v3.26.3
Shareholder Report, Average Annual Return (Details)
 6 Months Ended 12 Months Ended 60 Months Ended 114 Months Ended 120 Months Ended
Jun. 30, 2026
Jun. 30, 2026
Jun. 30, 2026
Jun. 30, 2026
Jun. 30, 2026
Needham Growth Fund | Institutional Class          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Institutional Class        
Average Annual Return, Percent   90.95% 14.51% 17.27%  
Needham Growth Fund | Retail Class          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Retail Class        
Average Annual Return, Percent   90.12% 14.01%   16.89%
Needham Aggressive Growth Fund | Institutional Class          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Institutional Class        
Average Annual Return, Percent   89.98% 23.39% 22.60%  
Needham Aggressive Growth Fund | Retail Class          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Retail Class        
Average Annual Return, Percent   89.23% 22.73%   22.98%
Needham Small Cap Growth Fund | Institutional Class          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Institutional Class        
Average Annual Return, Percent   114.10% 10.27% 19.96%  
Needham Small Cap Growth Fund | Retail Class          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Retail Class        
Average Annual Return, Percent   112.86% 9.66%   20.51%
Needham Growth Fund | Institutional Class | Russell 3000® Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell 3000® Index        
Average Annual Return, Percent   22.82% 12.31% 14.89%  
Needham Growth Fund | Retail Class | Russell 3000® Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell 3000® Index        
Average Annual Return, Percent   22.82% 12.31%   15.06%
Needham Aggressive Growth Fund | Institutional Class | Russell 3000® Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell 3000® Index        
Average Annual Return, Percent   22.82% 12.31% 14.89%  
Needham Aggressive Growth Fund | Retail Class | Russell 3000® Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell 3000® Index        
Average Annual Return, Percent   22.82% 12.31%   15.06%
Needham Small Cap Growth Fund | Institutional Class | Russell 3000® Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell 3000® Index        
Average Annual Return, Percent   22.82% 12.31% 14.89%  
Needham Small Cap Growth Fund | Retail Class | Russell 3000® Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell 3000® Index        
Average Annual Return, Percent   22.82% 12.31%   15.06%
Needham Growth Fund | Institutional Class | Russell Midcap® Growth Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell Midcap® Growth Index        
Average Annual Return, Percent   6.17% 6.03% 13.19%  
Needham Growth Fund | Institutional Class | S&P MidCap 400® Growth          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name S&P MidCap 400® Growth        
Average Annual Return, Percent   25.89% 9.07% 10.98%  
Needham Growth Fund | Retail Class | Russell Midcap® Growth Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell Midcap® Growth Index        
Average Annual Return, Percent   6.17% 6.03%   13.04%
Needham Growth Fund | Retail Class | S&P MidCap 400® Growth          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name S&P MidCap 400® Growth        
Average Annual Return, Percent   25.89% 9.07%   11.65%
Needham Aggressive Growth Fund | Institutional Class | Russell 2000® Growth Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell 2000® Growth Index        
Average Annual Return, Percent   38.74% 5.57% 11.19%  
Needham Aggressive Growth Fund | Retail Class | Russell 2000® Growth Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell 2000® Growth Index        
Average Annual Return, Percent   38.74% 5.57%   11.97%
Needham Small Cap Growth Fund | Institutional Class | Russell 2000® Growth Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell 2000® Growth Index        
Average Annual Return, Percent   38.74% 5.57% 11.19%  
Needham Small Cap Growth Fund | Retail Class | Russell 2000® Growth Index          
Average Annual Return [Line Items]          
Line Graph and Table Measure Name Russell 2000® Growth Index        
Average Annual Return, Percent   38.74% 5.57%   11.97%