v3.26.1
Shareholder Report, Average Annual Return (Details)
 6 Months Ended 12 Months Ended 34 Months Ended 47 Months Ended 52 Months Ended 58 Months Ended 59 Months Ended 60 Months Ended 61 Months Ended 81 Months Ended 86 Months Ended 120 Months Ended
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William Blair Growth Fund - CLASS I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Growth Fund Class I                      
Average Annual Return, Percent 7.51% 14.59%           8.88%       15.31%
William Blair Growth Fund - Class N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Growth Fund Class N                      
Average Annual Return, Percent 7.31% 14.22%           8.55%       14.97%
William Blair Growth Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Growth Fund Class R6                      
Average Annual Return, Percent 7.44% 14.63%           8.92%     14.42%  
William Blair Large Cap Growth Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Large Cap Growth Fund Class I                      
Average Annual Return, Percent 0.66% 6.22%           8.60%       16.37%
William Blair Large Cap Growth Fund - Class N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Large Cap Growth Fund Class N                      
Average Annual Return, Percent 0.54% 5.95%           8.32%       16.07%
William Blair Large Cap Growth Fund - CLASS R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Large Cap Growth Fund Class R6                      
Average Annual Return, Percent 0.70% 6.29%           8.65%     14.88%  
William Blair Mid Cap Value Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Mid Cap Value Fund Class I                      
Average Annual Return, Percent 15.01% 21.06%     8.80%              
William Blair Mid Cap Value Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Mid Cap Value Fund Class R6                      
Average Annual Return, Percent 15.01% 21.13%     8.85%              
William Blair Small-Mid Cap Core Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small-Mid Cap Core Fund Class I                      
Average Annual Return, Percent 15.58% 20.77%           5.67%   10.89%    
William Blair Small-Mid Cap Core Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small-Mid Cap Core Fund Class R6                      
Average Annual Return, Percent 15.56% 20.80%           5.71%   10.93%    
William Blair Small-Mid Cap Growth Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small-Mid Cap Growth Fund Class I                      
Average Annual Return, Percent 15.46% 24.10%           3.40%       11.42%
William Blair Small-Mid Cap Growth Fund - Class N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small-Mid Cap Growth Fund Class N                      
Average Annual Return, Percent 15.35% 23.85%           3.14%       11.14%
William Blair Small-Mid Cap Growth Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small-Mid Cap Growth Fund Class R6                      
Average Annual Return, Percent 15.49% 24.18%           3.45%     8.85%  
William Blair Small-Mid Cap Value Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small-Mid Cap Value Fund Class I                      
Average Annual Return, Percent 23.41% 32.50% 13.21%                  
William Blair Small-Mid Cap Value Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small-Mid Cap Value Fund Class R6                      
Average Annual Return, Percent 23.43% 32.47% 13.24%                  
William Blair Small Cap Growth Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small Cap Growth Fund Class I                      
Average Annual Return, Percent 22.27% 32.95%           6.61%       14.21%
William Blair Small Cap Growth Fund - Class N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small Cap Growth Fund Class N                      
Average Annual Return, Percent 22.16% 32.65%           6.36%       13.93%
William Blair Small Cap Growth Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small Cap Growth Fund Class R6                      
Average Annual Return, Percent 22.30% 33.00%           6.68%     11.82%  
William Blair Small Cap Value Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small Cap Value Fund Class I                      
Average Annual Return, Percent 23.32% 35.07%           5.99%       9.96%
William Blair Small Cap Value Fund - Class N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small Cap Value Fund Class N                      
Average Annual Return, Percent 23.18% 34.70%         6.86%          
William Blair Small Cap Value Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Small Cap Value Fund Class R6                      
Average Annual Return, Percent 23.36% 35.16%         7.23%          
William Blair Global Leaders Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Global Leaders Fund Class I                      
Average Annual Return, Percent 9.82% 16.45%           4.53%       11.48%
William Blair Global Leaders Fund - CLASS N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Global Leaders Fund Class N                      
Average Annual Return, Percent 9.69% 16.17%           4.25%       11.19%
William Blair Global Leaders Fund - CLASS R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Global Leaders Fund Class R6                      
