Investment Strategy |
Dec. 31, 2025 |
|---|---|
| William Blair Growth Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | 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 U.S. growth companies of all sizes that are expected to exhibit quality growth characteristics. The Fund invests primarily in equity securities issued by companies that typically have market capitalizations no smaller than the smallest capitalized company, and no larger than the largest capitalized company, included in the Russell 3000® 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 companies in the Russell 3000® Index, but that may have market capitalizations outside the range of companies included in the index. The Russell 3000® Index is a widely recognized, unmanaged index that measures the performance of the 3,000 largest U.S. companies. The size of companies in the Russell 3000® Index may change with market conditions. In addition, changes to the composition of the Russell 3000® Index can change the market capitalization range of the companies in the index. As of March 31, 2026, the Russell 3000® Index included securities issued by companies that ranged in size between $4 million and $4.2 trillion. The Russell 3000® Growth Index, the Fund’s performance benchmark, measures the performance of Russell 3000® companies with higher price‑to‑book ratios and higher forecasted growth values. 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.
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| William Blair Large Cap Growth Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | 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 large capitalized (“large cap”) companies. The Fund invests primarily in a portfolio of equity securities, including common stocks and other forms of equity investments (e.g., securities convertible into common stocks), of large 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 large cap company if it has a market capitalization no smaller than the smallest capitalized company included in the Russell 1000® 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 large cap companies, but that may have market capitalizations below the market capitalization of the smallest member of the Russell 1000® Index. The Russell 1000® Index is a widely recognized, unmanaged index that measures the performance of the 1,000 largest U.S. companies. The companies in the Russell 1000® Index are considered representative of large cap companies. The size of companies in the Russell 1000® Index may change with market conditions. In addition, changes to the composition of the Russell 1000® Index can change the market capitalization range of the companies included in the index. As of March 31, 2026, the Russell 1000® Index included securities issued by companies that ranged in size between $907 million and $4.2 trillion. The Russell 1000® Growth Index, the Fund’s performance benchmark, measures the performance of those Russell 1000 companies with a greater-than-average growth orientation. 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. THE FUND IS NON‑DIVERSIFIED UNDER THE INVESTMENT COMPANY ACT OF 1940, AS AMENDED, AND MAY INVEST A LARGER PERCENTAGE OF ITS ASSETS IN FEWER ISSUERS THAN DIVERSIFIED MUTUAL FUNDS.
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| William Blair Mid Cap Value Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | 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 medium-capitalized (“mid cap”) companies. For purposes of the Fund, the Adviser considers a company to be a mid cap company if it has a market capitalization no smaller than the smallest capitalized company, and 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 value prospects similar to mid cap companies, but that may have market capitalizations above the market capitalization of the largest member of the Russell Midcap® Index. The Fund may invest in equity securities listed on a national securities exchange or traded in the over‑the‑counter markets. The Fund invests primarily in common stocks, but it may also invest in other types of equity securities, including real estate investment trusts (“REITs”) and American Depositary Receipts (“ADRs”). The Russell Midcap® Index measures the performance of the mid cap segment of the U.S. equity universe. The Russell Midcap® Index is a subset of the Russell 1000® Index. It includes approximately 800 of the smallest securities based on a combination of their market cap and current index membership. The Russell Midcap® Index represents approximately 22% of the total market capitalization of the Russell 1000® companies as of March 31, 2026. The companies in the Russell Midcap® Index are considered representative of mid cap companies. 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 the companies included in the index. As of March 31, 2026, the Russell Midcap® Index included securities issued by companies that ranged in size between $907 million and $116.8 billion. The Russell Midcap® Value Index, the Fund’s performance benchmark, measures the performance of those Russell Midcap companies with lower price‑to‑book ratios and lower forecasted growth values. In selecting investments for the Fund, the Adviser typically looks to invest in companies with leading market share positions, shareholder oriented managements, and strong balance sheet and cash flow ratios. Usually, the shares of the companies the Adviser buys are selling at a price to earnings ratio below the average price to earnings ratio of the stocks that comprise the Russell Midcap® Index. In addition, the companies selected by the Adviser usually have higher returns on equity and capital than the average company in the Russell Midcap® Index. The Adviser screens the Fund’s universe of potential investments to identify potentially undervalued securities based on factors such as financial strength, earnings valuation, and earnings quality. The Adviser further narrows the list of potential investments through traditional fundamental security analysis, which may include interviews with company management and a review of the assessments and opinions of outside analysts and consultants. Securities are sold when the Adviser believes the shares have become relatively overvalued or it finds more attractive alternatives. The Adviser generally will not sell a security merely due to market appreciation outside the Fund’s target capitalization range if it believes the company has valuation upside potential.
