S000008595 [Member] Investment Strategy - William Blair International Small Cap Growth Fund	Dec. 31, 2025
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.

S000008595 [Member] Investment Strategy - William Blair International Small Cap Growth Fund

Dec. 31, 2025

PRINCIPAL INVESTMENT STRATEGIES:

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.