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Label Element Value
Prospectus [Line Items] rr_ProspectusLineItems  
Document Type dei_DocumentType 485BPOS
Document Period End Date dei_DocumentPeriodEndDate Dec. 31, 2018
Entity Registrant Name dei_EntityRegistrantName NEEDHAM FUNDS INC
Entity Central Index Key dei_EntityCentralIndexKey 0001002537
Amendment Flag dei_AmendmentFlag false
Document Creation Date dei_DocumentCreationDate Apr. 30, 2019
Document Effective Date dei_DocumentEffectiveDate Apr. 30, 2019
Prospectus Date rr_ProspectusDate Apr. 30, 2019
Entity Inv Company Type dei_EntityInvCompanyType N-1A
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 and hold shares of the Growth Fund.
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination April 30, 2020
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 most recent fiscal year, the Growth Fund’s portfolio turnover rate was 8% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 8.00%
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 or traded on the NASDAQ System. 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.
Risk [Heading] rr_RiskHeading Principal Investment Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock
Stock Investing and Market Risks. The Growth Fund invests primarily in equity securities that fluctuate in value. Political and economic news can influence market-wide trends. Other factors may cause price swings in a single company’s stock or the stocks of the companies within a given industry.

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.

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.

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.
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.

Sector Risks. To the extent the Growth Fund focuses its investments in securities of issuers in a particular market sector, such as technology companies or healthcare companies, the 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 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 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.
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.
Loss of money is a risk of investing in the Growth Fund.
Risk Lose Money [Text] rr_RiskLoseMoney Loss of money is a risk of investing in the Growth Fund.
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 22.23% (for the quarter ended 6/30/2009) and the lowest quarterly return was-25.79% (for the quarter ended 9/30/2011).
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel highest quarterly return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 22.23%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel lowest quarterly return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (25.79%)
Performance Table Heading rr_PerformanceTableHeading Average annual total returns for the periods ended December 31, 2018
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. After-tax returns are shown only for Retail Class shares. After-tax returns for Institutional Class shares will vary.
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 In certain cases, the figure representing “Return after Taxes on Distributions and Redemptions” may be higher than the other return figures for the same period, since a higher after-tax return results when a capital loss occurs upon redemption and provides an assumed tax deduction that benefits the investor.
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, 2018. 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. In certain cases, the figure representing “Return after Taxes on Distributions and Redemptions” may be higher than the other return figures for the same period, since a higher after-tax return results when a capital loss occurs upon redemption and provides an assumed tax deduction that benefits the investor.
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.
Needham Growth Fund | S&P 500® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel S&P 500® Index*
1 Year rr_AverageAnnualReturnYear01 (4.38%) [1]
5 Years rr_AverageAnnualReturnYear05 8.49% [1]
10 Years rr_AverageAnnualReturnYear10 13.12% [1]
Needham Growth Fund | S&P MidCap 400® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel S&P MidCap 400® Index*
1 Year rr_AverageAnnualReturnYear01 (11.08%) [1]
5 Years rr_AverageAnnualReturnYear05 6.03% [1]
10 Years rr_AverageAnnualReturnYear10 13.68% [1]
Needham Growth Fund | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Russell 2000® Index*
1 Year rr_AverageAnnualReturnYear01 (11.01%) [2]