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| William Blair Small-Mid Cap Core Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | 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, 2026, the Russell Midcap® Index included securities issued by companies that ranged in size between $907 million and $116.8 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 $11.6 billion, median capitalization of $1.5 billion and market capitalization of the largest company at $97.5 billion as of March 31, 2026. 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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| William Blair Small-Mid Cap Growth Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | 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, 2026, the Russell Midcap® Index included securities issued by companies that ranged in size between $907 million and $116.8 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 $11.6 billion, median capitalization of $1.5 billion and market capitalization of the largest company at $97.5 billion as of March 31, 2026. 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.
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| William Blair Small-Mid Cap Value Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | 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. 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 value prospects similar to small to mid cap companies, but that may have market capitalizations above the market capitalization of the largest member of the Russell Midcap® Index. The Fund may invest in equity securities listed on a national securities exchange or traded in the over‑the‑counter markets. The Fund invests primarily in common stocks, but it may also invest in other types of equity securities, including real estate investment trusts (“REITs”) and American Depositary Receipts (“ADRs”). 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, 2026, the Russell Midcap® Index included securities issued by companies that ranged in size between $907 million and $116.8 billion. The Russell 2500TM Value Index, the Fund’s performance benchmark, measures the performance of those Russell 2500TM companies with lower price‑to‑book ratios and lower forecasted growth values. In selecting investments for the Fund, the Adviser typically looks to invest in companies with leading market share positions, shareholder-oriented managements, and strong balance sheet and cash flow ratios. Usually, the shares of the companies the Adviser buys are selling at a price to earnings ratio below the average price to earnings ratio of the stocks that comprise the Russell 2500TM Value Index. In addition, the companies selected by the Adviser usually have higher returns on equity and capital than the average company in the Russell 2500TM Value Index. The Adviser screens the Fund’s universe of potential investments to identify potentially undervalued securities based on factors such as financial strength, earnings valuation and earnings quality. The Adviser further narrows the list of potential investments through traditional fundamental security analysis, which may include interviews with company management and a review of the assessments and opinions of outside analysts and consultants. Securities are sold when the Adviser believes the shares have become relatively overvalued or it finds more attractive alternatives. The Adviser generally will not sell a security merely due to market appreciation outside the Fund’s target capitalization range if it believes the company has valuation upside potential.
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| William Blair Small Cap Growth Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | 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”) 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 U.S. growth companies that are expected to exhibit quality growth characteristics. The Fund’s investments in small cap companies may include a significant weighting to micro‑cap companies (which, for purposes of the Fund, are companies with market capitalizations of $1 billion or less at the time of the Fund’s investment). For purposes of the Fund, the Adviser considers a company to be a small cap company if it has a market capitalization no larger than the largest capitalized company included in the Russell 2000® 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 companies, but that may have market capitalizations above the market capitalization of the largest member of the Russell 2000® Index. The Russell 2000® Index is a widely recognized, unmanaged index of common stocks that measures the performance of the 2,000 smallest companies in the Russell 3000® Index. The companies in the Russell 2000® Index are considered representative of small cap companies. The size of companies in the Russell 2000® Index may change with market conditions. In addition, changes to the composition of the Russell 2000® Index can change the market capitalization range of the companies included in the index. As of March 31, 2026, the Russell 2000® Index included securities issued by companies that ranged in size between $4 million and $38.3 billion. The Russell 2000® Growth Index, the Fund’s performance benchmark, measures the performance of those Russell 2000 companies with a greater-than-average growth orientation. 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.
