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Hennessy Gas Utility Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Hennessy Gas Utility Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock
The Hennessy Gas Utility Fund seeks income and capital appreciation.
Expense [Heading] rr_ExpenseHeading Fund Fees and Expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock
This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption SHAREHOLDER FEES (fees paid directly from your investment)
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)
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock
The 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 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 Fund’s performance.  During the most recent fiscal year, the Fund’s portfolio turnover rate was 38% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 38.00%
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 shares of the Fund with the cost of investing in other mutual funds.

The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that you reinvest all dividends and distributions, that your investment has a 5% return each year and that the Fund’s operating expenses remain the same.
Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption Although your actual costs may be higher or lower, based on the assumptions, your costs would be:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategy
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
The Fund intends to provide investment results that replicate the performance of the American Gas Association Stock Index (the “AGA Stock Index”).  The AGA Stock Index is maintained by the American Gas Association, a national trade association of natural gas companies, and is licensed exclusively to the Investment Manager for use as an investment strategy. The AGA Stock Index consists of all publicly-traded member companies of the American Gas Association, which include natural gas distribution, gas pipeline, diversified gas and combination gas and electric companies. The Fund invests in domestic U.S. securities, but such securities may include domestic securities of foreign issuers, including indirect investments such as American Depositary Receipts or other types of depositary receipts, which are U.S. dollar-denominated securities of foreign issuers traded in the U.S.  The stocks included in the Fund are chosen solely on the basis of their inclusion in the AGA Stock Index.

Under normal circumstances, the Fund intends to invest at least 85% of its net assets in the common stock of companies that have natural gas distribution and transmission operations and no attempt is made to actively manage the Fund’s portfolio by using economic, financial or market analysis.  The adverse financial situation of a company will not result in its elimination from the Fund’s portfolio unless the company is removed from the AGA Stock Index.  The percentage of the Fund’s assets invested in the stock of a particular company is approximately the same as the percentage weighting of such company in the AGA Stock Index.  The percentage weighting of each company in the AGA Stock Index is an amount equal to such company’s market capitalization (which is the number of its shares outstanding multiplied by the market price of its stock) multiplied by the percentage of such company’s assets devoted to natural gas distribution and transmission.  The latter component of this calculation is used to recognize the natural gas distribution and transmission component of the company’s asset base.

There is no predetermined acceptable range of the difference between the total return of the AGA Stock Index and the total return of the Fund.  Any difference is likely the result of various expenses incurred by the Fund, such as management fees, transaction costs and other operating expenses, as well as subscription and redemption activity.  On the other hand, the Fund does attempt to achieve a correlation of monthly returns with the AGA Stock Index of approximately 95% or better.  A correlation of 100% would mean the total return of the Fund’s assets would increase or decrease exactly the same as the total return of the AGA Stock Index.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock
As with any security, there are market and investment risks associated with an investment in the Fund. The value of an investment will fluctuate over time and it is possible to lose money. The principal risks of investing in the Fund include the following:

Market and Equity Investments Risk:  The market value of a security may move up or down, and these fluctuations may cause a security to be worth more or less than the price originally paid for it. Market risk may affect a single company, an industry, a sector of the economy or the market as a whole. The value of equity securities will fluctuate due to many factors, including the past and predicted earnings of the issuer, the quality of the issuer’s management, general market conditions, forecasts for the issuer’s industry and the value of the issuer’s assets.

Industry Concentration Risk: The Fund concentrates its investments within the natural gas distribution and transmission industry. Because of its narrow industry focus, the Fund’s performance is tied closely to and affected by developments and risks inherent in the natural gas industry.  Specifically, the natural gas companies in the Fund may be adversely affected by a decline or an increase in the price of natural gas, disruption of gas supplies transported on interstate pipelines, loss of supply for other reasons, failure of equipment, accidents and severe weather.  There are also a variety of hazards inherent in natural gas distribution activities, including the risk of explosions on natural gas distribution systems and other operating risks.  The profitability of the natural gas industry may also be sensitive to increased interest rates because of the industry’s capital intensive nature.

