member] | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
Risk [Text Block] | rr_RiskTextBlock | Equity Funds General Risk. The value of equity securities the Fund holds might decrease in response to the activities of an individual company or in response to general market, business and economic conditions. If this occurs, the Fund’s share price may also decrease. In addition, there is the risk that individual securities may not perform as expected or a strategy used by the Adviser may fail to produce its intended result.
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William Blair Small-Mid Cap Core Fund | Market Risk [Member] | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
Risk [Text Block] | rr_RiskTextBlock | Market Risk. The value of the Fund’s investments may go up or down, sometimes rapidly or unpredictably. The value of an investment may decline due to factors affecting securities markets generally or particular industries represented in the securities markets. The value of an investment may decline due to general market conditions that are not specifically related to a particular company, such as real or perceived adverse economic conditions, recessions, inflation, rapid interest rate changes, changes in the general outlook for corporate earnings, changes in interest or currency rates or adverse investor sentiment generally. Events such as war, geopolitical risk, acts of terrorism, social unrest, tariffs and other restrictions on trade, natural disasters, the spread of infectious illness or other public health threats, or the threat or potential of one or more such events and developments could also significantly impact the Fund and its investments. The value of an investment may also decline due to factors that affect a particular industry or industries, such as labor shortages or increased production costs and competitive conditions within an industry. During a general downturn in the securities markets, multiple asset classes may decline in value simultaneously. Geopolitical and other events may also disrupt securities markets and adversely affect global economies and markets and thereby decrease the value of the Fund’s investments.
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William Blair Small-Mid Cap Core Fund | Small and Mid Cap Company Risk [Member] | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
Risk [Text Block] | rr_RiskTextBlock | Small and Mid Cap Company Risk. Stocks of small and mid cap companies involve greater risk than those of larger, more established companies. This is because small and mid cap companies may be in earlier stages of development, may be dependent on a small number of products or services, may lack substantial capital reserves and/or do not have proven track records. Small and mid cap companies may be traded in low volumes. This can increase volatility and increase the risk that the Fund will not be able to sell a security on short notice at a reasonable price. The securities of small and mid cap companies may be more volatile and less liquid than securities of large capitalized companies.
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William Blair Small-Mid Cap Core Fund | Share Ownership Concentration Risk [Member] | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
Risk [Text Block] | rr_RiskTextBlock | Share Ownership Concentration Risk. To the extent that a significant portion of the Fund’s shares is held by a limited number of shareholders or their affiliates, there is a risk that the share trading activities of these shareholders could disrupt the Fund’s investment strategies, which could have adverse consequences for the Fund and other shareholders (e.g., by requiring the Fund to sell investments at inopportune times or causing the Fund to maintain larger-than-expected cash positions pending acquisition of investments).
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William Blair Small-Mid Cap Core Fund | Style Risk [Member] | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
Risk [Text Block] | rr_RiskTextBlock | Style Risk. Different investment styles (e.g., growth vs. value, quality bias, market capitalization focus) tend to shift in and out of favor depending on market conditions and investor sentiment, and at times when the investment style used by the Adviser for the Fund is out of favor, the Fund may underperform other equity funds that use different investment styles.
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William Blair Small-Mid Cap Core Fund | Focus Risk [Member] | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
Risk [Text Block] | rr_RiskTextBlock | Focus Risk. To the extent that the Fund focuses its investments in particular industries, asset classes or sectors of the economy, any market changes affecting companies in those industries, asset classes or sectors may impact the Fund’s performance.
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William Blair Small-Mid Cap Core Fund | Operational and Technology Risk [Member] | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
Risk [Text Block] | rr_RiskTextBlock | Operational and Technology Risk. Cyber-attacks, disruptions, or failures that affect the Fund’s service providers, counterparties, market participants, or issuers of securities held by the Fund may adversely affect the Fund and its shareholders, including by causing losses for the Fund or impairing Fund operations.