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| William Blair Small Cap Value Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | Under normal market conditions, 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”) companies. For purposes of the Fund, the Adviser considers a company to be a small cap company if it has a market capitalization no larger than the largest capitalized company included in the Russell 2000® 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 value prospects similar to small cap companies, but that may have market capitalizations above the market capitalization of the largest member of the Russell 2000® Index. The Fund may invest in equity securities listed on a national securities exchange or traded in the over‑the‑counter markets. The Fund invests primarily in common stocks, but it may also invest in other types of equity securities, including real estate investment trusts (“REITs”) and American Depositary Receipts (“ADRs”). The Russell 2000® Index is a widely recognized, unmanaged index of common stocks that measures the performance of the 2,000 smallest companies in the Russell 3000® Index. The companies in the Russell 2000® Index are considered representative of small cap companies. The size of companies in the Russell 2000® Index may change with market conditions. In addition, changes to the composition of the Russell 2000® Index can change the market capitalization range of the companies included in the index. As of March 31, 2026, the Russell 2000® Index included securities issued by companies that ranged in size between $4 million and $38.3 billion. The Russell 2000® Value Index, the Fund’s performance benchmark, measures the performance of those Russell 2000 companies with lower price‑to‑book ratios and lower forecasted growth values. In selecting investments for the Fund, the Adviser typically looks to invest in companies with leading market share positions, shareholder oriented managements, and strong balance sheet and cash flow ratios. Usually, the shares of the companies the Adviser buys are selling at a price to earnings ratio below the average price to earnings ratio of the stocks that comprise the Russell 2000® Index. In addition, the companies selected by the Adviser usually have higher returns on equity and capital than the average company in the Russell 2000® Index. The Adviser screens the Fund’s universe of potential investments to identify potentially undervalued securities based on factors such as financial strength, earnings valuation, and earnings quality. The Adviser further narrows the list of potential investments through traditional fundamental security analysis, which may include interviews with company management and a review of the assessments and opinions of outside analysts and consultants. Securities are sold when the Adviser believes the shares have become relatively overvalued or it finds more attractive alternatives. The Adviser generally will not sell a security merely due to market appreciation outside the Fund’s target capitalization range if it believes the company has valuation upside potential.
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| William Blair Global Leaders Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | Under normal market conditions, the Fund invests at least 80% of its total assets in a portfolio of equity securities, including common stocks and other forms of equity investments (e.g., securities convertible into common stocks), issued by companies of all sizes worldwide that the Adviser believes have above-average growth, profitability and quality characteristics. The Adviser seeks investment opportunities in companies at different stages of development, ranging from large, well-established companies to smaller companies at earlier stages of development, that are leaders in their country, industry or globally in terms of products, services or execution. The use of the term “global” in the Fund’s name is meant to emphasize that the Adviser looks for investment opportunities on a worldwide basis and the investment strategy is not constrained by the countries or regions in which companies are located. The Fund’s investments are normally allocated among at least five different countries. The Fund may invest in emerging markets, which include every country in the world except the United States, Canada, Japan, Australia, New Zealand, Hong Kong, Singapore and most Western European countries. In choosing investments, the Adviser performs fundamental company analysis and focuses on stock selection. The Adviser generally seeks equity securities, including common stocks, of companies that historically have had superior growth, profitability and quality relative to local markets and relative to companies within the same industry worldwide, and that are expected to continue such performance. Such companies generally will exhibit superior business fundamentals, including leadership in their field, quality products or services, distinctive marketing and distribution, pricing flexibility and revenue from products or services consumed on a steady, recurring basis. These business characteristics should be accompanied by management that is shareholder return-oriented and that uses conservative accounting policies. Companies with above-average returns on equity, strong balance sheets and consistent, above-average earnings growth will be the primary focus. Stock selection will take into account both local and global comparisons. The Adviser will vary the Fund’s sector and geographic diversification based upon the Adviser’s ongoing evaluation of economic, market and political trends throughout the world. In making decisions regarding country allocation, the Adviser will consider such factors as the conditions and growth potential of various economies and securities markets, currency exchange rates, technological developments in the various countries and other pertinent financial, social, national and political factors. THE FUND IS NON‑DIVERSIFIED UNDER THE INVESTMENT COMPANY ACT OF 1940, AS AMENDED, AND MAY INVEST A LARGER PERCENTAGE OF ITS ASSETS IN FEWER ISSUERS THAN DIVERSIFIED MUTUAL FUNDS.
