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Label Element Value
Needham Growth Fund  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk/Return [Heading] rr_RiskReturnHeading Needham Growth Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The Needham Growth Fund (the “Growth Fund”) seeks long-term, tax-efficient capital appreciation.
Expense [Heading] rr_ExpenseHeading Fees and Expenses of the Growth Fund
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock This table describes the fees and expenses you may pay if you buy, hold and sell shares of the Growth Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination April 29, 2025
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Growth Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Growth Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Growth Fund’s performance. During the fiscal year ended December 31, 2023, the Growth Fund’s portfolio turnover rate was 9% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 9.00%
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock
This example is intended to help you compare the cost of investing in the Growth Fund with the cost of investing in other mutual funds.
The example assumes that you invest $10,000 in the Growth Fund for the time periods indicated and then hold or redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year, that all dividends and distributions have been reinvested, and that the Growth Fund’s operating expenses remain the same (after giving effect to the fee waiver and expense reimbursement arrangement in year one only).
Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
Under normal conditions, the Growth Fund invests at least 65% of its total assets in the equity securities (principally, common stock) of domestic issuers listed on a nationally recognized securities exchange. The Growth Fund considers domestic issuers of equity securities to be companies located, organized, or with a majority of assets or business in the United States. The Growth Fund may, but is not required to, invest in the securities of companies of any market capitalization and from a variety of industries included in the technology, healthcare, energy and industrials, specialty retailing, media/leisure/cable/entertainment and business and consumer services sectors. These are some of the sectors within the economy which the Adviser believes will have significant long-term growth rates and often include the stocks of rapidly growing companies with a variety of market capitalizations. Although the Growth Fund’s investments have typically been most heavily weighted in the information technology and healthcare sectors, the allocation of the Growth Fund’s assets among the various sectors may change at any time. The Growth Fund may engage in short sales. The Growth Fund may make a profit or loss depending upon whether the market price of the security sold short decreases or increases between the date of the short sale and the date on which the Growth Fund replaces the borrowed security.
It is the policy of the Growth Fund generally to not engage in trading for short-term gains, and the Adviser employs other measures to maximize tax efficiency to the extent consistent with the Growth Fund’s investment strategies, including using: (a) the specific identification method to calculate the tax basis for shares of Growth Fund portfolio holdings to seek to minimize taxable gains or offset other gains; and (b) net short-term capital gains to offset Growth Fund expenses which would otherwise be non-deductible by the Growth Fund. During certain periods, market forces may cause the Adviser, seeking to act in the best interests of the Growth Fund, to manage the Growth Fund in a manner that may not maximize tax efficiency, such as if the Growth Fund experiences extreme inflows and outflows from an unusually high volume of purchase and redemption activity, resulting in high portfolio turnover. The Adviser seeks to balance tax efficiency with the overall best interests of the Growth Fund.
Strategy Portfolio Concentration [Text] rr_StrategyPortfolioConcentration Under normal conditions, the Growth Fund invests at least 65% of its total assets in the equity securities (principally, common stock) of domestic issuers listed on a nationally recognized securities exchange.
Risk [Heading] rr_RiskHeading Principal Investment Risks
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Bar Chart and Performance Table
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock
The information in the bar chart and table that follows provides some indication of the risks of investing in the Growth Fund by showing changes in the performance of the Growth Fund’s Retail Class shares from year to year and by showing how the Growth Fund’s average annual returns for 1, 5 and 10 years and for the life of the Growth Fund compare to those of broad measures of market performance.
The Growth Fund’s past performance (before and after taxes) is not necessarily an indication of how the Growth Fund will perform in the future. Updated performance information is available on the Growth Fund’s website at www.needhamfunds.com.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The information in the bar chart and table that follows provides some indication of the risks of investing in the Growth Fund by showing changes in the performance of the Growth Fund’s Retail Class shares from year to year and by showing how the Growth Fund’s average annual returns for 1, 5 and 10 years and for the life of the Growth Fund compare to those of broad measures of market performance.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.needhamfunds.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Growth Fund’s past performance (before and after taxes) is not necessarily an indication of how the Growth Fund will perform in the future.
Bar Chart [Heading] rr_BarChartHeading Calendar Year Total Returns as of December 31 – Retail Class
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock During the ten-year period shown in the above chart, the highest quarterly return was 30.69% (for the quarter ended June 30, 2020) and the lowest quarterly return was -22.34% (for the quarter ended June 30, 2022).
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel highest quarterly return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 30.69%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel lowest quarterly return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Jun. 30, 2022
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (22.34%)
Performance Table Heading rr_PerformanceTableHeading Average annual total returns for the periods ended December 31, 2023
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes Comparative indices reflect no deductions for fees, expenses, or taxes.
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not include the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred Your actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their shares in a tax-deferred account (including a 401(k) or IRA), or to investors who are tax-exempt.
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown After-tax returns are shown only for Retail Class shares. After-tax returns for Institutional Class shares will vary.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock
The following table shows the average annual returns of the Growth Fund’s shares, and the change in value of certain broad-based market indices over various periods ended December 31, 2023. The index information is intended to permit you to compare the Growth Fund’s performance to several broad measures of market performance.
After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not include the impact of state and local taxes.
Your actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their shares in a tax-deferred account (including a 401(k)
or IRA), or to investors who are tax-exempt. After-tax returns are shown only for Retail Class shares. After-tax returns for Institutional Class shares will vary.
Performance Table Footnotes rr_PerformanceTableFootnotesTextBlock Source: London Stock Exchange Group plc and its group undertakings (collectively, the “LSE Group”). © LSE Group 2022. All rights in the Russell 2000® Index (the “Index”) vest in the relevant LSE Group company which owns the Index. The Index is calculated by or on behalf of FTSE International Limited or its affiliate, agent or partner. Neither the LSE Group nor its licensors accept any liability for any errors or omissions in the Index; no party may rely on the Index returns shown; and the LSE Group makes no claim, prediction, warranty or representation about the Needham Growth Fund or the suitability of the Index with respect to the Needham Growth Fund. No further distribution of data from the LSE Group is permitted without the relevant LSE Group company’s express written consent. The LSE Group is not connected to the Needham Growth Fund and does not promote, sponsor or endorse the Needham Growth Fund or the content of this prospectus.
Needham Growth Fund | Risk Lose Money [Member]  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Loss of money is a risk of investing in the Growth Fund.
Needham Growth Fund | Focus Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Focus Risks. Although the Growth Fund is classified as “diversified” under the Investment Company Act of 1940, as amended (the “1940 Act”), the Growth Fund may invest its assets in a smaller number of issuers than other, more diversified, funds. The Growth Fund’s net asset value (“NAV”) may be more vulnerable to changes in the market value of a single issuer or group of issuers and may be relatively more susceptible to adverse effects from any single corporate, industry, economic, market, political or regulatory occurrence than if the Growth Fund’s investments consisted of securities issued by a larger number of issuers.
Needham Growth Fund | Growth Investing Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Growth Investing Risks. The Growth Fund invests in stocks believed by the Adviser to have the potential for growth, but that may not realize such perceived potential for extended periods of time or may never realize such perceived growth potential. Such stocks may be more volatile than other stocks because they can be more sensitive to investor perceptions of the issuing company’s growth potential.
