v3.25.1
Label Element Value
William Blair Small-Mid Cap Growth Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading WILLIAM BLAIR SMALL‑MID CAP GROWTH FUND SUMMARY
Objective [Heading] rr_ObjectiveHeading INVESTMENT OBJECTIVE:
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The William Blair Small‑Mid Cap Growth 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 63% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 63.00%
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 market conditions, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in stocks of small capitalized (“small cap”) and medium capitalized (“mid cap”) companies. The Fund invests primarily in a diversified portfolio of equity securities, including common stocks and other forms of equity investments (e.g., securities convertible into common stocks), of small cap and mid cap U.S. growth companies that are expected to exhibit quality growth characteristics. For purposes of the Fund, the Adviser considers a company to be a small cap or 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 and growth prospects 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 $351.9 million and $89 billion. The Russell 2500TM Growth Index, the Fund’s performance benchmark, measures the performance of those Russell 2500TM companies with higher price‑to‑book ratios and higher forecasted growth values. The Russell 2500TM Index 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 quality growth 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 (for example, this may 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), (c) the company should participate in an industry expected to grow rapidly due to economic factors or technological change or should grow through market share gains in its industry and (d) the company should have a strong management team.
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 Growth 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 Growth 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 N shares performance has varied in each of the last ten calendar years.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock Highest QuarterlyReturn26.45% (2Q20)Lowest QuarterlyReturn(21.07)% (2Q22)
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 N shares and on a before‑tax basis for Class I and Class R6 shares. After‑tax returns for Class I and 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 N shares and on a before‑tax basis for Class I and Class R6 shares. After‑tax returns for Class I and 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 Growth 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 Growth 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.
William Blair Small-Mid Cap Growth 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.
William Blair Small-Mid Cap Growth 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.
William Blair Small-Mid Cap Growth 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).
William Blair Small-Mid Cap Growth 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 growth 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.
William Blair Small-Mid Cap Growth 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.
William Blair Small-Mid Cap Growth 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.
William Blair Small-Mid Cap Growth Fund | Class N  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Redemption Fee rr_RedemptionFeeOverRedemption none
Management Fee rr_ManagementFeesOverAssets 0.94%
Distribution (Rule 12b-1) Fee rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses rr_OtherExpensesOverAssets 0.17%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.36%
Fee Waiver and/or Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.12% [1]
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement rr_NetExpensesOverAssets 1.24%
1 Year rr_ExpenseExampleYear01 $ 126
3 Years rr_ExpenseExampleYear03 419
5 Years rr_ExpenseExampleYear05 733
10 Years rr_ExpenseExampleYear10 $ 1,625
2015 rr_AnnualReturn2015 4.47%
2016 rr_AnnualReturn2016 6.45%
2017 rr_AnnualReturn2017 28.57%
2018 rr_AnnualReturn2018 (2.29%)
2019 rr_AnnualReturn2019 30.41%
2020 rr_AnnualReturn2020 32.04%
2021 rr_AnnualReturn2021 8.27%
2022 rr_AnnualReturn2022 (23.11%)
2023 rr_AnnualReturn2023 17.64%
2024 rr_AnnualReturn2024 11.21%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest QuarterlyReturn
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 26.45%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest QuarterlyReturn
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Jun. 30, 2022
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (21.07%)
1 Year rr_AverageAnnualReturnYear01 11.21%
5 Years rr_AverageAnnualReturnYear05 7.54%
10 Years rr_AverageAnnualReturnYear10 10.11%
William Blair Small-Mid Cap Growth 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 0.94%
Distribution (Rule 12b-1) Fee rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.17%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.11%
Fee Waiver and/or Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.12% [1]
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement rr_NetExpensesOverAssets 0.99%
1 Year rr_ExpenseExampleYear01 $ 101
3 Years rr_ExpenseExampleYear03 341
5 Years rr_ExpenseExampleYear05 600
10 Years rr_ExpenseExampleYear10 $ 1,341
1 Year rr_AverageAnnualReturnYear01 11.51%
5 Years rr_AverageAnnualReturnYear05 7.81%
10 Years rr_AverageAnnualReturnYear10 10.39%
William Blair Small-Mid Cap Growth 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 0.94%
Distribution (Rule 12b-1) Fee rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.04%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.98%
Fee Waiver and/or Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.04% [1]
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement rr_NetExpensesOverAssets 0.94%
1 Year rr_ExpenseExampleYear01 $ 96
3 Years rr_ExpenseExampleYear03 308
5 Years rr_ExpenseExampleYear05 538
10 Years rr_ExpenseExampleYear10 $ 1,198
1 Year rr_AverageAnnualReturnYear01 11.54%
5 Years rr_AverageAnnualReturnYear05 7.86%
Since Share Class Inception rr_AverageAnnualReturnSinceInception 8.40%
Inception Date rr_AverageAnnualReturnInceptionDate May 02, 2019
William Blair Small-Mid Cap Growth Fund | Return After Taxes on Distributions | Class N  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 7.74%
5 Years rr_AverageAnnualReturnYear05 5.64%
10 Years rr_AverageAnnualReturnYear10 8.39%
William Blair Small-Mid Cap Growth Fund | Return After Taxes on Distributions and Sale of Fund Shares | Class N  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 8.66%
5 Years rr_AverageAnnualReturnYear05 5.72%
10 Years rr_AverageAnnualReturnYear10 7.96%
William Blair Small-Mid Cap Growth Fund | Russell 3000® Index (reflects no deduction for fees, expenses or taxes) | Class I  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 23.81%
5 Years rr_AverageAnnualReturnYear05 13.86%
10 Years rr_AverageAnnualReturnYear10 12.55%
William Blair Small-Mid Cap Growth 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 Share Class Inception rr_AverageAnnualReturnSinceInception 14.26%
Inception Date rr_AverageAnnualReturnInceptionDate May 02, 2019
William Blair Small-Mid Cap Growth Fund | Russell 2500™ Growth Index (reflects no deduction for fees, expenses or taxes) | Class I  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 13.90%
5 Years rr_AverageAnnualReturnYear05 8.08%
10 Years rr_AverageAnnualReturnYear10 9.45%
William Blair Small-Mid Cap Growth Fund | Russell 2500™ Growth Index (reflects no deduction for fees, expenses or taxes) | Class R6  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 13.90%
5 Years rr_AverageAnnualReturnYear05 8.08%
Since Share Class Inception rr_AverageAnnualReturnSinceInception 8.74%
Inception Date rr_AverageAnnualReturnInceptionDate May 02, 2019
[1]
William Blair Investment Management, LLC (the “Adviser”) has entered into a contractual agreement with the Fund to waive fees and/or reimburse expenses in order to limit the Fund’s operating expenses (excluding interest expenses, taxes, brokerage commissions, acquired fund fees and expenses, dividend and interest expenses on short sales, other investment-related costs and extraordinary expenses, such as litigation and other expenses not incurred in the ordinary course of the Fund’s business) to 1.24%, 0.99% and 0.94% of average daily net assets for Class N, Class I and Class R6 shares, respectively, until April 30, 2026. The Adviser may not terminate this arrangement prior to April 30, 2026 without the approval of the Fund’s Board of Trustees.