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| William Blair International Leaders Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | Under normal market conditions, 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), issued by companies of all sizes domiciled outside the U.S. that the Adviser believes have above-average growth, profitability and quality characteristics. Under normal market conditions, the Fund typically holds a limited number of securities (i.e., 40‑70 securities). The Adviser seeks investment opportunities in companies at different stages of development ranging from large, well-established companies to smaller companies at earlier stages of development, that are leaders in their country, industry or globally in terms of products, services or execution. The Fund’s investments are normally allocated among at least six different countries and no more than 50% of the Fund’s equity holdings may be invested in securities of issuers in one country at any given time. Normally, the Fund’s investments will be divided among Continental Europe, the United Kingdom, Canada, Japan and the markets of the Pacific Basin. The Fund may invest the greater of 40% of its net assets or twice the emerging markets component of the MSCI All Country World ex USA Investable Market Index (“ACWI ex USA IMI”) (net) in emerging markets, which include every country in the world except the United States, Canada, Japan, Australia, New Zealand, Hong Kong, Singapore and most Western European countries.In choosing investments, the Adviser performs fundamental company analysis and focuses on stock selection. The Adviser generally seeks equity securities, including common stocks, of companies that historically have had superior growth, profitability and quality relative to local markets and relative to companies within the same industry worldwide, and that are expected to continue such performance. Such companies generally will exhibit superior business fundamentals, including leadership in their field, quality products or services, distinctive marketing and distribution, pricing flexibility and revenue from products or services consumed on a steady, recurring basis. These business characteristics should be accompanied by management that is shareholder return-oriented and that uses conservative accounting policies. Companies with above-average returns on equity, strong balance sheets and consistent, above-average earnings growth will be the primary focus. Stock selection will take into account both local and global comparisons.The Adviser will vary the Fund’s sector and geographic diversification based upon the Adviser’s ongoing evaluation of economic, market and political trends throughout the world. In making decisions regarding country allocation, the Adviser will consider such factors as the conditions and growth potential of various economies and securities markets, currency exchange rates, technological developments in the various countries and other pertinent financial, social, national and political factors. |
| William Blair International Growth Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | Under normal market conditions, the Fund invests at least 80% of its total assets in a diversified portfolio of equity securities, including common stocks and other forms of equity investments (e.g., securities convertible into common stocks), issued by companies of all sizes domiciled outside the U.S. that the Adviser believes have above-average growth, profitability and quality characteristics. The Adviser seeks investment opportunities in companies at different stages of development ranging from large, well-established companies to smaller companies at earlier stages of development. The Fund’s investments are normally allocated among at least six different countries and no more than 50% of the Fund’s equity holdings may be invested in securities of issuers in one country at any given time. Normally, the Fund’s investments will be divided among Continental Europe, the United Kingdom, Canada, Japan and the markets of the Pacific Basin. The Fund may invest the greater of 35% of its net assets or twice the emerging markets component of the MSCI All Country World ex USA Investable Market Index (“ACWI ex USA IMI”) (net) in emerging markets, which include every country in the world except the United States, Canada, Japan, Australia, New Zealand, Hong Kong, Singapore and most Western European countries.In choosing investments, the Adviser performs fundamental company analysis and focuses on stock selection. The Adviser generally seeks equity securities, including common stocks, of companies that historically have had superior growth, profitability and quality relative to local markets and relative to companies within the same industry worldwide, and that are expected to continue such performance. Such companies generally will exhibit superior business fundamentals, including leadership in their field, quality products or services, distinctive marketing and distribution, pricing flexibility and revenue from products or services consumed on a steady, recurring basis. These business characteristics should be accompanied by management that is shareholder return-oriented and that uses conservative accounting policies. Companies with above-average returns on equity, strong balance sheets and consistent, above-average earnings growth will be the primary focus. Stock selection will take into account both local and global comparisons.The Adviser will vary the Fund’s sector and geographic diversification based upon the Adviser’s ongoing evaluation of economic, market and political trends throughout the world. In making decisions regarding country allocation, the Adviser will consider such factors as the conditions and growth potential of various economies and securities markets, currency exchange rates, technological developments in the various countries and other pertinent financial, social, national and political factors. |