Needham Growth Fund | Market Capitalization Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Market Capitalization Risks. To the extent the Growth Fund emphasizes stocks of small, mid or large cap companies, it will assume the associated risks. At any given time, any of these market capitalizations may be out of favor with investors. Larger companies may be less responsive to changes and opportunities affecting their business than are small and mid cap companies, though small and mid cap companies tend to have less established operating histories, less predictable earnings and revenues (some companies may be experiencing significant losses), and more volatile share prices than those of larger companies.
Needham Growth Fund | Sector Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock
Sector Risks. Business and economic developments affecting a market sector, such as technology or healthcare companies, or a portion of a market sector in which the Growth Fund invests a significant portion of its assets would likely have a greater effect on the Growth Fund than those same developments would have on a fund invested in a wider spectrum of market sectors.

Companies in the technology sector or portions of the technology sector such as semiconductors and/or semiconductor equipment generally operate in intensely competitive markets. This level of competition can put pressure on the prices of their products and services, which could adversely affect their profitability. Also, because technological development in many areas increases at a rapid rate, these companies often produce products with very short life cycles and face the risk of product obsolescence. Other risks of investments in technology companies, including semiconductor and semiconductor equipment companies, include worldwide competition, changes in consumer preferences, problems with product compatibility, the effects of economic slowdowns, and changes in government regulation.
The value of equity securities of healthcare companies may fluctuate because of changes in the regulatory and competitive environment in which they operate. Many healthcare companies offer products and services that are subject to governmental regulation and may be adversely affected by changes in governmental policies or laws. Failure to obtain regulatory approvals or changes in governmental policies regarding funding or subsidies may also adversely affect the value of the equity securities of healthcare companies. Healthcare companies may also be strongly affected by scientific or technological developments and their products may quickly become obsolete. Furthermore, these companies may be adversely affected by product liability-related lawsuits. The securities of companies in these sectors may experience more price volatility than securities of companies in other sectors.
Needham Growth Fund | Short Sales Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Short Sales Risks. Short sales present the risk that the price of the security sold short will increase in value between the time of the short sale and the time the Growth Fund must purchase the security to return it to the lender. The Growth Fund may not be able to close a short position at a favorable price or time and the loss of value on a short sale is potentially unlimited.
Needham Growth Fund | Small Company Investment Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Small Company Investment Risks. The Growth Fund often invests in smaller companies that may have limited product lines, markets or financial resources. These smaller companies may trade at a lower volume than more widely held securities and may fluctuate in value more sharply than those of other securities.
Needham Growth Fund | Stock Investing And Market Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock
Stock Investing and Market Risks. The Growth Fund invests primarily in equity securities that fluctuate in value. There is no guarantee that the Growth Fund will achieve its investment objective of long-term, tax-efficient capital appreciation. Fund losses may be incurred due to declines in one or more markets in which Fund investments are made. These declines may be the result of, among other things, political, regulatory, market, economic or social developments affecting the relevant market(s). In addition, turbulence and reduced liquidity in financial markets may negatively affect many issuers, which could have an adverse effect on your Fund investment. Global economies and financial markets are increasingly interconnected, and conditions and events in one country, region or financial market may adversely impact issuers worldwide. As a result, local, regional or global events such as war, acts of terrorism, the spread of infectious illness or other public health issues, recessions or other events could have a significant negative impact on global economic and market conditions. The coronavirus disease 2019 (COVID-19) global pandemic and the aggressive responses taken by many governments or voluntarily imposed by private parties, including closing borders, restricting travel and imposing prolonged quarantines or similar restrictions, as well as the closure of, or operational changes to, many retail and other businesses, have had negative impacts, and in many cases severe negative impacts, on
markets worldwide. It is not known how long such impacts, or any future impacts of other significant events described above, will or would last, but there could be a prolonged period of global economic slowdown, which may impact your Fund investment.
Needham Growth Fund | Tax-Efficiency Management Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock
Tax-Efficiency Management Risks. Because the Adviser balances investment considerations and tax-efficiency considerations, the pretax performance of the Growth Fund may be lower than the performance of similar funds that are not managed for tax-efficiency. Even though tax-efficient strategies are being used, they may not
actually reduce the amount of taxable income and capital gains distributed by the Growth Fund to shareholders. For example, at times a high percentage of the Growth Fund’s net asset value may consist of unrealized capital gains, which represent a potential future tax liability to shareholders, or during certain periods market forces may cause the Adviser, seeking to act in the best interests of the Growth Fund, to manage the Growth Fund in a manner that may not maximize tax efficiency.
Needham Growth Fund | Retail Class  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol NEEGX
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 1.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.29%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.79%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.04%) [1]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 1.75%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 178
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 559
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 966
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 2,102
Annual Return 2014 rr_AnnualReturn2014 8.98%
Annual Return 2015 rr_AnnualReturn2015 (5.07%)
Annual Return 2016 rr_AnnualReturn2016 10.57%
Annual Return 2017 rr_AnnualReturn2017 8.32%
Annual Return 2018 rr_AnnualReturn2018 (10.26%)
Annual Return 2019 rr_AnnualReturn2019 42.31%
Annual Return 2020 rr_AnnualReturn2020 41.59%
Annual Return 2021 rr_AnnualReturn2021 27.68%
Annual Return 2022 rr_AnnualReturn2022 (33.66%)
Annual Return 2023 rr_AnnualReturn2023 26.85%
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 26.85%
5 Years rr_AverageAnnualReturnYear05 16.71%
10 Years rr_AverageAnnualReturnYear10 9.18%
Since Inception rr_AverageAnnualReturnSinceInception 12.44%
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 01, 1996
Needham Growth Fund | Retail Class | After Taxes on Distributions  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions
1 Year rr_AverageAnnualReturnYear01 26.85%
5 Years rr_AverageAnnualReturnYear05 15.31%
10 Years rr_AverageAnnualReturnYear10 7.50%
Since Inception rr_AverageAnnualReturnSinceInception 11.06%
Needham Growth Fund | Retail Class | After Taxes on Distributions and Sales  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions       and Redemption
1 Year rr_AverageAnnualReturnYear01 15.89%
5 Years rr_AverageAnnualReturnYear05 13.34%
10 Years rr_AverageAnnualReturnYear10 7.01%
Since Inception rr_AverageAnnualReturnSinceInception 10.59%
Needham Growth Fund | Retail Class | S&P 500® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel S&P 500® Index* [2]
1 Year rr_AverageAnnualReturnYear01 26.29% [2]
5 Years rr_AverageAnnualReturnYear05 15.69% [2]
10 Years rr_AverageAnnualReturnYear10 12.03% [2]
Since Inception rr_AverageAnnualReturnSinceInception 9.61% [2]
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 01, 1996 [2]
Needham Growth Fund | Retail Class | S&P MidCap 400® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel S&P MidCap 400® Index* [2]
1 Year rr_AverageAnnualReturnYear01 16.44% [2]
5 Years rr_AverageAnnualReturnYear05 12.62% [2]
10 Years rr_AverageAnnualReturnYear10 9.27% [2]
Since Inception rr_AverageAnnualReturnSinceInception 11.08% [2]
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 01, 1996 [2]
Needham Growth Fund | Retail Class | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Russell 2000® Index* [2]
1 Year rr_AverageAnnualReturnYear01 16.93% [2]
5 Years rr_AverageAnnualReturnYear05 9.97% [2]
10 Years rr_AverageAnnualReturnYear10 7.16% [2]
Since Inception rr_AverageAnnualReturnSinceInception 8.30% [2]
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 01, 1996 [2]
Needham Growth Fund | Institutional Class  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol NEEIX
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 1.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.25%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.50%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.29%) [1]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 1.21%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 123
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 446
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 791
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,766
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 27.30%
5 Years rr_AverageAnnualReturnYear05 17.24%
Since Inception rr_AverageAnnualReturnSinceInception 11.73%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016
Needham Growth Fund | Institutional Class | S&P 500® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel S&P 500® Index* [2]
1 Year rr_AverageAnnualReturnYear01 26.29% [2]
5 Years rr_AverageAnnualReturnYear05 15.69% [2]
10 Years rr_AverageAnnualReturnYear10 12.03% [2]
Since Inception rr_AverageAnnualReturnSinceInception 13.41% [2]
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016 [2]
Needham Growth Fund | Institutional Class | S&P MidCap 400® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel S&P MidCap 400® Index* [2]
1 Year rr_AverageAnnualReturnYear01 16.44% [2]
5 Years rr_AverageAnnualReturnYear05 12.62% [2]
10 Years rr_AverageAnnualReturnYear10 9.27% [2]
Since Inception rr_AverageAnnualReturnSinceInception 9.37% [2]
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016 [2]
Needham Growth Fund | Institutional Class | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Russell 2000® Index* [2]
1 Year rr_AverageAnnualReturnYear01 16.93% [2]
5 Years rr_AverageAnnualReturnYear05 9.97% [2]
10 Years rr_AverageAnnualReturnYear10 7.16% [2]
Since Inception rr_AverageAnnualReturnSinceInception 7.33% [2]
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016 [2]
Needham Aggressive Growth Fund  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk/Return [Heading] rr_RiskReturnHeading Needham Aggressive Growth Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The Needham Aggressive Growth Fund (the “Aggressive Growth Fund”) seeks long-term, tax-efficient capital appreciation.
