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S000072753 [Member] Investment Risks - William Blair China Growth Fund
Dec. 31, 2025
Investing Through Stock Connect Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
Investing Through Stock Connect Risk. Investing in China A‑Shares through the Stock Connect program is subject to trading, clearance, settlement, and other procedures, which could pose risks to the Fund. Trading through Stock Connect is subject to market-wide trading volume and market cap quota limitations, each of which may restrict or preclude the Fund’s ability to invest in A‑Shares through Stock Connect. A primary feature of Stock Connect is the application of the home market’s laws and rules applicable to investors in A‑Shares. Therefore, the Fund’s investments in Stock Connect A‑Shares are generally subject to PRC securities regulations and listing rules, among other restrictions. Additionally, restrictions on the timing of permitted trading activity in A‑Shares, including the imposition of local holidays in either Hong Kong or mainland China and restrictions on purchasing and selling the same security on the same day, may subject the Fund to the risk of price fluctuations of China A‑Shares at times when the Fund is unable to add to or exit its position.
Equity Funds General Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
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.
Foreign Investment Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
Foreign Investment Risk. The risks of foreign investments may include less publicly available information, less stringent investor protections and disclosure standards, less governmental regulation and supervision of foreign stock exchanges, brokers and issuers, share registration and custody, less stringent or a lack of uniform accounting, auditing and financial reporting standards, practices and requirements, the possibility of expropriation, seizure or nationalization, confiscatory taxation, limits on repatriation, adverse changes in investment or exchange control regulations, political instability, restrictions on the flow of international capital, imposition of foreign withholding or other taxes, fluctuating currencies, inflation, difficulty in obtaining and enforcing judgments against foreign entities or other adverse political, social or diplomatic developments that could affect the Fund’s investments. Foreign investments may be less liquid and their prices more volatile than the securities of U.S. companies. The Fund is expected to incur operating expenses that are higher than those of
mutual funds investing exclusively in U.S. securities due to the higher custodial fees associated with foreign securities investments.
Emerging Markets Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
Emerging Markets Risk. Foreign investment risk is typically magnified in emerging markets, which are the less developed and developing nations.
Market Risks [Member]  
Prospectus [Line Items]  
Risk [Text Block]
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. Local, regional or global events such as war, military conflict, geopolitical disputes, acts of terrorism, social unrest, tariffs and other restrictions on trade, sanctions, supply chain disruptions, increased government spending, natural disasters, the spread of infectious illness or other public health issues, or the threat or potential threat 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.
Currency Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
Currency Risk. The value of the Fund’s portfolio may be affected by changes in exchange rates or control regulations. Currency exchange rates may fluctuate significantly over short periods of time for a number of reasons, including changes in interest rates. If a non‑U.S. currency gains against the U.S. dollar, the value of the security increases in U.S. dollar terms. If a non‑U.S. currency declines against the U.S. dollar, the value of the security decreases in U.S. dollar terms.
Focus Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
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.
Liquidity Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
Liquidity Risk. Investments that trade less frequently can be more difficult or more costly to buy, or to sell, than more liquid or active investments. It may not be possible to sell or otherwise dispose of illiquid securities both at the price and within a time period deemed desirable by the Fund.
Operational and Technology Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
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.
Share Ownership Concentration Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
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). In addition, separate accounts managed by the Adviser may invest in the Fund and, therefore, the Adviser at times may have discretionary authority over a significant portion of the assets of the Fund. In such instances, the Adviser’s
decision to make changes to or rebalance its clients’ allocations in the separate accounts may substantially impact the Fund’s performance.
Smaller Company Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
Smaller Company Risk. Stocks of smaller companies involve greater risk than those of larger, more established companies. This is because smaller 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. Smaller 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 smaller companies may be more volatile and less liquid than securities of large capitalized companies.
Style Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
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 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.
Chinese Investment Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block]
Chinese Investment Risk. Because the fund focuses its investments in China, the Fund’s performance is expected to be closely tied to social, political, and economic conditions in China and to be more volatile than the performance of more geographically diversified funds. Investing in Chinese investments involves a higher degree of risk and special considerations not typically associated with investing in other more established economies or
securities markets. The Fund’s investment exposure to China may subject the Fund, to a greater extent than if investments were made in developed countries, to the risks of adverse securities markets, exchange rates and social, political, regulatory, economic, or environmental events and natural disasters that may occur in the China region. The economy, industries, and securities and currency markets of China are particularly vulnerable to the region’s dependence on exports and international trade and increasing competition from Asia’s other low‑cost emerging economies. The imposition of tariffs or other trade barriers, trade restrictions, sanctions, or investment prohibitions by the U.S. or foreign governments on exports from China may also have an adverse impact on Chinese issuers. There is also a risk that the U.S. Government or other governments may sanction Chinese issuers or otherwise prohibit U.S. persons (such as the Fund) from investing in certain Chinese issuers, such prohibitions and sanctions may be permanent or may expand over time, which may negatively affect the liquidity and price of their securities. In addition, currency fluctuations, currency convertibility, interest rate fluctuations and higher rates of inflation as a result of internal social unrest or conflicts with other countries have had, and may continue to have, negative effects on the economies and securities markets of China. The government of the PRC exercises significant control over the economy in mainland China and may at any time alter or discontinue economic reforms. Hong Kong and Macau do not exercise the same level of control over their economies as does the PRC with respect to mainland China, but changes to their political and economic relationships with the PRC could adversely impact the Fund’s investments in Hong Kong and Macau.
Concerns regarding a trade war between China and the United States have triggered and may continue to trigger a significant reduction in international trade, supply chain diversification away from Chinese producers, the oversupply of certain manufactured goods, substantial price reductions of goods and possible failure of individual companies and/or large segments of China’s export industry, all of which may have a negative impact on the Fund’s investments. Events such as these and their consequences are difficult to predict and it is unclear whether further tariffs, trade restrictions, sanctions, or investment prohibitions may be imposed or other escalating actions may be taken in the future.
Risk Lose Money [Member]  
Prospectus [Line Items]  
Risk [Text Block] The Fund’s returns will vary, and you could lose money by investing in the Fund.
Risk Nondiversified Status [Member]  
Prospectus [Line Items]  
Risk [Text Block]
Non‑Diversification Risk. The Fund is non‑diversified, meaning that it is permitted to invest a larger percentage of its assets in fewer issuers than diversified mutual funds. Thus, the Fund may be more susceptible to adverse developments affecting any single issuer held in its portfolio and may be more susceptible to greater losses because of these developments.