| William Blair Institutional International Growth Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | Under normal market conditions, the Fund invests at least 80% of its total assets in a diversified portfolio of equity securities, including common stocks and other forms of equity investments (e.g., securities convertible into common stocks), issued by companies of all sizes domiciled outside the U.S. that the Adviser believes have above-average growth, profitability and quality characteristics. The Adviser seeks investment opportunities in companies at different stages of development ranging from large, well-established companies to smaller companies at earlier stages of development. The Fund’s investments are normally allocated among at least six different countries and no more than 50% of the Fund’s equity holdings may be invested in securities of issuers in one country at any given time. Normally, the Fund’s investments will be divided among Continental Europe, the United Kingdom, Canada, Japan and the markets of the Pacific Basin. The Fund may invest the greater of 35% of its net assets or twice the emerging markets component of the MSCI All Country World ex USA Investable Market Index (“ACWI ex USA IMI”) (net) in emerging markets, which include every country in the world except the United States, Canada, Japan, Australia, New Zealand, Hong Kong, Singapore and most Western European countries.In choosing investments, the Adviser performs fundamental company analysis and focuses on stock selection. The Adviser generally seeks equity securities, including common stocks, of companies that historically have had superior growth, profitability and quality relative to local markets and relative to companies within the same industry worldwide, and that are expected to continue such performance. Such companies generally will exhibit superior business fundamentals, including leadership in their field, quality products or services, distinctive marketing and distribution, pricing flexibility and revenue from products or services consumed on a steady, recurring basis. These business characteristics should be accompanied by management that is shareholder return-oriented and that uses conservative accounting policies. Companies with above-average returns on equity, strong balance sheets and consistent, above-average earnings growth will be the primary focus. Stock selection will take into account both local and global comparisons.The Adviser will vary the Fund’s sector and geographic diversification based upon the Adviser’s ongoing evaluation of economic, market and political trends throughout the world. In making decisions regarding country allocation, the Adviser will consider such factors as the conditions and growth potential of various economies and securities markets, currency exchange rates, technological developments in the various countries and other pertinent financial, social, national and political factors. |
| William Blair International Small Cap Growth Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | 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 capitalization (“small 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), issued by foreign small cap companies that the Adviser believes have above-average growth, profitability and quality characteristics. For purposes of the Fund, William Blair Investment Management, LLC (the “Adviser”) considers a company to be a small cap company if it has a float adjusted market capitalization at the time of purchase no larger than the greater of $5 billion or the largest capitalized company included in the MSCI All Country World (“ACWI”) ex USA Small Cap Index (net). Securities of companies whose float adjusted market capitalizations no longer meet this definition of small cap company after purchase may continue to be held in the Fund. The Fund’s investments are normally allocated among at least six different countries and no more than 50% of the Fund’s equity holdings may be invested in securities of issuers in one country at any given time. Normally, the Fund’s investments will be divided among Continental Europe, the United Kingdom, Canada, Japan and the markets of the Pacific Basin. The Fund may invest the greater of 35% of its net assets or twice the emerging markets component of the MSCI ACWI ex USA Small Cap Index (net) in emerging markets, which include every country in the world except the United States, Canada, Japan, Australia, New Zealand, Hong Kong, Singapore and most Western European countries. As of March 31, 2026, the MSCI ACWI ex USA Small Cap Index (net) included securities issued by companies that ranged in size between $41 million and $12.4 billion.In choosing investments, the Adviser performs fundamental company analysis and focuses on stock selection. The Adviser generally seeks equity securities, including common stocks, of companies that historically have had superior growth, profitability and quality relative to local markets and relative to companies within the same industry worldwide, and that are expected to continue such performance. Such companies generally will exhibit superior business fundamentals, including leadership in their field, quality products or services, distinctive marketing and distribution, pricing flexibility and revenue from products or services consumed on a steady, recurring basis. These business characteristics should be accompanied by management that is shareholder return-oriented and that uses conservative accounting policies. Companies with above-average returns on equity, strong balance sheets and consistent, above-average earnings growth will be the primary focus. Stock selection will take into account both local and global comparisons.The Adviser will vary the Fund’s sector and geographic diversification based upon the Adviser’s ongoing evaluation of economic, market and political trends throughout the world. In making decisions regarding country allocation, the Adviser will consider such factors as the conditions and growth potential of various economies and securities markets, currency exchange rates, technological developments in the various countries and other pertinent financial, social, national and political factors. |