Non-Diversification Risk: Because the Fund is non-diversified (meaning that compared to diversified mutual funds, the Fund may invest a greater percentage of its assets in a more limited number of issuers), the Fund’s shares may be more susceptible to adverse changes in the value of a particular security than would be the shares of a diversified mutual fund.  Thus, the Fund may be more volatile because each stock in which it invests could have greater impact on the Fund’s performance.

Foreign Securities Risk:  The Fund may invest in the securities of foreign issuers, including American Depositary Receipts, which are U.S. dollar-denominated securities of foreign issuers traded in the U.S.  There are specific risks associated with investing in the securities of foreign companies not typically associated with investing in domestic companies.  Risks include fluctuations in the exchange rates of foreign currencies, which may affect the U.S. Dollar value of a security, and the possibility of substantial price volatility as a result of political and economic instability in the foreign country.

Index Tracking Risk:  While the Fund seeks to track the performance of the AGA Stock Index as closely as possible, the Fund’s return may not always be able to match or achieve a high correlation due to such factors as the expenses incurred by the Fund, such as management fees, transaction costs and other operating expenses that are not incurred by the AGA Stock Index, or the possibility that the Fund may experience significant subscriptions or redemptions.  In addition, the Fund may not be fully invested at all times as a result of cash flows into the Fund or reserves of cash that are maintained in order to cover operating expenses and meet redemption requests.
Risk Lose Money [Text] rr_RiskLoseMoney The value of an investment will fluctuate over time and it is possible to lose money.
Risk Nondiversified Status [Text] rr_RiskNondiversifiedStatus Because the Fund is non-diversified (meaning that compared to diversified mutual funds, the Fund may invest a greater percentage of its assets in a more limited number of issuers), the Fund’s shares may be more susceptible to adverse changes in the value of a particular security than would be the shares of a diversified mutual fund. Thus, the Fund may be more volatile because each stock in which it invests could have greater impact on the Fund’s performance.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance Information
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock
The following performance information provides some indication of the risks of investing in the Fund by showing changes in its performance from year to year and how the Fund’s average annual returns for 1, 5, and 10 years compare with those of an index reflecting a broad measure of market performance, the S&P 500 Index, as well as an additional index that reflects the market sector in which the Fund invests, the AGA Stock Index.  For additional information on these indices, please see “Descriptions of Indices” on page 60 of this Prospectus.  Performance information is only shown for Investor Class shares because Institutional Class shares were not offered prior to the date of this Prospectus.  The Investor Class and Institutional Class shares will have substantially similar annual returns because the shares are invested in the same portfolio of securities and the annual returns will differ only to the extent that the Classes do not have the same expenses.  The Fund is the successor to the FBR Gas Utility Index Fund (the “Predecessor Gas Utility Fund”).  The performance information provided for the periods on or prior to October 26, 2012, is historical information for the Predecessor Gas Utility Fund, which had the same investment objective and same investment strategy as the Fund. The Fund’s past performance (before and after taxes) is not necessarily an indication of future performance.  Performance may be higher or lower in the future. Updated performance information is available on the Hennessy Funds’ website (hennessyfunds.com).
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The following performance information provides some indication of the risks of investing in the Fund by showing changes in its performance from year to year and how the Fund’s average annual returns for 1, 5 and 10 years compare with those of an index reflecting a broad measure of market performance, the S&P 500 Index, as well as an additional index that reflects the market sector in which the Fund invests, the AGA Stock Index.
Performance Additional Market Index [Text] rr_PerformanceAdditionalMarketIndex We use the AGA Stock Index as an additional index because it compares the Fund’s performance with the returns of an index reflecting the performance of investments similar to those of the Fund. The AGA Stock Index is calculated monthly and provided by the American Gas Association.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress hennessyfunds.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Fund’s past performance (before and after taxes) is not necessarily an indication of future performance.
Bar Chart [Heading] rr_BarChartHeading HENNESSY GAS UTILITY FUND CALENDAR YEAR TOTAL RETURNS OF INVESTOR SHARES
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
For the period shown in the bar chart, the Fund’s highest quarterly return was 14.73% for the quarter ended June 30, 2009, and the lowest quarterly return was -16.40% for the quarter ended September 30, 2008.
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel highest quarterly return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 14.73%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel lowest quarterly return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (16.40%)
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees, expenses or taxes)
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred Actual after-tax returns depend on an investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock
Performance of the Fund’s Institutional Class will differ from that of the Fund’s Investor Class shares as the share classes have different expenses.
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock
We use the AGA Stock Index as an additional index because it compares the Fund’s performance with the returns of an index reflecting the performance of investments similar to those of the Fund. The AGA Stock Index is calculated monthly and provided by the American Gas Association.

