Needham Growth Fund | Institutional Class Needham Growth Fund | Institutional Class | Russell 3000® Index Needham Growth Fund | Institutional Class | Russell Midcap® Growth Index Needham Growth Fund | Institutional Class | S&P MidCap 400® Growth Information Technology Industrials Health Care [Member] Communication Services Consumer Discretionary Financials Materials Real Estate [Member] Energy Cash & Other Investment Type [Axis] Dreyfus Treasury Securities Cash Management PDF Solutions, Inc. Corning, Inc. Lam Research Corp. Vicor Corp. Vishay Intertechnology, Inc. FormFactor, Inc. Marvell Technology, Inc. MKS, Inc. Vertiv Holdings Co. Needham Growth Fund | Retail Class Needham Growth Fund | Retail Class | Russell 3000® Index Needham Growth Fund | Retail Class | Russell Midcap® Growth Index Needham Growth Fund | Retail Class | S&P MidCap 400® Growth Needham Aggressive Growth Fund | Institutional Class Needham Aggressive Growth Fund | Institutional Class | Russell 3000® Index Needham Aggressive Growth Fund | Institutional Class | Russell 2000® Growth Index Consumer Staples Arteris, Inc. nLight, Inc. Vishay Precision Group, Inc. CECO Environmental Corp. Oil-Dri Corp. of America Needham Aggressive Growth Fund | Retail Class Needham Aggressive Growth Fund | Retail Class | Russell 3000® Index Needham Aggressive Growth Fund | Retail Class | Russell 2000® Growth Index Needham Small Cap Growth Fund | Institutional Class Needham Small Cap Growth Fund | Institutional Class | Russell 3000® Index Needham Small Cap Growth Fund | Institutional Class | Russell 2000® Growth Index Utilities Badger Meter, Inc. Ambarella, Inc. Harmonic, Inc. Veeco Instruments, Inc. Calix, Inc. CEVA, Inc. Needham Small Cap Growth Fund | Retail Class Needham Small Cap Growth Fund | Retail Class | Russell 3000® Index Needham Small Cap Growth Fund | Retail Class | Russell 2000® Growth Index Net Assets Investment Company, Portfolio Turnover