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.

S000069167 [Member] Investment Strategy - William Blair Emerging Markets Debt 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 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.