5 Years rr_AverageAnnualReturnYear05 4.41% [2]
10 Years rr_AverageAnnualReturnYear10 11.97% [2]
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%
Component2 Other Expenses rr_Component2OtherExpensesOverAssets 0.84%
Component3 Other Expenses rr_Component3OtherExpensesOverAssets 0.42%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 1.26%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 2.76%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets none [3]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 2.76%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 279
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 856
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,459
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 3,090
Annual Return 2009 rr_AnnualReturn2009 46.87%
Annual Return 2010 rr_AnnualReturn2010 31.37%
Annual Return 2011 rr_AnnualReturn2011 (10.94%)
Annual Return 2012 rr_AnnualReturn2012 12.80%
Annual Return 2013 rr_AnnualReturn2013 34.68%
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%)
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 (10.26%)
5 Years rr_AverageAnnualReturnYear05 2.14%
10 Years rr_AverageAnnualReturnYear10 11.25%
Since Inception rr_AverageAnnualReturnSinceInception 11.53%
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 (13.29%)
5 Years rr_AverageAnnualReturnYear05 0.22%
10 Years rr_AverageAnnualReturnYear10 9.93%
Since Inception rr_AverageAnnualReturnSinceInception 10.15%
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 01, 1996
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 (3.61%)
5 Years rr_AverageAnnualReturnYear05 1.74%
10 Years rr_AverageAnnualReturnYear10 9.43%
Since Inception rr_AverageAnnualReturnSinceInception 9.86%
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 01, 1996
Needham Growth Fund | Retail Class | S&P 500® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 8.33% [1]
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 01, 1996 [1]
Needham Growth Fund | Retail Class | S&P MidCap 400® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 10.75% [1]
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 01, 1996 [1]
Needham Growth Fund | Retail Class | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 7.94% [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
Component2 Other Expenses rr_Component2OtherExpensesOverAssets 0.81%
Component3 Other Expenses rr_Component3OtherExpensesOverAssets 0.42%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 1.23%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 2.48%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.27%) [3]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 2.21%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 224
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 747
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,296
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 2,796
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 (9.83%)
Since Inception rr_AverageAnnualReturnSinceInception (0.91%)
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016
Needham Growth Fund | Institutional Class | S&P 500® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 7.92% [1]
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016 [1]
Needham Growth Fund | Institutional Class | S&P MidCap 400® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 1.66% [1]
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016 [1]
Needham Growth Fund | Institutional Class | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 1.00% [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 and hold shares of the Aggressive Growth Fund.
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination April 30, 2020
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 most recent fiscal year, the Aggressive Growth Fund’s portfolio turnover rate was 8% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 8.00%
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 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 or traded on the NASDAQ System. 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.
Risk [Heading] rr_RiskHeading Principal Investment Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock
Stock Investing and Market Risks. The Aggressive Growth Fund invests primarily in equity securities that fluctuate in value. Political and economic news can influence market-wide trends. Other factors may cause price swings in a single company’s stock or the stocks of the companies within a given industry.