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William Blair Small-Mid Cap Core Fund | Class I | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
Maximum Sales Charge (Load) Imposed on Purchases | rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice | none | ||
Redemption Fee | rr_RedemptionFeeOverRedemption | none | ||
Management Fee | rr_ManagementFeesOverAssets | 0.90% | ||
Distribution (Rule 12b-1) Fee | rr_DistributionAndService12b1FeesOverAssets | none | ||
Other Expenses | rr_OtherExpensesOverAssets | 0.25% | ||
Total Annual Fund Operating Expenses | rr_ExpensesOverAssets | 1.15% | ||
Fee Waiver and/or Expense Reimbursement | rr_FeeWaiverOrReimbursementOverAssets | 0.20% | [1] | |
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement | rr_NetExpensesOverAssets | 0.95% | ||
1 Year | rr_ExpenseExampleYear01 | $ 97 | ||
3 Years | rr_ExpenseExampleYear03 | 346 | ||
5 Years | rr_ExpenseExampleYear05 | 614 | ||
10 Years | rr_ExpenseExampleYear10 | $ 1,380 | ||
2020 | rr_AnnualReturn2020 | 20.60% | ||
2021 | rr_AnnualReturn2021 | 26.63% | ||
2022 | rr_AnnualReturn2022 | (17.11%) | ||
2023 | rr_AnnualReturn2023 | 13.26% | ||
2024 | rr_AnnualReturn2024 | 12.04% | ||
Highest Quarterly Return, Label | rr_HighestQuarterlyReturnLabel | Highest QuarterlyReturn | ||
Highest Quarterly Return, Date | rr_BarChartHighestQuarterlyReturnDate | Dec. 31, 2020 | ||
Highest Quarterly Return | rr_BarChartHighestQuarterlyReturn | 28.67% | ||
Lowest Quarterly Return, Label | rr_LowestQuarterlyReturnLabel | Lowest QuarterlyReturn | ||
Lowest Quarterly Return, Date | rr_BarChartLowestQuarterlyReturnDate | Mar. 31, 2020 | ||
Lowest Quarterly Return | rr_BarChartLowestQuarterlyReturn | (26.97%) | ||
1 Year | rr_AverageAnnualReturnYear01 | 12.04% | ||
5 Years | rr_AverageAnnualReturnYear05 | 9.94% | ||
Since Fund Inception | rr_AverageAnnualReturnSinceInception | 10.84% | ||
Inception Date | rr_AverageAnnualReturnInceptionDate | Oct. 01, 2019 | ||
William Blair Small-Mid Cap Core Fund | Class R6 | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
Maximum Sales Charge (Load) Imposed on Purchases | rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice | none | ||
Redemption Fee | rr_RedemptionFeeOverRedemption | none | ||
Management Fee | rr_ManagementFeesOverAssets | 0.90% | ||
Distribution (Rule 12b-1) Fee | rr_DistributionAndService12b1FeesOverAssets | none | ||
Other Expenses | rr_OtherExpensesOverAssets | 0.11% | ||
Total Annual Fund Operating Expenses | rr_ExpensesOverAssets | 1.01% | ||
Fee Waiver and/or Expense Reimbursement | rr_FeeWaiverOrReimbursementOverAssets | 0.11% | [1] | |
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement | rr_NetExpensesOverAssets | 0.90% | ||
1 Year | rr_ExpenseExampleYear01 | $ 92 | ||
3 Years | rr_ExpenseExampleYear03 | 311 | ||
5 Years | rr_ExpenseExampleYear05 | 547 | ||
10 Years | rr_ExpenseExampleYear10 | $ 1,226 | ||
1 Year | rr_AverageAnnualReturnYear01 | 12.16% | ||
5 Years | rr_AverageAnnualReturnYear05 | 9.99% | ||
Since Fund Inception | rr_AverageAnnualReturnSinceInception | 10.88% | ||
Inception Date | rr_AverageAnnualReturnInceptionDate | Oct. 01, 2019 | ||
William Blair Small-Mid Cap Core Fund | Return After Taxes on Distributions | Class I | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
1 Year | rr_AverageAnnualReturnYear01 | 11.68% | ||
5 Years | rr_AverageAnnualReturnYear05 | 9.87% | ||
Since Fund Inception | rr_AverageAnnualReturnSinceInception | 10.76% | ||
Inception Date | rr_AverageAnnualReturnInceptionDate | Oct. 01, 2019 | ||
William Blair Small-Mid Cap Core Fund | Return After Taxes on Distributions and Sale of Fund Shares | Class I | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
1 Year | rr_AverageAnnualReturnYear01 | 7.38% | ||
5 Years | rr_AverageAnnualReturnYear05 | 7.89% | ||
Since Fund Inception | rr_AverageAnnualReturnSinceInception | 8.65% | ||
Inception Date | rr_AverageAnnualReturnInceptionDate | Oct. 01, 2019 | ||
William Blair Small-Mid Cap Core Fund | Russell 3000® Index (reflects no deduction for fees, expenses or taxes) | Class R6 | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
1 Year | rr_AverageAnnualReturnYear01 | 23.81% | ||
5 Years | rr_AverageAnnualReturnYear05 | 13.86% | ||
Since Fund Inception | rr_AverageAnnualReturnSinceInception | 15.03% | ||
Inception Date | rr_AverageAnnualReturnInceptionDate | Oct. 01, 2019 | ||
William Blair Small-Mid Cap Core Fund | Russell 2500™ Index (reflects no deduction for fees, expenses or taxes) | Class R6 | ||||
Risk/Return: | rr_RiskReturnAbstract | |||
1 Year | rr_AverageAnnualReturnYear01 | 12.00% | ||
5 Years | rr_AverageAnnualReturnYear05 | 8.77% | ||
Since Fund Inception | rr_AverageAnnualReturnSinceInception | 10.04% | ||
Inception Date | rr_AverageAnnualReturnInceptionDate | Oct. 01, 2019 | ||
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