S000008605 [Member] Annual Total Returns |
12 Months Ended | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
Dec. 31, 2025 |
Dec. 31, 2024 |
Dec. 31, 2023 |
Dec. 31, 2022 |
Dec. 31, 2021 |
Dec. 31, 2020 |
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| William Blair Institutional International Growth Fund | Institutional Class | ||||||||||
| Prospectus [Line Items] | ||||||||||
| Annual Return [Percent] | 18.05% | 2.41% | 15.20% | (28.28%) | 9.39% | 32.47% | 30.75% | (17.50%) | 29.53% | (2.40%) |