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.

Small Company Investment Risks. The Aggressive 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.

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.

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.

Sector Risks. To the extent that the Aggressive Growth Fund focuses its investments in securities of issuers in a particular market sector, such as technology companies or healthcare companies, the Aggressive 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.

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.

Loss of money is a risk of investing in the Aggressive Growth Fund.
Risk Lose Money [Text] rr_RiskLoseMoney Loss of money is a risk of investing in the Aggressive Growth Fund.
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 19.89% (for the quarter ended 6/30/2009) and the lowest quarterly return was -26.93% (for the quarter ended 9/30/2011).
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel highest quarterly return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 19.89%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel lowest quarterly return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (26.93%)
Performance Table Heading rr_PerformanceTableHeading Average annual total returns for the periods ended December 31, 2018
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. After-tax returns are shown only for Retail Class shares.
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.
Needham Aggressive Growth Fund | S&P 500® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel S&P 500® Index*
1 Year rr_AverageAnnualReturnYear01 (4.38%) [4]
5 Years rr_AverageAnnualReturnYear05 8.49% [4]
10 Years rr_AverageAnnualReturnYear10 13.12% [4]
Needham Aggressive Growth Fund | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Russell 2000® Index*
1 Year rr_AverageAnnualReturnYear01 (11.01%) [5]
5 Years rr_AverageAnnualReturnYear05 4.41% [5]
10 Years rr_AverageAnnualReturnYear10 11.97% [5]
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%
Component2 Other Expenses rr_Component2OtherExpensesOverAssets 0.96%
Component3 Other Expenses rr_Component3OtherExpensesOverAssets 0.51%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 1.47%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 2.97%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.06%) [6]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 2.91%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 294
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 913
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,557
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 3,285
Annual Return 2009 rr_AnnualReturn2009 31.18%
Annual Return 2010 rr_AnnualReturn2010 39.42%
Annual Return 2011 rr_AnnualReturn2011 (13.77%)
Annual Return 2012 rr_AnnualReturn2012 14.61%
Annual Return 2013 rr_AnnualReturn2013 36.26%
Annual Return 2014 rr_AnnualReturn2014 7.13%
Annual Return 2015 rr_AnnualReturn2015 (7.05%)
Annual Return 2016 rr_AnnualReturn2016 22.18%
Annual Return 2017 rr_AnnualReturn2017 8.73%
Annual Return 2018 rr_AnnualReturn2018 (15.80%)
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 (15.80%)
5 Years rr_AverageAnnualReturnYear05 2.18%
10 Years rr_AverageAnnualReturnYear10 10.62%
Since Inception rr_AverageAnnualReturnSinceInception 7.87%
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 (18.45%)
5 Years rr_AverageAnnualReturnYear05 0.31%
10 Years rr_AverageAnnualReturnYear10 9.56%
Since Inception rr_AverageAnnualReturnSinceInception 6.90%
Inception Date rr_AverageAnnualReturnInceptionDate Sep. 04, 2001
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 (7.16%)
5 Years rr_AverageAnnualReturnYear05 1.74%
10 Years rr_AverageAnnualReturnYear10 8.84%
Since Inception rr_AverageAnnualReturnSinceInception 6.58%
Inception Date rr_AverageAnnualReturnInceptionDate Sep. 04, 2001
Needham Aggressive Growth Fund | Retail Class | S&P 500® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 6.82% [4]
Inception Date rr_AverageAnnualReturnInceptionDate Sep. 04, 2001 [4]
Needham Aggressive Growth Fund | Retail Class | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 7.74% [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
Component2 Other Expenses rr_Component2OtherExpensesOverAssets 0.95%
Component3 Other Expenses rr_Component3OtherExpensesOverAssets 0.59%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 1.54%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 2.79%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.44%) [6]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 2.35%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 238
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 824
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,435
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 3,087
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 (15.36%)
Since Inception rr_AverageAnnualReturnSinceInception (3.78%)
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016
Needham Aggressive Growth Fund | Institutional Class | S&P 500® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 7.92% [4]
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016 [4]
Needham Aggressive Growth Fund | Institutional Class | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 1.00% [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 and hold shares of the Small Cap Growth Fund.
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination April 30, 2020
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 most recent fiscal year, the Small Cap Growth Fund’s portfolio turnover rate was 103% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 103.00%
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 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). Although your actual costs may be higher or lower, based on these assumptions, your costs would be:
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 or traded on the NASDAQ System that have market capitalizations not exceeding $5 billion at the time of investment. 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 $5 billion. Although there is no minimum limitation, the market capitalization range of the Small Cap Growth Fund’s investments at the time of investment will typically fall within the range of the companies listed on the Russell 2000® Index, which as of May 11, 2018 consists of securities with market capitalizations between $159.2 million and $5.0 billion.

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 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.
Risk [Heading] rr_RiskHeading Principal Investment Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock
Stock Investing and Market Risks. The Small Cap Growth Fund invests primarily in equity securities that fluctuate in value. Political and economic news can influence market-wide trends. Other factors may cause price swings in a single company’s stock or the stocks of the companies within a given industry.

Small Company Investment Risks. The Small Cap 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. The Small Cap Growth Fund may engage in active and frequent trading of portfolio securities.

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.

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.

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.

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.

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.

Loss of money is a risk of investing in the Small Cap Growth Fund.

Risk Lose Money [Text] rr_RiskLoseMoney Loss of money is a risk of investing in the Small Cap Growth Fund.
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 21.00% (for the quarter ended 6/30/2009) and the lowest quarterly return was -23.72% (for the quarter ended 9/30/2011).
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel highest quarterly return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 21.00%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel lowest quarterly return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn 23.72%
Performance Table Heading rr_PerformanceTableHeading Average annual total returns for the periods ended December 31, 2018
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 Small Cap Growth Fund’s shares and the change in value of certain broad-based market indices over various periods ended December 31, 2018. 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. In certain cases, the figure representing “Return after Taxes on Distributions and Redemptions” may be higher than the other return figures for the same period, since a higher after-tax return results when a capital loss occurs upon redemption and provides an assumed tax deduction that benefits the investor.

