v3.26.1
Average Annual Total Returns
 12 Months Ended 60 Months Ended 108 Months Ended 120 Months Ended 283 Months Ended 292 Months Ended 360 Months Ended
Mar. 31, 2026
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Russell 3000® Index                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] [1],[2] Russell 3000® Index(a)(b)              
Average Annual Return, Percent   17.15% [1],[3],[4] 13.15% [1],[3],[4] 14.45% [1],[2],[3] 14.29% [1],[3],[4] 10.16% [3],[4] 9.76% [1],[2] 10.24%
Russell MidCap® Growth Index                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] [5],[6] Russell MidCap® Growth Index(a)(b)              
Average Annual Return, Percent [5],[6]   8.66% 6.65% 13.08% 12.49%     9.96%
S&P MidCap 400 Growth                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] [5],[6] S&P MidCap 400 Growth(a)(b)              
Average Annual Return, Percent [5],[6]   7.50% 9.12% 9.66% 10.72%     11.06%
Bloomberg US 3000 Index                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] [1],[2],[3],[5],[6] Bloomberg US 3000 Index(a)(b)              
Average Annual Return, Percent   17.21% [1],[3],[4],[5],[6] 13.09% [1],[2],[3],[5],[6] 14.44% [1],[2],[3],[5],[6] 14.27% [1],[2],[3],[5],[6] 10.29% [3],[4] 9.87% [1],[2] 10.23% [5],[6]
Bloomberg US Mid Growth Index                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] [5],[6] Bloomberg US Mid Growth Index(a)(b)              
Average Annual Return, Percent [5],[6]   10.28% 6.89% 12.35% 11.83%     10.22%
Bloomberg US 400 Index                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] [5],[6] Bloomberg US 400 Index(a)(b)              
Average Annual Return, Percent [5],[6]   11.22% 7.49% 10.40% 10.75%     10.66%
Russell 2000® Growth Index                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] [1],[4] Russell 2000® Growth Index(a)(b)              
Average Annual Return, Percent   13.01% [1],[2],[3] 3.18% [1],[3],[4] 9.38% [1],[3],[4] 9.57% [1],[3],[4] 8.72% [3],[4] 8.37% [1],[2]  
Bloomberg US 2000 Growth Index                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] [1],[4] Bloomberg US 2000 Growth Index(a)(b)              
Average Annual Return, Percent   12.82% [1],[3],[4] 3.88% [1],[2],[3] 9.63% [1],[3],[4] 10.03% [1],[2],[3] 9.42% [3],[4] 9.29% [1],[2]  
Needham Growth Fund, Institutional Class                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return Before Taxes              
Average Annual Return, Percent   9.60% 6.52% 11.85%        
Performance Inception Date Dec. 30, 2016              
Needham Growth Fund, Retail Class                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return Before Taxes              
Average Annual Return, Percent   9.06% 6.06%   11.26%     12.39%
Performance Inception Date Jan. 01, 1996              
Needham Growth Fund, Retail Class | After Taxes on Distributions                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return After Taxes on Distributions              
Average Annual Return, Percent   7.16% 5.09%   9.58%     11.01%
Needham Growth Fund, Retail Class | After Taxes on Distributions and Sales                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return After Taxes on Distributions       and Redemption              
Average Annual Return, Percent   6.62% 4.68%   8.80%     10.55%
Needham Aggressive Growth Fund, Institutional Class                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return Before Taxes              
Average Annual Return, Percent   27.09% 15.33% 17.18%        
Performance Inception Date Dec. 30, 2016              
Needham Aggressive Growth Fund, Retail Class                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return Before Taxes              
Average Annual Return, Percent   26.51% 14.69%   17.07%   12.09%  
Performance Inception Date Sep. 04, 2001              
Needham Aggressive Growth Fund, Retail Class | After Taxes on Distributions                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return After Taxes on Distributions              
Average Annual Return, Percent   25.84% 14.20%   15.73%   11.12%  
Needham Aggressive Growth Fund, Retail Class | After Taxes on Distributions and Sales                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return After Taxes on Distributions and Redemption              
Average Annual Return, Percent   16.12% 11.74%   13.88%   10.27%  
Needham Small Cap Growth Fund, Institutional Class                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return Before Taxes              
Average Annual Return, Percent   11.16% 0.92% 13.02%        
Performance Inception Date Dec. 30, 2016              
Needham Small Cap Growth Fund, Retail Class                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return Before Taxes              
Average Annual Return, Percent   10.50% 0.36%   14.14% 11.09%    
Performance Inception Date May 22, 2002              
Needham Small Cap Growth Fund, Retail Class | After Taxes on Distributions                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return After Taxes on Distributions              
Average Annual Return, Percent   10.50% (1.33%)   11.08% 9.15%    
Needham Small Cap Growth Fund, Retail Class | After Taxes on Distributions and Sales                
Prospectus [Line Items]                
Average Annual Return, Label [Optional Text] Return After Taxes on Distributions and       Redemption              
Average Annual Return, Percent   6.22% (0.22%)   10.39% 8.82%    
[1] The Aggressive Growth Fund has selected the Bloomberg indices to replace the Russell indices, effective March 31, 2026, because the Aggressive Growth Fund believes the Bloomberg indices are more representative of and more suited to the Aggressive Growth Fund’s portfolio. Performance of the Russell indices will not be shown in the future.
[2] Comparative indices reflect no deductions for fees, expenses, or taxes.
[3] The Small Cap Growth Fund has selected the Bloomberg indices to replace the Russell indices, effective March 31, 2026, because the Small Cap Growth Fund believes the Bloomberg indices are more representative of and more suited to the Small Cap Growth Fund’s portfolio. Performance of the Russell indices will not be shown in the future.
[4] Comparative indices reflect no deductions for fees, expenses, or taxes.
[5] The Growth Fund has selected the Bloomberg indices to replace the Russell and S&P indices, effective March 31, 2026, because the Growth Fund believes the Bloomberg indices are more representative of and more suited to the Growth Fund’s portfolio. Performance of the Russell and S&P indices will not be shown in the future.
[6] Comparative indices reflect no deductions for fees, expenses, or taxes.