S000008597 [Member] Performance Management - William Blair Small Cap Value Fund |
Dec. 31, 2025 |
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| Prospectus [Line Items] | |
| Bar Chart and Performance Table [Heading] | FUND PERFORMANCE HISTORY: |
| Performance Narrative [Text Block] | The information below provides some indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual total returns for the periods indicated compare with those of a broad measure of market performance and an additional index. The Russell 3000® Index represents a broad measure of market performance, while the Russell 2000® Value Index is generally representative of the market sectors or types of investments in which the Fund invests. The Fund’s past performance (before and after taxes) does not necessarily indicate how it will perform in the future. On July 16, 2021, the Fund acquired the assets and assumed the liabilities of the ICM Small Company Portfolio (the “Predecessor Fund”), a series of The Advisors’ Inner Circle Fund, in a reorganization (the “Reorganization”). In the Reorganization, former shareholders of the Predecessor Fund received Class I shares of the Fund. The Predecessor Fund was advised by Investment Counselors of Maryland, LLC, which was acquired by the Adviser. The Predecessor Fund’s (Institutional Class shares) performance and financial history have been adopted by Class I shares of the Fund following the Reorganization and are being used going forward from the date of the Reorganization. The performance of Class I shares of the Fund therefore reflects the performance of the Predecessor Fund prior to the Reorganization. The performance of the Predecessor Fund has not been restated to reflect the annual operating expenses of Class I shares of the Fund, which were different than those of the Predecessor Fund. Because the Fund had different fees and expenses than the Predecessor Fund, the Predecessor Fund would therefore have had different performance results if it was subject to the Fund’s fees and expenses. The Fund’s past performance (including the performance of the Predecessor Fund), before and after taxes, is not necessarily an indication of how the Fund will perform in the future and does not guarantee future results. For more recent performance information, go to https://im.williamblair.com/investments/mutual-funds or call 1‑800‑635‑2886.
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| Performance Past Does Not Indicate Future [Text] | The Fund’s past performance (before and after taxes) does not necessarily indicate how it will perform in the future. |
| Performance Information Illustrates Variability of Returns [Text] | The information below provides some indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual total returns for the periods indicated compare with those of a broad measure of market performance and anadditional index. |
| Performance Additional Market Index [Text] | The information below provides some indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual total returns for the periods indicated compare with those of a broad measure of market performance and anadditional index. The Russell 3000® Index represents a broad measure of market performance, while the Russell 2000® Value Index is generally representative of the market sectors or types of investments in which the Fund invests. |
| Bar Chart Narrative [Text Block] | The bar chart below provides an illustration of how the Fund’s Class I shares performance has varied in each of the last ten calendar years. |
| Bar Chart [Heading] | Annual Total Returns. |
| Bar Chart Closing [Text Block] | Highest QuarterlyReturn29.75% ()Lowest Quarterly Return(35.02)% () |
| Performance Table Heading | Average Annual Total Returns (For the periods ended December 31, 2025). |
| Performance Table Narrative | The table below shows returns on a before‑tax and after‑tax basis for Class I shares and on a before‑tax basis for Class N and Class R6 shares. After‑tax returns for Class N and Class R6 shares will vary. After‑tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. In some instances, the “Return After Taxes on Distributions and Sale of Fund Shares” may be greater than the “Return Before Taxes” because the investor is assumed to be able to use the capital loss on the sale of Fund shares to offset other taxable capital gains. Actual after‑tax returns depend on an investor’s tax situation and may differ from those shown. After‑tax returns are not relevant to investors who hold their Fund shares through tax‑deferred arrangements, such as 401(k) plans or individual retirement accounts. |
| Performance Table Uses Highest Federal Rate | After‑tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. |
| Performance Table Not Relevant to Tax Deferred | After‑tax returns are not relevant to investors who hold their Fund shares through tax‑deferred arrangements, such as 401(k) plans or individual retirement accounts. |
| Performance Table Explanation after Tax Higher | In some instances, the “Return After Taxes on Distributions and Sale of Fund Shares” may be greater than the “Return Before Taxes” because the investor is assumed to be able to use the capital loss on the sale of Fund shares to offset other taxable capital gains. |
| Performance Table One Class of after Tax Shown [Text] | The table below shows returns on a before‑tax and after‑tax basis for Class I shares and on a before‑tax basis for Class N and Class R6 shares. After‑tax returns for Class N and Class R6 shares will vary. |
| Performance Availability Website Address [Text] | https://im.williamblair.com/investments/mutual-funds |
| Performance Availability Phone [Text] | 1‑800‑635‑2886 |
| Class I | |
| Prospectus [Line Items] | |
| Highest Quarterly Return, Label [Optional Text] | Highest QuarterlyReturn |
| Highest Quarterly Return | 29.75% |
| Highest Quarterly Return, Date | Dec. 31, 2020 |
| Lowest Quarterly Return, Label [Optional Text] | Lowest QuarterlyReturn |
| Lowest Quarterly Return | (35.02%) |
| Lowest Quarterly Return, Date | Mar. 31, 2020 |