Expense [Heading] rr_ExpenseHeading Fees and Expenses of the Aggressive Growth Fund
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock This table describes the fees and expenses you may pay if you buy, hold and sell shares of the Aggressive Growth Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination April 29, 2025
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Aggressive Growth Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Aggressive Growth Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Aggressive Growth Fund’s performance. During the fiscal year ended December 31, 2023, the Aggressive Growth Fund’s portfolio turnover rate was 7% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 7.00%
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text] rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees Acquired Fund Fees and Expenses (“AFFE”) are indirect fees and expenses that the Aggressive Growth Fund incurs from investing in shares of other funds, including money market funds. Please note that the amount of Total Annual Fund Operating Expenses shown in the above table will differ from the "Financial Highlights" section of the Prospectus, which reflects the operating expenses of the Fund and does not include indirect expenses such as AFFE.
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock
This example is intended to help you compare the cost of investing in the Aggressive Growth Fund with the cost of investing in other mutual funds.

The example assumes that you invest $10,000 in the Aggressive Growth Fund for the time periods indicated and then hold or redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year, that all dividends and distributions have been reinvested, and that the Aggressive Growth Fund’s operating expenses remain the same (after giving
effect to the fee waiver and expense reimbursement arrangement in year one only).
Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
Under normal conditions, the Aggressive Growth Fund invests at least 65% of its total assets in the equity securities (principally, common stock) of domestic issuers listed on a nationally recognized securities exchange. The Aggressive Growth Fund considers domestic issuers of equity securities to be companies located, organized, or with a majority of assets or business in the United States. The Aggressive Growth Fund invests principally in markets and industries with strong growth potential, focusing primarily on the market leaders in these areas as these companies often garner a disproportionate share of the positive financial returns. Although the Aggressive Growth Fund may invest in companies of any size, the Aggressive Growth Fund’s investment strategy may result in a focus on smaller companies. The Aggressive Growth Fund may, but is not required to, invest in stocks from a variety of industries included in the technology, healthcare, energy and industrials, specialty retailing, media/leisure/cable/entertainment and business and consumer services sectors. Although the Aggressive Growth Fund’s investments have typically been most heavily weighted in the information technology and healthcare sectors, the allocation of the Aggressive Growth Fund’s assets among the various sectors may change at any time. The Aggressive Growth Fund may engage in short sales. The Aggressive Growth Fund may make a profit or loss depending upon whether the market price of the security sold short decreases or increases between the date of the short sale and the date on which the Aggressive Growth Fund replaces the borrowed security.

It is the policy of the Aggressive Growth Fund generally to not engage in trading for short-term gains, and the Adviser employs other measures to maximize tax efficiency to the extent consistent with the Aggressive Growth Fund’s investment strategies, including using: (a) the specific identification method to calculate the tax basis for shares of Aggressive Growth Fund portfolio holdings to seek to minimize taxable gains or offset other gains; and (b) net short-term capital gains to offset Aggressive Growth Fund expenses which would otherwise be non-deductible by the Aggressive Growth Fund. During certain periods, market forces may cause the Adviser, seeking to act in the best interests of the Aggressive Growth Fund, to manage the Aggressive Growth Fund in a manner that may not maximize tax efficiency, such as if the Aggressive Growth Fund experiences extreme inflows and outflows from an unusually high volume of purchase and redemption activity, resulting in high portfolio turnover. The Adviser seeks to balance tax efficiency with the overall best interests of the Aggressive Growth Fund.
Strategy Portfolio Concentration [Text] rr_StrategyPortfolioConcentration Under normal conditions, the Aggressive Growth Fund invests at least 65% of its total assets in the equity securities (principally, common stock) of domestic issuers listed on a nationally recognized securities exchange. The Aggressive Growth Fund considers domestic issuers of equity securities to be companies located, organized, or with a majority of assets or business in the United States.
Risk [Heading] rr_RiskHeading Principal Investment Risks
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Bar Chart and Performance Table
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock
The information in the bar chart and table that follows provides some indication of the risks of investing in the Aggressive Growth Fund by showing changes in the performance of the Aggressive Growth Fund’s Retail Class shares from year to year and by showing how the Aggressive Growth Fund’s average annual returns for 1, 5 and 10 years and for the life of the Aggressive Growth Fund compare to those of broad measures of market performance.

The Aggressive Growth Fund’s past performance (before and after taxes) is not necessarily an indication of how the Aggressive Growth Fund will perform in the future. Updated performance information is available on the Aggressive Growth Fund’s website at www.needhamfunds.com.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The information in the bar chart and table that follows provides some indication of the risks of investing in the Aggressive Growth Fund by showing changes in the performance of the Aggressive Growth Fund’s Retail Class shares from year to year and by showing how the Aggressive Growth Fund’s average annual returns for 1, 5 and 10 years and for the life of the Aggressive Growth Fund compare to those of broad measures of market performance.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.needhamfunds.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Aggressive Growth Fund’s past performance (before and after taxes) is not necessarily an indication of how the Aggressive Growth Fund will perform in the future.
Bar Chart [Heading] rr_BarChartHeading Calendar Year Total Returns as of December 31 – Retail Class
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
During the ten-year period shown in the above chart, the highest quarterly return was 34.85% (for the quarter ended June 30, 2020) and the lowest quarterly return was -19.72% (for the quarter ended June 30, 2022).