| William Blair Emerging Markets Leaders Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | Under normal market conditions, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in emerging markets securities. 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), issued by emerging market companies of all sizes that the Adviser believes have above-average growth, profitability and quality characteristics. Under normal market conditions, the Fund typically holds a limited number of securities (i.e., 50‑80 securities). The Adviser seeks investment opportunities in companies at different stages of development, ranging from large, well-established companies to smaller companies at earlier stages of development, that are leaders in their country, industry or globally in terms of products, services or execution. Emerging market companies, for purposes of the Fund, are companies organized under the laws of an emerging market country or that have securities traded principally on an exchange or over‑the‑counter in an emerging market country. Currently, emerging markets include every country in the world except the United States, Canada, Japan, Australia, New Zealand, Hong Kong, Singapore and most Western European countries. The Fund’s investments are normally allocated among at least six different countries and no more than 50% of the Fund’s equity holdings may be invested in securities of issuers in one country at any given time.In choosing investments, the Adviser performs fundamental company analysis and focuses on stock selection. The Adviser generally seeks equity securities, including common stocks, of emerging market companies that historically have had superior growth, profitability and quality relative to local markets and relative to companies within the same industry worldwide, and that are expected to continue such performance. Such companies generally will exhibit superior business fundamentals, including leadership in their field, quality products or services, distinctive marketing and distribution, pricing flexibility and revenue from products or services consumed on a steady, recurring basis. These business characteristics should be accompanied by management that is shareholder return-oriented and that uses conservative accounting policies. Companies with above-average returns on equity, strong balance sheets and consistent, above-average earnings growth at reasonable valuation levels will be the primary focus. Stock selection will take into account both local and global comparisons.The Adviser will vary the Fund’s sector and geographic diversification based upon the Adviser’s ongoing evaluation of economic, market and political trends throughout the world. In making decisions regarding country allocation, the Adviser will consider such factors as the conditions and growth potential of various economies and securities markets, currency exchange rates, technological developments in the various countries and other pertinent financial, social, national and political factors. |
| William Blair Emerging Markets Growth Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | Under normal market conditions, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in emerging markets securities. 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), issued by emerging market companies of all sizes that the Adviser believes have above-average growth, profitability and quality characteristics. The Adviser seeks investment opportunities in companies at different stages of development ranging from large, well-established companies to smaller companies at earlier stages of development. Emerging market companies, for purposes of the Fund, are companies organized under the laws of an emerging market country or that have securities traded principally on an exchange or over‑the‑counter in an emerging market country. Currently, emerging markets include every country in the world except the United States, Canada, Japan, Australia, New Zealand, Hong Kong, Singapore and most Western European countries. The Fund’s investments are normally allocated among at least six different countries and no more than 50% of the Fund’s equity holdings may be invested in securities of issuers in one country at any given time.In choosing investments, the Adviser performs fundamental company analysis and focuses on stock selection. The Adviser generally seeks equity securities, including common stocks, of emerging market companies that historically have had superior growth, profitability and quality relative to local markets and relative to companies within the same industry worldwide, and that are expected to continue such performance. Such companies generally will exhibit superior business fundamentals, including leadership in their field, quality products or services, distinctive marketing and distribution, pricing flexibility and revenue from products or services consumed on a steady, recurring basis. These business characteristics should be accompanied by management that is shareholder return-oriented and that uses conservative accounting policies. Companies with above-average returns on equity, strong balance sheets and consistent, above-average earnings growth will be the primary focus. Stock selection will take into account both local and global comparisons.The Adviser will vary the Fund’s sector and geographic diversification based upon the Adviser’s ongoing evaluation of economic, market and political trends throughout the world. In making decisions regarding country allocation, the Adviser will consider such factors as the conditions and growth potential of various economies and securities markets, currency exchange rates, technological developments in the various countries and other pertinent financial, social, national and political factors. |