The after-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Caption rr_AverageAnnualReturnCaption AVERAGE ANNUAL TOTAL RETURNS (for the periods ended December 31, 2016)
Hennessy Gas Utility Fund | AGA Stock Index (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel AGA Stock Index (reflects no deduction for fees, expenses or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 22.27%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 11.45%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 9.69%
Hennessy Gas Utility Fund | S&P 500 Index (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel S&P 500 Index (reflects no deduction for fees, expenses or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 11.96%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 14.66%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 6.95%
Hennessy Gas Utility Fund | Investor Class  
Risk/Return: rr_RiskReturnAbstract  
SHAREHOLDER FEES (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fees rr_ManagementFeesOverAssets 0.40%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.15%
Shareholder Servicing rr_Component1OtherExpensesOverAssets 0.10%
Remaining Other Expenses rr_Component3OtherExpensesOverAssets 0.36%
Other Expenses rr_OtherExpensesOverAssets 0.46%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.01%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 103
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 322
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 558
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,236
Annual Return 2007 rr_AnnualReturn2007 14.03%
Annual Return 2008 rr_AnnualReturn2008 (28.30%)
Annual Return 2009 rr_AnnualReturn2009 23.92%
Annual Return 2010 rr_AnnualReturn2010 12.40%
Annual Return 2011 rr_AnnualReturn2011 25.15%
Annual Return 2012 rr_AnnualReturn2012 7.45%
Annual Return 2013 rr_AnnualReturn2013 25.42%
Annual Return 2014 rr_AnnualReturn2014 21.20%
Annual Return 2015 rr_AnnualReturn2015 (16.01%)
Annual Return 2016 rr_AnnualReturn2016 20.70%
Label rr_AverageAnnualReturnLabel Hennessy Gas Utility Fund Returns before taxes
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 20.70%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 10.61%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 8.97%
Hennessy Gas Utility Fund | Investor Class | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Hennessy Gas Utility Fund Returns after taxes on distributions
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 19.21%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 9.33%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 7.86%
Hennessy Gas Utility Fund | Investor Class | After Taxes on Distributions and Sale of Fund Shares  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Hennessy Gas Utility Fund Returns after taxes on distributions and sale of Fund shares
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 12.03%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 8.23%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 7.16%
Hennessy Gas Utility Fund | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
SHAREHOLDER FEES (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fees rr_ManagementFeesOverAssets 0.40%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Shareholder Servicing rr_Component1OtherExpensesOverAssets none
Remaining Other Expenses rr_Component3OtherExpensesOverAssets 0.24%
Other Expenses rr_OtherExpensesOverAssets 0.24% [1]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.64%
Other Expenses, New Fund, Based on Estimates [Text] rr_OtherExpensesNewFundBasedOnEstimates “Other Expenses” is estimated based on expenses from fiscal year 2016 for Investor Class shares of the Fund, as adjusted to account for anticipated class-level differences in expenses of Institutional Class shares.
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 65
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 205
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 357
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 798
[1] "Other Expenses" is estimated based on expenses from fiscal year 2016 for Investor Class shares of the Fund, as adjusted to account for anticipated class-level differences in expenses of Institutional Class shares.