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel highest quarterly return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 34.85%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel lowest quarterly return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Jun. 30, 2022
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (19.72%)
Performance Table Heading rr_PerformanceTableHeading Average annual total returns for the periods ended December 31, 2023
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes Comparative indices reflect no deductions for fees, expenses, or taxes.
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not include the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred Your actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their shares in a tax-deferred account (including a 401(k) or IRA), or to investors who are tax-exempt.
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown After-tax returns are shown only for Retail Class shares. After-tax returns for Institutional Class shares may vary.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock
The following table shows the average annual returns of the Aggressive Growth Fund’s shares and the change in value of certain broad-based market indices over various periods ended December 31, 2023. The index information is intended to permit you to compare the Aggressive Growth Fund’s performance to several broad measures of market performance.
After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not include the impact of state and local taxes.
Your actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their shares in a tax-deferred account (including a 401(k) or IRA), or to investors who are tax-exempt. After-tax returns are shown only for Retail Class shares. After-tax returns for Institutional Class shares may vary.
Performance Table Footnotes rr_PerformanceTableFootnotesTextBlock Source: London Stock Exchange Group plc and its group undertakings (collectively, the “LSE Group”). © LSE Group 2022. All rights in the Russell 2000® Index and Russell 2000® Growth Index (each, the “Index”) vest in the relevant LSE Group company which owns the Index. The Index is calculated by or on behalf of FTSE International Limited or its affiliate, agent or partner. Neither the LSE Group nor its licensors accept any liability for any errors or omissions in the Index; no party may rely on the Index returns shown; and the LSE Group makes no claim, prediction, warranty or representation about the Needham Aggressive Growth Fund or the suitability of the Index with respect to the Needham Aggressive Growth Fund. No further distribution of data from the LSE Group is permitted without the relevant LSE Group company’s express written consent. The LSE Group is not connected to the Needham Aggressive Growth Fund and does not promote, sponsor or endorse the Needham Aggressive Growth Fund or the content of this prospectus.
Needham Aggressive Growth Fund | Risk Lose Money [Member]  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Loss of money is a risk of investing in the Aggressive Growth Fund.
Needham Aggressive Growth Fund | Focus Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Focus Risks. Although the Aggressive Growth Fund is classified as “diversified” under the 1940 Act, the Aggressive Growth Fund may invest its assets in a smaller number of issuers than other, more diversified, funds. The Aggressive Growth Fund’s NAV may be more vulnerable to changes in the market value of a single issuer or group of issuers and may be relatively more susceptible to adverse effects from any single corporate, industry, economic, market, political or regulatory occurrence than if the Aggressive Growth Fund’s investments consisted of securities issued by a larger number of issuers.
Needham Aggressive Growth Fund | Growth Investing Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Growth Investing Risks. The Aggressive Growth Fund invests in stocks believed by the Adviser to have the potential for growth, but that may not realize such perceived potential for extended periods of time or may never realize such perceived growth potential. Such stocks may be more volatile than other stocks because they can be more sensitive to investor perceptions of the issuing company’s growth potential.
Needham Aggressive Growth Fund | Market Capitalization Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Market Capitalization Risks. To the extent the Aggressive Growth Fund emphasizes stocks of small, mid or large cap companies, it will assume the associated risks. At any given time, any of these market capitalizations may be out of favor with investors. Larger companies may be less responsive to changes and opportunities affecting their business than are small and mid cap companies, though small and mid cap companies tend to have less established operating histories, less predictable earnings and revenues (some companies may be experiencing significant losses), and more volatile share prices than those of larger companies.
Needham Aggressive Growth Fund | Sector Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock
Sector Risks. Business and economic developments affecting a market sector, such as technology or healthcare companies, or a portion of a market sector in which the Aggressive Growth Fund invests a significant portion of its assets would likely have a greater effect on the Aggressive Growth Fund than those same developments would have on a fund invested in a wider spectrum of market sectors.

Companies in the technology sector or portions of the technology sector such as semiconductors and/or semiconductor equipment generally operate in intensely competitive markets. This level of competition can put pressure on the prices of their products and services, which could adversely affect their profitability. Also, because technological development in many areas increases at a rapid rate, these companies often produce products with very short life cycles and face the risk of product obsolescence. Other risks of investments in technology companies, including semiconductor and semiconductor equipment companies, include worldwide competition, changes in consumer preferences, problems with product compatibility, the effects of economic slowdowns, and changes in government regulation.

The value of equity securities of healthcare companies may fluctuate because of changes in the regulatory and competitive environment in which they operate. Many healthcare companies offer products and services that are subject to governmental regulation and may be adversely affected by changes in governmental policies or laws. Failure to obtain regulatory approvals or changes in governmental policies regarding funding or subsidies may also adversely affect the value of the equity securities of healthcare companies. Healthcare companies may also be strongly affected by scientific or technological developments and their products may quickly become obsolete. Furthermore, these companies may be adversely affected by product liability-related lawsuits. The
securities of companies in these sectors may experience more price volatility than securities of companies in other sectors.
Needham Aggressive Growth Fund | Short Sales Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Short Sales Risks. Short sales present the risk that the price of the security sold short will increase in value between the time of the short sale and the time the Aggressive Growth Fund must purchase the security to return it to the lender. The Aggressive Growth Fund may not be able to close a short position at a favorable price or time and the loss of value on a short sale is potentially unlimited.
Needham Aggressive Growth Fund | Small Company Investment Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Small Company Investment Risks. Investments in smaller companies may offer greater opportunities for capital appreciation than larger companies, but may also involve certain special risks. These companies may have limited product lines, limited operating histories, markets or financial resources and may be dependent on small or inexperienced management groups. The trading volume of small company securities (which tends to be lower than that of securities of larger companies) may make such securities more difficult to sell than securities of larger companies, and the lack of an efficient market for small company securities may make them difficult to value. Securities of smaller companies also may fluctuate in value more sharply than those of larger companies.
Needham Aggressive Growth Fund | Stock Investing And Market Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock
Stock Investing and Market Risks. The Aggressive Growth Fund invests primarily in equity securities that fluctuate in value. There is no guarantee that the Aggressive Growth Fund will achieve its investment objective of long-term, tax-efficient capital appreciation. Fund losses may be incurred due to declines in one or more markets in which Fund investments are made. These declines may be the result of, among other things, political, regulatory, market, economic or social developments affecting the relevant market(s). In addition, turbulence and reduced liquidity in financial markets may negatively affect many issuers, which could have an adverse effect on your Fund investment. Global economies and financial markets are increasingly interconnected, and conditions and events in one country, region or financial market may adversely impact issuers worldwide. As a result, local, regional or global events such as war, acts of terrorism, the spread of infectious illness or other public health issues, recessions or other events could have a significant negative
impact on global economic and market conditions. The coronavirus disease 2019 (COVID-19) global pandemic and the aggressive responses taken by many governments or voluntarily imposed by private parties, including closing borders, restricting travel and imposing prolonged quarantines or similar restrictions, as well as the closure of, or operational changes to, many retail and other businesses, have had negative impacts, and in many cases severe negative impacts, on markets worldwide. It is not known how long such impacts, or any future impacts of other significant events described above, will or would last, but there could be a prolonged period of global economic slowdown, which may impact your Fund investment.