Average Annual Return, Percent 9.82% 16.58%           4.57%       11.54%
William Blair International Leaders Fund - CLASS I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name International Leaders Fund Class I                      
Average Annual Return, Percent 16.58% 23.86%           3.65%       9.50%
William Blair International Leaders Fund - CLASS N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name International Leaders Fund Class N                      
Average Annual Return, Percent 16.39% 23.51%           3.39%       9.22%
William Blair International Leaders Fund - CLASS R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name International Leaders Fund Class R6                      
Average Annual Return, Percent 16.57% 23.90%           3.70%       9.57%
William Blair International Growth Fund - CLASS I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name International Growth Fund Class I                      
Average Annual Return, Percent 16.78% 21.88%           3.24%       8.98%
William Blair International Growth Fund - Class N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name International Growth Fund Class N                      
Average Annual Return, Percent 16.62% 21.58%           2.98%       8.68%
William Blair International Growth Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name International Growth Fund Class R6                      
Average Annual Return, Percent 16.80% 21.93%           3.30%     9.24%  
William Blair Institutional International Growth Fund - Institutional Class                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Institutional International Growth Fund                      
Average Annual Return, Percent 16.64% 21.72%           3.27%       9.09%
William Blair International Small Cap Growth Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name International Small Cap Growth Fund Class I                      
Average Annual Return, Percent 10.46% 7.69%           (0.19%)       6.32%
William Blair International Small Cap Growth Fund - Class N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name International Small Cap Growth Fund Class N                      
Average Annual Return, Percent 10.25% 7.40%           (0.45%)       6.03%
William Blair International Small Cap Growth Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name International Small Cap Growth Fund Class R6                      
Average Annual Return, Percent 10.38% 7.77%           (0.14%)       6.40%
William Blair Emerging Markets Leaders Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets Leaders Fund Class I                      
Average Annual Return, Percent 18.29% 33.92%           2.11%       7.96%
William Blair Emerging Markets Leaders Fund - Class N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets Leaders Fund Class N                      
Average Annual Return, Percent 18.21% 33.50%           1.86%       7.68%
William Blair Emerging Markets Leaders Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets Leaders Fund Class R6                      
Average Annual Return, Percent 18.32% 33.81%           2.15%       8.00%
William Blair Emerging Markets Growth Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets Growth Fund Class I                      
Average Annual Return, Percent 35.04% 54.80%           5.19%       11.26%
William Blair Emerging Markets Growth Fund - Class N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets Growth Fund Class N                      
Average Annual Return, Percent 34.83% 54.39%           4.92%       10.98%
William Blair Emerging Markets Growth Fund R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets Growth Fund Class R6                      
Average Annual Return, Percent 35.11% 54.98%           5.25%       11.34%
Emerging Markets ex China Growth Fund-Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets ex China Growth Fund Class I                      
Average Annual Return, Percent 53.38% 80.97%   27.79%                
Emerging Markets ex China Growth Fund-Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets ex China Growth Fund Class R6                      
Average Annual Return, Percent 53.40% 81.01%   27.87%                
Emerging Markets Small Cap Growth Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets Small Cap Growth Fund Class I                      
Average Annual Return, Percent 21.23% 32.38%           6.02%       9.45%
Emerging Markets Small Cap Growth Fund - Class N                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets Small Cap Growth Fund Class N                      
Average Annual Return, Percent 21.06% 32.02%           5.76%       9.16%
Emerging Markets Small Cap Growth Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets Small Cap Growth Fund Class R6                      
Average Annual Return, Percent 21.29% 32.46%           6.08%       9.52%
China Growth Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name China Growth Fund Class I                      
Average Annual Return, Percent 16.08% 29.57%       (5.04%)            
China Growth Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name China Growth Fund Class R6                      
Average Annual Return, Percent 16.00% 29.60%       (5.08%)            
