v3.26.1
S000008603 [Member] Annual Fund Operating Expenses - William Blair Small-Mid Cap Growth Fund
Dec. 31, 2025
Prospectus [Line Items]  
Fee Waiver or Reimbursement over Assets, Date of Termination April 30, 2027
Class N  
Prospectus [Line Items]  
Management Fees (as a percentage of Assets) 0.94%
Distribution and Service (12b-1) Fees 0.25%
Other Expenses (as a percentage of Assets): 0.18%
Expenses (as a percentage of Assets) 1.37%
Fee Waiver or Reimbursement 0.13% [1]
Net Expenses (as a percentage of Assets) 1.24%
Class I  
Prospectus [Line Items]  
Management Fees (as a percentage of Assets) 0.94%
Distribution and Service (12b-1) Fees 0.00%
Other Expenses (as a percentage of Assets): 0.18%
Expenses (as a percentage of Assets) 1.12%
Fee Waiver or Reimbursement 0.13% [1]
Net Expenses (as a percentage of Assets) 0.99%
Class R6  
Prospectus [Line Items]  
Management Fees (as a percentage of Assets) 0.94%
Distribution and Service (12b-1) Fees 0.00%
Other Expenses (as a percentage of Assets): 0.05%
Expenses (as a percentage of Assets) 0.99%
Fee Waiver or Reimbursement 0.05% [1]
Net Expenses (as a percentage of Assets) 0.94%
[1]
William Blair Investment Management, LLC (the “Adviser”) has entered into a contractual agreement with the Fund to waive fees and/or reimburse expenses in order to limit the Fund’s operating expenses (excluding interest expenses, taxes, brokerage commissions, acquired fund fees and expenses, dividend and interest expenses on short sales, other investment-related costs and extraordinary expenses, such as litigation and other expenses not incurred in the ordinary course of the Fund’s business) to 1.24%, 0.99% and 0.94% of average daily net assets for Class N, Class I and Class R6 shares, respectively, until April 30, 2027. The Adviser may not terminate this arrangement prior to April 30, 2027 without the approval of the Fund’s Board of Trustees.