Needham Aggressive Growth Fund | Tax-Efficiency Management Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Tax-Efficiency Management Risks. Because the Adviser balances investment considerations and tax-efficiency considerations, the pretax performance of the Aggressive Growth Fund may be lower than the performance of similar funds that are not managed for tax-efficiency. Even though tax-efficient strategies are being used, they may not actually reduce the amount of taxable income and capital gains distributed by the Aggressive Growth Fund to shareholders. For example, at times a high percentage of the Aggressive Growth Fund’s net asset value may consist of unrealized capital gains, which represent a potential future tax liability to shareholders, or during certain periods market forces may cause the Adviser, seeking to act in the best interests of the Aggressive Growth Fund, to manage the Aggressive Growth Fund in a manner that may not maximize tax efficiency.
Needham Aggressive Growth Fund | Retail Class  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol NEAGX
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 1.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Recoupment of Waived Fees and/or Reimbursements rr_Component1OtherExpensesOverAssets 0.08% [3]
All Remaining Other Expenses rr_Component2OtherExpensesOverAssets 0.28%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.36%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.04% [4]
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.90%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.08%) [3]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 1.82%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 185
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 589
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,019
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 2,216
Annual Return 2014 rr_AnnualReturn2014 7.18%
Annual Return 2015 rr_AnnualReturn2015 (7.09%)
Annual Return 2016 rr_AnnualReturn2016 22.18%
Annual Return 2017 rr_AnnualReturn2017 8.73%
Annual Return 2018 rr_AnnualReturn2018 (15.80%)
Annual Return 2019 rr_AnnualReturn2019 43.93%
Annual Return 2020 rr_AnnualReturn2020 51.39%
Annual Return 2021 rr_AnnualReturn2021 37.54%
Annual Return 2022 rr_AnnualReturn2022 (27.53%)
Annual Return 2023 rr_AnnualReturn2023 37.65%
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 37.65%
5 Years rr_AverageAnnualReturnYear05 24.49%
10 Years rr_AverageAnnualReturnYear10 12.78%
Since Inception rr_AverageAnnualReturnSinceInception 11.39%
Inception Date rr_AverageAnnualReturnInceptionDate Sep. 04, 2001
Needham Aggressive Growth Fund | Retail Class | After Taxes on Distributions  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions
1 Year rr_AverageAnnualReturnYear01 37.65%
5 Years rr_AverageAnnualReturnYear05 23.23%
10 Years rr_AverageAnnualReturnYear10 11.18%
Since Inception rr_AverageAnnualReturnSinceInception 10.36%
Needham Aggressive Growth Fund | Retail Class | After Taxes on Distributions and Sales  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions and Redemption
1 Year rr_AverageAnnualReturnYear01 22.29%
5 Years rr_AverageAnnualReturnYear05 19.90%
10 Years rr_AverageAnnualReturnYear10 9.98%
Since Inception rr_AverageAnnualReturnSinceInception 9.58%
Needham Aggressive Growth Fund | Retail Class | Russell 2000® Growth Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Russell 2000® Growth Index* [5]
1 Year rr_AverageAnnualReturnYear01 18.66% [5]
5 Years rr_AverageAnnualReturnYear05 9.22% [5]
10 Years rr_AverageAnnualReturnYear10 7.16% [5]
Since Inception rr_AverageAnnualReturnSinceInception 7.87% [5]
Inception Date rr_AverageAnnualReturnInceptionDate Sep. 04, 2001 [5]
Needham Aggressive Growth Fund | Retail Class | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Russell 2000® Index* [5]
1 Year rr_AverageAnnualReturnYear01 16.93% [5]
5 Years rr_AverageAnnualReturnYear05 9.97% [5]
10 Years rr_AverageAnnualReturnYear10 7.16% [5]
Since Inception rr_AverageAnnualReturnSinceInception 8.24% [5]
Inception Date rr_AverageAnnualReturnInceptionDate Sep. 04, 2001 [5]
Needham Aggressive Growth Fund | Institutional Class  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol NEAIX
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 1.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Recoupment of Waived Fees and/or Reimbursements rr_Component1OtherExpensesOverAssets none [3]
All Remaining Other Expenses rr_Component2OtherExpensesOverAssets 0.27%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.27%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.04% [4]
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.56%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.34%) [3]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 1.22%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 124
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 459
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 818
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,828
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 38.37%
5 Years rr_AverageAnnualReturnYear05 25.26%
Since Inception rr_AverageAnnualReturnSinceInception 16.16%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016
Needham Aggressive Growth Fund | Institutional Class | Russell 2000® Growth Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Russell 2000® Growth Index* [5]
1 Year rr_AverageAnnualReturnYear01 18.66% [5]
5 Years rr_AverageAnnualReturnYear05 9.22% [5]
10 Years rr_AverageAnnualReturnYear10 7.16% [5]
Since Inception rr_AverageAnnualReturnSinceInception 8.07% [5]
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016 [5]
Needham Aggressive Growth Fund | Institutional Class | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Russell 2000® Index* [5]
1 Year rr_AverageAnnualReturnYear01 16.93% [5]
5 Years rr_AverageAnnualReturnYear05 9.97% [5]
10 Years rr_AverageAnnualReturnYear10 7.16% [5]
Since Inception rr_AverageAnnualReturnSinceInception 7.33% [5]
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016 [5]
Needham Small Cap Growth Fund  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk/Return [Heading] rr_RiskReturnHeading Needham Small Cap Growth Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The Needham Small Cap Growth Fund (the “Small Cap Growth Fund”) seeks long-term, tax-efficient capital appreciation.
Expense [Heading] rr_ExpenseHeading Fees and Expenses of the Small Cap Growth Fund
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock This table describes the fees and expenses you may pay if you buy, hold and sell shares of the Small Cap Growth Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination April 29, 2025
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Small Cap Growth Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Small Cap Growth Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Small Cap Growth Fund’s performance. During the fiscal year ended December 31, 2023, the Small Cap Growth Fund’s portfolio turnover rate was 126% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 126.00%
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text] rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees Acquired Fund Fees and Expenses (“AFFE”) are indirect fees and expenses that the Small Cap Growth Fund incurs from investing in shares of other funds, including money market funds. Please note that the amount of Total Annual Fund Operating Expenses shown in the above table will differ from the “Financial Highlights” section of the Prospectus, which reflects the operating expenses of the Fund and does not include indirect expenses such as AFFE.
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock
This example is intended to help you compare the cost of investing in the Small Cap Growth Fund with the cost of investing in other mutual funds.

The example assumes that you invest $10,000 in the Small Cap Growth Fund for the time periods indicated and then hold or redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year, that all dividends and distributions have been reinvested, and that the Small Cap Growth Fund’s operating expenses remain the same (after giving
effect to the fee waiver and expense reimbursement arrangement in year one only).
Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption Although your actual costs may be higher or lower, based on these assumptions, your costs would be:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
Under normal conditions, the Small Cap Growth Fund invests at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in the equity securities (principally, common stock) of domestic issuers listed on a nationally recognized securities exchange that, at the time of investment by the Small Cap Growth Fund, have market capitalizations not exceeding $8 billion (the “80% Policy”). The Small Cap Growth Fund considers domestic issuers of equity securities to be companies located, organized, or with a majority of assets or business in the United States. For purposes of the 80% Policy, the Small Cap Growth Fund may continue to hold securities of an issuer if, after the time of the Small Cap Growth Fund’s investment, the issuer’s market capitalization exceeds $8 billion. Although the Adviser will seek to invest the Small Cap Growth Fund’s assets in accordance with its 80% Policy, during certain periods, which may be prolonged periods of time, market conditions, the availability of attractive investment opportunities that the Adviser believes are appropriate investments for the Small Cap Growth Fund and/or high levels of new investments into the Small Cap Growth Fund can lead to periods of higher cash levels that cause the Small Cap Growth Fund’s investments to be constituted of less than 80% of its net assets in accordance with the 80% Policy. During such periods, the Small Cap Growth Fund may not achieve its investment objective.

