S000075391 [Member] Investment Strategy - William Blair Mid Cap Value Fund	Dec. 31, 2025
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.

S000075391 [Member] Investment Strategy - William Blair Mid Cap Value Fund

Dec. 31, 2025

PRINCIPAL INVESTMENT STRATEGIES:

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.