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.
Needham Small Cap Growth Fund | S&P 500® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel S&P 500® Index*
1 Year rr_AverageAnnualReturnYear01 (4.38%) [7]
5 Years rr_AverageAnnualReturnYear05 8.49% [7]
10 Years rr_AverageAnnualReturnYear10 13.12% [7]
Needham Small Cap Growth Fund | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Label rr_AverageAnnualReturnLabel Russell 2000® Index*
1 Year rr_AverageAnnualReturnYear01 (11.01%) [8]
5 Years rr_AverageAnnualReturnYear05 4.41% [8]
10 Years rr_AverageAnnualReturnYear10 11.97% [8]
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%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.69%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.08% [9]
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 2.27%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.34%) [10]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 1.93%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 196
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 677
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,184
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 2,579
Annual Return 2009 rr_AnnualReturn2009 41.18%
Annual Return 2010 rr_AnnualReturn2010 36.89%
Annual Return 2011 rr_AnnualReturn2011 (16.10%)
Annual Return 2012 rr_AnnualReturn2012 8.53%
Annual Return 2013 rr_AnnualReturn2013 27.91%
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 11.81%
Annual Return 2018 rr_AnnualReturn2018 (5.07%)
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 (5.07%)
5 Years rr_AverageAnnualReturnYear05 5.03%
10 Years rr_AverageAnnualReturnYear10 11.15%
Since Inception rr_AverageAnnualReturnSinceInception 9.39%
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 (10.13%)
5 Years rr_AverageAnnualReturnYear05 2.17%
10 Years rr_AverageAnnualReturnYear10 9.47%
Since Inception rr_AverageAnnualReturnSinceInception 7.76%
Inception Date rr_AverageAnnualReturnInceptionDate May 22, 2002
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 (1.09%)
5 Years rr_AverageAnnualReturnYear05 3.29%
10 Years rr_AverageAnnualReturnYear10 8.89%
Since Inception rr_AverageAnnualReturnSinceInception 7.69%
Inception Date rr_AverageAnnualReturnInceptionDate May 22, 2002
Needham Small Cap Growth Fund | Retail Class | S&P 500® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 7.33% [7]
Inception Date rr_AverageAnnualReturnInceptionDate May 22, 2002 [7]
Needham Small Cap Growth Fund | Retail Class | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 7.66% [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
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.70%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.08% [9]
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 2.03%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.77%) [10]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 1.26%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 128
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 562
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,022
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 2,297
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 (4.32%)
Since Inception rr_AverageAnnualReturnSinceInception 3.60%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016
Needham Small Cap Growth Fund | Institutional Class | S&P 500® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 7.92% [7]
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016 [7]
Needham Small Cap Growth Fund | Institutional Class | Russell 2000® Index  
Prospectus [Line Items] rr_ProspectusLineItems  
Since Inception rr_AverageAnnualReturnSinceInception 1.00% [8]
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2016 [8]
[1] Comparative indices reflect no deductions for fees, expenses, or taxes.
[2] Source: London Stock Exchange Group plc and its group undertakings (collectively, the “LSE Group”). © LSE Group 2019. 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.
[3] Reflects a contractual agreement by Needham Investment Management LLC (the “Adviser”) to waive its fee and, if necessary, reimburse the Growth Fund until April 30, 2020 to the extent Total Annual Fund Operating Expenses exceed 1.95% and 1.40% of the average daily net assets of Retail Class shares and Institutional Class shares, respectively, of the Growth Fund (the “Expense Cap”). This agreement can only be amended or terminated by agreement of 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 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 recovery 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. Any such recovery 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.
[4] Comparative indices reflect no deductions for fees, expenses, or taxes.
[5] Source: London Stock Exchange Group plc and its group undertakings (collectively, the “LSE Group”). © LSE Group 2019. 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 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.
[6] 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 30, 2020 to the extent Total Annual Fund Operating Expenses exceed 1.95% and 1.40% of the average daily net assets of Retail Class shares and Institutional Class shares, respectively, of the Aggressive Growth Fund (the “Expense Cap”). This agreement can only be amended or terminated by agreement of 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 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 recovery 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. Any such recovery 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.
[7] Comparative indices reflect no deductions for fees, expenses, or taxes.
[8] Source: London Stock Exchange Group plc and its group undertakings (collectively, the “LSE Group”). © LSE Group 2019. 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 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.
[9] Acquired Fund Fees and Expenses 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 Acquired Fund Fees and Expenses.
[10] 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 30, 2020 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”). This agreement can only be amended or terminated by agreement of 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 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 recovery 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. Any such recovery 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.