The Small Cap Growth Fund invests, in general, in companies with strong growth potential that, for a variety of reasons, including the market’s inefficiencies, are trading at a discount to their underlying value where a catalyst is in place to eliminate that discount. The Small Cap Growth Fund may, but is not required to, invest in stocks from a variety of industries included in the technology, healthcare, energy and industrials, specialty retailing, media/leisure/cable/entertainment and business and consumer services sectors. These are some of the sectors within the economy which the Adviser believes will have significant long-term growth rates and often include the stocks of rapidly growing companies. Although the Small Cap Growth Fund’s investments have typically been most heavily weighted in the information technology and healthcare sectors, the allocation of the Small Cap Growth Fund’s assets among the various sectors may change at any time. The Small Cap Growth Fund may engage in active and frequent trading of portfolio securities.The Small Cap Growth Fund may engage in short sales. The Small Cap Growth Fund may make a profit or loss depending upon whether the market price of the security sold short decreases or increases between the date of the short sale and the date on which the Small Cap Growth Fund replaces the borrowed security.

The Adviser employs certain measures to maximize tax efficiency to the extent consistent with the Small Cap Growth Fund’s investment strategies, including using: (a) the specific identification method to calculate the tax basis for shares of Small Cap Growth Fund portfolio holdings to seek to minimize taxable gains or offset other gains; and (b) net short-term capital gains to offset Small Cap Growth Fund expenses which would otherwise be non-deductible by the Small Cap Growth Fund. During certain periods, market forces may cause the Adviser, seeking to act in the best interests of the Small Cap Growth Fund, to manage the Small Cap Growth Fund in a manner that may not maximize tax efficiency, such as if the Small Cap Growth Fund experiences extreme inflows and outflows from an unusually high volume of purchase and redemption activity, resulting in high portfolio turnover. The Adviser seeks to balance tax efficiency with the overall best interests of the Small Cap Growth Fund.
Strategy Portfolio Concentration [Text] rr_StrategyPortfolioConcentration Under normal conditions, the Small Cap Growth Fund invests at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in the equity securities (principally, common stock) of domestic issuers listed on a nationally recognized securities exchange that, at the time of investment by the Small Cap Growth Fund, have market capitalizations not exceeding $8 billion (the “80% Policy”).
Risk [Heading] rr_RiskHeading Principal Investment Risks
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Bar Chart and Performance Table
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock
The information in the bar chart and table that follow provides some indication of the risks of investing in the Small Cap Growth Fund by showing changes in the performance of the Small Cap Growth Fund’s Retail Class shares from year to year and by showing how the Small Cap Growth Fund’s average annual returns for 1, 5 and 10 years and for the life of the Small Cap Growth Fund compare to those of broad measures of market performance.
The Small Cap Growth Fund’s past performance (before and after taxes) is not necessarily an indication of how the Small Cap Growth Fund will perform in the future. Updated performance information is available on the Small Cap Growth Fund’s website at www.needhamfunds.com.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The information in the bar chart and table that follow provides some indication of the risks of investing in the Small Cap Growth Fund by showing changes in the performance of the Small Cap Growth Fund’s Retail Class shares from year to year and by showing how the Small Cap Growth Fund’s average annual returns for 1, 5 and 10 years and for the life of the Small Cap Growth Fund compare to those of broad measures of market performance.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.needhamfunds.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Small Cap Growth Fund’s past performance (before and after taxes) is not necessarily an indication of how the Small Cap Growth Fund will perform in the future.
Bar Chart [Heading] rr_BarChartHeading Calendar Year Total Returns as of December 31 – Retail Class
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
During the ten-year period shown in the above chart, the highest quarterly return was 36.32% (for the quarter ended December 31, 2020) and the lowest quarterly return was -26.13% (for the quarter ended June 30, 2022).
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel highest quarterly return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 36.32%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel lowest quarterly return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Jun. 30, 2022
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (26.13%)
Performance Table Heading rr_PerformanceTableHeading Average annual total returns for the periods ended December 31, 2023
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes Comparative indices reflect no deductions for fees, expenses, or taxes.
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not include the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred Your actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their shares in a tax-deferred account (including a 401(k) or IRA), or to investors who are tax-exempt.
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown After-tax returns are shown only for Retail Class shares. After-tax returns for Institutional Class shares will vary.
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher Return after taxes on distributions and redemption of Fund shares may be higher than returns before taxes or returns after taxes on distributions due to an assumed tax benefits from losses on a redemptions of the Fund’s shares at the end of the period.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock
The following table shows the average annual returns of the Small Cap Growth Fund’s shares and the change in value of certain broad-based market indices over various periods ended December 31, 2023. The index information is intended to permit you to compare the Small Cap Growth Fund’s performance to several broad measures of market performance.

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not include the impact of state and local taxes.
Your actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their shares in a tax-deferred account (including a 401(k) or IRA), or to investors who are tax-exempt. After-tax returns are shown only for Retail Class shares. After-tax returns for Institutional Class shares will vary. Return after taxes on distributions and redemption of Fund shares may be higher than returns before taxes or returns after taxes on distributions due to an assumed tax benefits from losses on a redemptions of the Fund’s shares at the end of the period.
Performance Table Footnotes rr_PerformanceTableFootnotesTextBlock Source: London Stock Exchange Group plc and its group undertakings (collectively, the “LSE Group”). © LSE Group 2022. All rights in the Russell 2000® Growth Index (the “Index”) vest in the relevant LSE Group company which owns the Index. The Index is calculated by or on behalf of FTSE International Limited or its affiliate, agent or partner. Neither the LSE Group nor its licensors accept any liability for any errors or omissions in the Index; no party may rely on the Index returns shown; and the LSE Group makes no claim, prediction, warranty or representation about the Small Cap Growth Fund or the suitability of the Index with respect to the Small Cap Growth Fund. No further distribution of data from the LSE Group is permitted without the relevant LSE Group company’s express written consent. The LSE Group is not connected to the Small Cap Growth Fund and does not promote, sponsor or endorse the Small Cap Growth Fund or the content of this prospectus.
Needham Small Cap Growth Fund | Risk Lose Money [Member]  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Loss of money is a risk of investing in the Small Cap Growth Fund.
Needham Small Cap Growth Fund | Focus Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Focus Risks. Although the Small Cap Growth Fund is classified as “diversified” under the 1940 Act, the Small Cap Growth Fund may invest its assets in a smaller number of issuers than other, more diversified, funds. The Small Cap Growth Fund’s NAV may be more vulnerable to changes in the market value of a single issuer or group of issuers and may be relatively more susceptible to adverse effects from any single corporate, industry, economic, market, political or regulatory occurrence than if the Small Cap Growth Fund’s investments consisted of securities issued by a larger number of issuers.