| William Blair Emerging Markets ex China Growth Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in emerging markets securities, excluding companies with their principal office in the People’s Republic of China (“PRC”). (For the avoidance of doubt, the PRC shall include mainland China, Hong Kong and Macau but exclude Taiwan.) 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), issued by emerging market companies (excluding companies with their principal office in the PRC) of all sizes that the Adviser believes have above-average growth, profitability and quality characteristics. The Adviser seeks investment opportunities in companies at different stages of development ranging from large, well-established companies to smaller companies at earlier stages of development. Emerging market companies, for purposes of the Fund, are companies organized under the laws of an emerging market country or that have securities traded principally on an exchange or over‑the‑counter in an emerging market country. Currently, for purposes of the Fund, emerging markets include every country in the world except the United States, Canada, Japan, Australia, New Zealand, Hong Kong, Singapore and most Western European countries. The Fund’s investments are normally allocated among at least six different countries and no more than 50% of the Fund’s equity holdings may be invested in securities of issuers in one country at any given time.In choosing investments, the Adviser performs fundamental company analysis and focuses on stock selection. The Adviser generally seeks equity securities, including common stocks, of emerging market companies that historically have had superior growth, profitability and quality relative to local markets and relative to companies within the same industry worldwide, and that are expected to continue such performance. Such companies generally will exhibit superior business fundamentals, including leadership in their field, quality products or services, distinctive marketing and distribution, pricing flexibility and revenue from products or services consumed on a steady, recurring basis. These business characteristics should be accompanied by management that is shareholder return-oriented and that uses conservative accounting policies. Companies with above-average returns on equity, strong balance sheets and consistent, above-average earnings growth will be the primary focus. Stock selection will take into account both local and global comparisons.The Adviser will vary the Fund’s sector and geographic diversification based upon the Adviser’s ongoing evaluation of economic, market and political trends throughout the world. In making decisions regarding country allocation, the Adviser will consider such factors as the conditions and growth potential of various economies and securities markets, currency exchange rates, technological developments in the various countries and other pertinent financial, social, national and political factors. |
| William Blair Emerging Markets Small Cap Growth Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | Under normal market conditions, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in equity securities of emerging market small capitalization (“small 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), issued by emerging market small cap companies that the Adviser believes have above average growth, profitability and quality characteristics. For purposes of the Fund, the Adviser considers a company to be a small cap company if it has a float adjusted market capitalization at the time of purchase no larger than the greater of $5 billion or the largest capitalized company included in the MSCI Emerging Markets Small Cap Index (net). Securities of companies whose float adjusted market capitalizations no longer meet this definition of small cap after purchase may continue to be held in the Fund. Emerging market companies, for purposes of the Fund, are companies organized under the laws of an emerging market country or that have securities traded principally on an exchange or over‑the‑counter in an emerging market country. Currently, emerging markets include every country in the world except the United States, Canada, Japan, Australia, New Zealand, Hong Kong, Singapore and most Western European countries. The Fund’s investments are normally allocated among at least six different countries and no more than 50% of the Fund’s equity holdings may be invested in securities of issuers in one country at any given time. In choosing investments, the Adviser performs fundamental company analysis and focuses on stock selection. The Adviser generally seeks equity securities, including common stocks, of emerging market companies that historically have had superior growth, profitability and quality relative to local markets and relative to companies within the same industry worldwide, and that are expected to continue such performance. Such companies generally will exhibit superior business fundamentals, including leadership in their field, quality products or services, distinctive marketing and distribution, pricing flexibility and revenue from products or services consumed on a steady, recurring basis. These business characteristics should be accompanied by management that is shareholder return-oriented and that uses conservative accounting policies. Companies with above-average returns on equity, strong balance sheets and consistent, above-average earnings growth will be the primary focus. Stock selection will take into account both local and global comparisons. The Adviser will vary the Fund’s sector and geographic diversification based upon the Adviser’s ongoing evaluation of economic, market and political trends throughout the world. In making decisions regarding country allocation, the Adviser will consider such factors as the conditions and growth potential of various economies and securities markets, currency exchange rates, technological developments in the various countries and other pertinent financial, social, national and political factors.