Emerging Markets Debt Fund - Class I                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets Debt Fund Class I                      
Average Annual Return, Percent 5.25% 14.57%           4.28% 4.27%      
Emerging Markets Debt Fund - Class R6                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Emerging Markets Debt Fund Class R6                      
Average Annual Return, Percent 5.28% 14.64%           4.34% 4.33%      
StandardPoors500IndexMember                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name S&P 500® Index                      
Average Annual Return, Percent 10.21% 22.32%           13.41%     15.84% 15.51%
Russell 1000® Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Russell 1000® Index                      
Average Annual Return, Percent 10.33% 22.02%           12.66%     15.54% 15.30%
Russell 3000® Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Russell 3000® Index                      
Average Annual Return, Percent 10.88% 22.82% 22.07%   15.25%   12.82% 12.31%   15.92% 15.26% 15.06%
MSCI All Country World (net) [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name MSCI All Country World (net)                      
Average Annual Return, Percent 11.25% 23.67%           10.98%       12.78%
MSCI All Country World ex-U.S. IMI (net) [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name MSCI All Country World ex-U.S. IMI (net)                      
Average Annual Return, Percent 13.05% 26.56%           8.44%     9.85% 9.81%
MSCI Emerging Markets Index (net) [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name MSCI Emerging Markets Index (net)                      
Average Annual Return, Percent 23.85% 43.51%           7.20%       10.07%
MSCI Emerging Markets IMI (net) [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name MSCI Emerging Markets IMI (net)                      
Average Annual Return, Percent 22.41% 40.30%           7.17%       9.98%
MSCI Emerging Markets ex-China IMI (net) [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name MSCI Emerging Markets ex-China IMI (net)                      
Average Annual Return, Percent 35.25% 56.45%   22.38%                
MSCI China All Shares Index (net) [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name MSCI China All Shares Index (net)                      
Average Annual Return, Percent (5.05%) 9.54%       (1.44%)            
Bloomberg Global Aggregate Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Bloomberg Global Aggregate Index                      
Average Annual Return, Percent (0.21%) 0.62%           (1.55%) (1.68%)      
Russell 3000 Growth Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Russell 3000® Growth Index                      
Average Annual Return, Percent 5.88% 18.24%           13.17%     17.95% 18.13%
Russell 1000® Growth Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Russell 1000® Growth Index                      
Average Annual Return, Percent 5.33% 17.71%           13.71%     18.47% 18.58%
Russell Midcap® Value Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Russell Midcap® Value Index                      
Average Annual Return, Percent 17.58% 26.62%     10.96%              
Russell 2500™ Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Russell 2500™ Index                      
Average Annual Return, Percent 22.71% 36.72%           8.30%   12.91%    
Russell 2500™ Growth Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Russell 2500™ Growth Index                      
Average Annual Return, Percent 19.66% 32.94%           4.98%     11.07% 12.56%
Russell 2500® Value Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Russell 2500® Value Index                      
Average Annual Return, Percent 24.16% 38.54% 20.73%                  
Russell 2000® Growth Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Russell 2000® Growth Index                      
Average Annual Return, Percent 22.18% 38.74%           5.57%     10.68% 11.97%
Russell 2000® Value Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name Russell 2000® Value Index                      
Average Annual Return, Percent 22.99% 43.01%         10.12% 8.23%       10.89%
MSCI All Country World IMI (net) [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name MSCI All Country World IMI (net)                      
Average Annual Return, Percent 11.77% 24.22%           10.57%       12.54%
MSCI All Country World ex-U.S. Small Cap Index (net) [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name MSCI All Country World ex-U.S. Small Cap Index (net)                      
Average Annual Return, Percent 9.10% 19.83%           6.30%       9.10%
MSCI Emerging Markets Small Cap Index (net) [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name MSCI Emerging Markets Small Cap Index (net)                      
Average Annual Return, Percent 12.89% 20.89%           7.16%       9.48%
JPMorgan Emerging Markets Bond Index (EMBI) Global Diversified Index [Member]                        
Average Annual Return [Line Items]                        
Line Graph and Table Measure Name JPMorgan Emerging Markets Bond Index (EMBI) Global Diversified Index                      
Average Annual Return, Percent 3.32% 11.78%           2.58% 2.72%