Needham Small Cap Growth Fund | Growth Investing Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Growth Investing Risks. The Small Cap Growth Fund invests in stocks believed by the Adviser to have the potential for growth, but that may not realize such perceived potential for extended periods of time or may never realize such perceived growth potential. Such stocks may be more volatile than other stocks because they can be more sensitive to investor perceptions of the issuing company’s growth potential.
Needham Small Cap Growth Fund | Portfolio Turnover Risk Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Portfolio Turnover Risk. The Small Cap Growth Fund may engage in short-term trading, which could produce higher transaction costs and taxable distributions, and lower the Small Cap Growth Fund’s after-tax performance.
Needham Small Cap Growth Fund | Sector Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock
Sector Risks. To the extent that the Small Cap Growth Fund focuses its investments in securities of issuers in a particular market sector, such as technology companies or healthcare companies, the Small Cap Growth Fund will be significantly affected by developments in that sector.

Technology companies generally operate in intensely competitive markets. This level of competition can put pressure on the prices of their products and services which could adversely affect their profitability. Also, because technology development in many areas increases at a rapid rate, these companies often produce products with very short life cycles and face the risk of product obsolescence. Other risks include worldwide competition, changes in consumer preferences, problems with product compatibility, the effects of economic slowdowns, and changes in government regulation.

The value of equity securities of healthcare companies may fluctuate because of changes in the regulatory and competitive environment in which they operate. Many healthcare companies offer products and services that are subject to governmental regulation and may be adversely affected by changes in governmental policies or laws. Failure to obtain regulatory approvals or changes in governmental policies regarding funding or subsidies may also adversely affect the value of the equity securities of healthcare companies. Healthcare companies may also be strongly affected by scientific or technological developments and their products may quickly become obsolete. Furthermore, these companies may be adversely affected by product liability-related lawsuits. The
securities of companies in these sectors may experience more price volatility than securities of companies in other sectors.
Needham Small Cap Growth Fund | Short Sales Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Short Sales Risks. Short sales present the risk that the price of the security sold short will increase in value between the time of the short sale and the time the Small Cap Growth Fund must purchase the security to return it to the lender. The Small Cap Growth Fund may not be able to close a short position at a favorable price or time and the loss of value on a short sale is potentially unlimited.
Needham Small Cap Growth Fund | Small Company Investment Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Small Company Investment Risks. Investments in smaller companies may offer greater opportunities for capital appreciation than larger companies, but may also involve certain special risks. These companies may have limited product lines, limited operating histories, markets or financial resources and may be dependent on small or inexperienced management groups. The trading volume of small company securities (which tends to be lower than that of securities of larger companies) may make such securities more difficult to sell than securities of larger companies, and the lack of an efficient market for small company securities may make them difficult to value. Securities of smaller companies also may fluctuate in value more sharply than those of larger companies.
Needham Small Cap Growth Fund | Stock Investing And Market Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Stock Investing and Market Risks. The Small Cap Growth Fund invests primarily in equity securities that fluctuate in value. There is no guarantee that the Small Cap Growth Fund will achieve its investment objective of long-term, tax-efficient capital appreciation. Fund losses may be incurred due to declines in one or more markets in which Fund investments are made. These declines may be the result of, among other things, political, regulatory, market, economic or social developments affecting the relevant market(s). In addition, turbulence and reduced liquidity in financial markets may negatively affect many issuers, which could have an adverse effect on your Fund investment. Global economies and financial markets are increasingly interconnected, and conditions and events in one country, region or financial market may adversely impact issuers worldwide. As a result, local, regional or global events such as war, acts of terrorism, the spread of infectious illness or other public health issues, recessions or other events could have a significant negative impact on global economic and market conditions. The coronavirus disease 2019 (COVID-19) global pandemic and the aggressive responses taken by many governments or voluntarily imposed by private parties, including closing borders, restricting travel and imposing prolonged quarantines or similar restrictions, as well as the closure of, or operational changes to, many retail and other businesses, have had negative impacts, and in many cases severe negative impacts, on markets worldwide. It is not known how long such impacts, or any future impacts of other significant events described above, will or would last, but there could be a prolonged period of global economic slowdown, which may impact your Fund investment.
Needham Small Cap Growth Fund | Tax-Efficiency Management Risks Member  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock Tax-Efficiency Management Risks. Because the Adviser balances investment considerations and tax-efficiency considerations, the pretax performance of the Small Cap Growth Fund may be lower than the performance of similar funds that are not managed for tax-efficiency. Even though tax-efficient strategies are being used, they may not actually reduce the amount of taxable income and capital gains distributed by the Small Cap Growth Fund to shareholders. For example, at times a high percentage of the Small Cap Growth Fund’s net asset value may consist of unrealized capital gains, which represent a potential future tax liability to shareholders, or during certain periods market forces may cause the Adviser, seeking to act in the best interests of the Small Cap Growth Fund, to manage the Small Cap Growth Fund in a manner that may not maximize tax efficiency.
Needham Small Cap Growth Fund | Retail Class  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol NESGX
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 1.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Recoupment of Waived Fees and/or Reimbursements rr_Component1OtherExpensesOverAssets 0.04% [6]
All Remaining Other Expenses rr_Component2OtherExpensesOverAssets 0.34%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.38%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.04% [7]
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.92%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.09%) [6]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 1.83%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 186
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 594
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,028
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 2,236
Annual Return 2014 rr_AnnualReturn2014 0.80%
Annual Return 2015 rr_AnnualReturn2015 (8.96%)
Annual Return 2016 rr_AnnualReturn2016 31.26%
Annual Return 2017 rr_AnnualReturn2017 11.88%
Annual Return 2018 rr_AnnualReturn2018 (5.13%)
Annual Return 2019 rr_AnnualReturn2019 54.45%
Annual Return 2020 rr_AnnualReturn2020 71.35%
Annual Return 2021 rr_AnnualReturn2021 10.98%
Annual Return 2022 rr_AnnualReturn2022 (30.33%)
Annual Return 2023 rr_AnnualReturn2023 5.68%
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 5.68%
5 Years rr_AverageAnnualReturnYear05 16.68%
10 Years rr_AverageAnnualReturnYear10 10.70%
Since Inception rr_AverageAnnualReturnSinceInception 11.04%
Inception Date rr_AverageAnnualReturnInceptionDate May 22, 2002
Needham Small Cap Growth Fund | Retail Class | After Taxes on Distributions  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions
1 Year rr_AverageAnnualReturnYear01 5.68%
5 Years rr_AverageAnnualReturnYear05 12.87%
10 Years rr_AverageAnnualReturnYear10 7.39%
Since Inception rr_AverageAnnualReturnSinceInception 8.92%
Needham Small Cap Growth Fund | Retail Class | After Taxes on Distributions and Sales  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions and       Redemption
1 Year rr_AverageAnnualReturnYear01 3.36%
5 Years rr_AverageAnnualReturnYear05 12.38%
10 Years rr_AverageAnnualReturnYear10 7.47%
Since Inception rr_AverageAnnualReturnSinceInception 8.78%
Needham Small Cap Growth Fund | Retail Class | Russell 2000® Growth Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Russell 2000® Growth Index* [8]
1 Year rr_AverageAnnualReturnYear01 18.66% [8]
5 Years rr_AverageAnnualReturnYear05 9.22% [8]
10 Years rr_AverageAnnualReturnYear10 7.16% [8]
Since Inception rr_AverageAnnualReturnSinceInception 8.23% [8]
Inception Date rr_AverageAnnualReturnInceptionDate May 22, 2002 [8]
Needham Small Cap Growth Fund | Institutional Class  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol NESIX
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 1.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Recoupment of Waived Fees and/or Reimbursements rr_Component1OtherExpensesOverAssets none [6]
All Remaining Other Expenses rr_Component2OtherExpensesOverAssets 0.32%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.32%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.04% [7]
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.61%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.39%) [6]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 1.22%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 124
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 470
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 839
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,878
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 5.85%
5 Years rr_AverageAnnualReturnYear05 17.33%
Since Inception rr_AverageAnnualReturnSinceInception 13.23%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016
Needham Small Cap Growth Fund | Institutional Class | Russell 2000® Growth Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Russell 2000® Growth Index* [8]
1 Year rr_AverageAnnualReturnYear01 18.66% [8]
5 Years rr_AverageAnnualReturnYear05 9.22% [8]
10 Years rr_AverageAnnualReturnYear10 7.16% [8]
Since Inception rr_AverageAnnualReturnSinceInception 8.07% [8]
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016 [8]
[1] Acquired Fund Fees and Expenses (“AFFE”) are indirect fees and expenses that the Growth Fund incurs from investing in shares of other funds, including money market funds. Please note that the amount of Total Annual Fund Operating Expenses shown in the above table will differ from the "Financial Highlights" section of the Prospectus, which reflects the operating expenses of the Fund and does not include indirect expenses such as AFFE.