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| William Blair China Growth Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in securities of companies with their principal office in the People’s Republic of China (“PRC”). For the avoidance of doubt, the PRC shall include mainland China, Hong Kong and Macau but exclude Taiwan. The Fund may pursue this objective through purchases of class A‑Shares of companies listed on the Shanghai and Shenzhen Stock Exchanges, H‑Shares listed in Hong Kong, American Depositary Receipts, Global Depositary Receipts and dual listed securities. The Fund invests primarily in a portfolio of equity securities, including common stocks and other forms of equity investments (e.g., securities convertible into common stocks), of companies of all sizes that the Adviser believes have above-average growth, profitability and quality characteristics. The Adviser seeks investment opportunities in companies at different stages of development ranging from large, well-established companies to smaller companies at earlier stages of development. In choosing investments, the Adviser performs fundamental company analysis and focuses on stock selection. The Adviser generally seeks equity securities, including common stocks, of Chinese companies that historically have had superior growth, profitability and quality relative to local markets and relative to companies within the same industry worldwide, and that are expected to continue such performance. Such companies generally will exhibit superior business fundamentals, including leadership in their field, quality products or services, distinctive marketing and distribution, pricing flexibility and revenue from products or services consumed on a steady, recurring basis. These business characteristics should be accompanied by management that is shareholder return-oriented and that uses conservative accounting policies. Companies with above-average returns on equity, strong balance sheets and consistent, above-average earnings growth will be the primary focus. As noted above, the Fund will invest in China A‑Shares. The Fund may do so through the Shanghai-Hong Kong and Shenzhen-Hong Kong Stock Connect Programs (“Stock Connect”). Stock Connect is a securities trading and clearing program between either the Shanghai Stock Exchange (“SSE”) or Shenzhen Stock Exchange (“SZSE”), and any of the Stock Exchange of Hong Kong Limited (“SEHK”), China Securities Depository or Clearing Corporation Limited (“ChinaClear”) and Hong Kong Securities Clearing Company Limited (“HKSCC”). Stock Connect is designed to permit mutual stock market access between mainland China and Hong Kong by allowing investors to trade and settle shares on each market via their local exchanges. Trading through Stock Connect is subject to daily quotas that limit the maximum daily net purchases on any particular day. Accordingly, the Fund’s direct investments in China A‑Shares may be limited by such daily quotas. Investments in China A‑Shares may also be made through the use of the Investment Manager’s Qualified Foreign Institutional Investor (“QFII”) license. THE FUND IS NON‑DIVERSIFIED UNDER THE INVESTMENT COMPANY ACT OF 1940, AS AMENDED, AND MAY INVEST A LARGER PERCENTAGE OF ITS ASSETS IN FEWER ISSUERS THAN DIVERSIFIED MUTUAL FUNDS.
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| William Blair Emerging Markets Debt Fund | |
| Prospectus [Line Items] | |
| Strategy [Heading] | PRINCIPAL INVESTMENT STRATEGIES: |
| Strategy Narrative [Text Block] | Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in debt instruments that are economically tied to emerging market countries, which may be represented by forwards or derivatives such as options, futures contracts or swap agreements. Emerging markets include every country in the world except countries included in the MSCI World Index. Debt securities in which the Fund invests include fixed-rate and floating-rate bonds issued by various public (governmental) and private (corporate) issuers (including private placements and restricted securities). The Fund invests a significant portion of its assets in sovereign debt securities (debt securities issued or guaranteed by foreign sovereign governments or their agencies, authorities or political subdivisions or instrumentalities, and supranational agencies) and debt securities of quasi-sovereign issuers (entities owned by a sovereign government). The Fund may also invest in debt securities issued or guaranteed by foreign corporations and foreign financial institutions. The Fund may invest in debt securities of any credit rating, including investment grade securities, below investment grade securities (commonly referred to as “high yield” or “junk bonds”), and unrated securities. The Fund may invest up to a maximum of 10% of the Fund’s net assets in distressed or defaulted debt securities. The Fund may invest in securities of any maturity. The majority of the Fund’s investments are generally made in securities of issuers based in South and Central America (including the Caribbean), Central Europe, Eastern Europe, Asia, Africa and the Middle East. More specifically, the Fund invests in countries where the Adviser is able to assess the specific political and economic risks and in countries that have undertaken certain economic reforms and reached certain growth objectives. Countries in which the Fund invests include frontier markets (emerging markets that are early in their development). The Fund invests in assets denominated in the currencies of economically developed and politically stable countries that are members of the Organisation for Economic Co‑operation and Development (OECD), as well as in assets denominated in local currency. The Adviser may, but is not required to, hedge the currency risk associated with the Fund’s investments. As part of its investment strategy, the Fund may utilize derivatives, including futures and forward contracts, swaps (including credit default swaps and total return swaps), credit derivatives, and currency-related derivatives. Derivatives are primarily utilized to hedge interest rate duration risk and foreign exchange risk. Additionally, the Fund may maintain assets in cash, deposit, call or current accounts or invest in short-term instruments, such as money market funds, U.S. or other government securities, certificates of deposit, bankers’ acceptances or similar temporary investments, to meet the expense needs of the Fund and/or to fund withdrawals or for such other purposes as may be determined by the Adviser. The Adviser measures the Fund’s performance against the JPMorgan Emerging Markets Bond Index (EMBI) Global Diversified, which is the Fund’s “benchmark” for purposes of its investment objective. The Fund is actively managed within its objective and is not constrained by a benchmark. THE FUND IS NON‑DIVERSIFIED UNDER THE INVESTMENT COMPANY ACT OF 1940, AS AMENDED, AND MAY INVEST A LARGER PERCENTAGE OF ITS ASSETS IN FEWER ISSUERS THAN DIVERSIFIED MUTUAL FUNDS.
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