(b)Reflects a contractual agreement by Needham Investment Management LLC (the “Adviser”) to waive its fee and, if necessary, reimburse the Growth Fund until April 29, 2025 to the extent Total Annual Fund Operating Expenses exceed 1.95% and 1.21% of the average daily net assets of Retail Class shares and Institutional Class shares, respectively, of the Growth Fund (the “Expense Cap”). To the extent that it is necessary for the Adviser to waive receipt of its management fee or reimburse the Growth Fund’s common expenses, the amount of the waiver or reimbursement will be applied equally to each share class of the Fund. This agreement can only be amended or terminated by agreement of The Needham Funds, Inc. (the “Company”), upon approval of the Company’s Board of Directors and the Adviser, and will terminate automatically in the event of termination of the Investment Advisory Agreement between the Adviser and the Company, on behalf of the Growth Fund. For a period of up to 36 months (the “Recoupment Period”) from the time of any waiver or reimbursement pursuant to this agreement, the Adviser may recoup from the Growth Fund fees waived and expenses reimbursed to the extent that such recoupment would not cause the Total Annual Fund Operating Expenses of the Growth Fund to exceed the lesser of the Expense Cap in effect (i) at the time of the waiver or reimbursement, or (ii) at the time of recoupment, during each 12-month period ended April 30 in the Recoupment Period. Any such recoupment will not include interest. The Expense Cap limitation on Total Annual Fund Operating Expenses excludes taxes, interest, brokerage, dividends on short positions, fees and expenses of “acquired funds” and extraordinary items but includes the management fee.
[2] Comparative indices reflect no deductions for fees, expenses, or taxes.
[3] Reflects a contractual agreement by Needham Investment Management LLC (the “Adviser”) to waive its fee and, if necessary, reimburse the Aggressive Growth Fund until April 29, 2025 to the extent Total Annual Fund Operating Expenses exceed 1.85% and 1.18% of the average daily net assets of Retail Class shares and Institutional Class shares, respectively, of the Aggressive Growth Fund (the “Expense Cap”). To the extent that it is necessary for the Adviser to waive receipt of its management fee or reimburse the Aggressive Growth Fund’s common expenses, the amount of the waiver or reimbursement will be applied equally to each share class of the Fund. This agreement can only be amended or terminated by agreement of The Needham Funds, Inc. (the “Company”), upon approval of the Company’s Board of Directors and the Adviser, and will terminate automatically in the event of termination of the Investment Advisory Agreement between the Adviser and the Company, on behalf of the Aggressive Growth Fund. For a period of up to 36 months (the “Recoupment Period”) from the time of any waiver or reimbursement pursuant to this agreement, the Adviser may recoup from the Aggressive Growth Fund fees waived and expenses reimbursed to the extent that such recoupment would not cause the Total Annual Fund Operating Expenses of the Aggressive Growth Fund to exceed the lesser of the Expense Cap in effect (i) at the time of the waiver or reimbursement, or (ii) at the time of recoupment, during each 12-month period ended April 30 in the Recoupment Period. Any such recoupment will not include interest. The Expense Cap limitation on Total Annual Fund Operating Expenses excludes taxes, interest, brokerage, dividends on short positions, AFFE, and extraordinary items but includes the management fee.
[4] Acquired Fund Fees and Expenses (“AFFE”) are indirect fees and expenses that the Aggressive Growth Fund incurs from investing in shares of other funds, including money market funds. Please note that the amount of Total Annual Fund Operating Expenses shown in the above table will differ from the "Financial Highlights" section of the Prospectus, which reflects the operating expenses of the Fund and does not include indirect expenses such as AFFE.
[5] Comparative indices reflect no deductions for fees, expenses, or taxes.
[6] Reflects a contractual agreement by Needham Investment Management LLC {the “Adviser”) to waive its fee and, if necessary, reimburse the Small Cap Growth Fund until April 29, 2025 to the extent Total Annual Fund Operating Expenses exceed 1.85% and 1.18% of the average daily net assets of Retail Class shares and Institutional Class shares, respectively, of the Small Cap Growth Fund (the “Expense Cap”). To the extent that it is necessary for the Adviser to waive receipt of its management fee or reimburse the Small Cap Growth Fund’s common expenses, the amount of the waiver or reimbursement will be applied equally to each share class of the Fund. This agreement can only be amended or terminated by agreement of The Needham Funds, Inc. (the “Company”), upon approval of the Company’s Board of Directors and the Adviser, and will terminate automatically in the event of termination of the Investment Advisory Agreement between the Adviser and the Company, on behalf of the Small Cap Growth Fund. For a period of up to 36 months (the “Recoupment Period”) from the time of any waiver or reimbursement pursuant to this agreement, the Adviser may recoup from the Small Cap Growth Fund fees waived and expenses reimbursed to the extent that such recoupment would not cause the Total Annual Fund Operating Expenses of the Small Cap Growth Fund to exceed the lesser of the Expense Cap in effect (i) at the time of the waiver or reimbursement, or (ii) at the time of recoupment, during each 12-month period ended April 30 in the Recoupment Period. Any such recoupment will not include interest. The Expense Cap limitation on Total Annual Fund Operating Expenses excludes taxes, interest, brokerage, dividends on short positions, AFFE, and extraordinary items but includes the management fee.
[7] Acquired Fund Fees and Expenses (“AFFE”) are indirect fees and expenses that the Small Cap Growth Fund incurs from investing in shares of other funds, including money market funds. Please note that the amount of Total Annual Fund Operating Expenses shown in the above table will differ from the “Financial Highlights” section of the Prospectus, which reflects the operating expenses of the Fund and does not include indirect expenses such as AFFE.
[8] Comparative indices reflect no deductions for fees, expenses, or taxes.