Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	JANUS INVESTMENT FUND
Prospectus Date	rr_ProspectusDate	Jan. 26, 2018
Janus Henderson Forty Fund | Class D Shares
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	FUND SUMMARY Janus Henderson Forty Fund
Objective [Heading]	rr_ObjectiveHeading	INVESTMENT OBJECTIVE
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Janus Henderson Forty Fund seeks long-term growth of capital.
Expense [Heading]	rr_ExpenseHeading	FEES AND EXPENSES OF THE FUND
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.
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	February 1, 2019
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 56% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	56.00%
Expense Example [Heading]	rr_ExpenseExampleHeading	EXAMPLE:
Expense Example by Year [Heading]	rr_ExpenseExampleByYearHeading	The Example assumes that you invest $10,000 in the Fund for the time periods indicated, reinvest all dividends and distributions, and then redeem all of your Shares at the end of each period. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses without waivers remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is based on expenses without waivers. The Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds.
Strategy [Heading]	rr_StrategyHeading	PRINCIPAL INVESTMENT STRATEGIES
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Fund pursues its investment objective by normally investing primarily in a core group of 20-40 common stocks selected for their growth potential. The Fund may invest in companies of any size, from larger, well-established companies to smaller, emerging growth companies. The Fund may also invest in foreign securities, which may include investments in emerging markets. As of September 30, 2017, the Fund held stocks of 42 companies. Of these holdings, 20 comprised approximately 64.10% of the Fund’s holdings. The portfolio managers apply a “bottom up” approach in choosing investments. In other words, the portfolio managers look at companies one at a time to determine if a company is an attractive investment opportunity and if it is consistent with the Fund’s investment policies. The Fund may lend portfolio securities on a short-term or long-term basis, in an amount equal to up to one-third of its total assets as determined at the time of the loan origination.
Risk [Heading]	rr_RiskHeading	PRINCIPAL INVESTMENT RISKS
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	The biggest risk is that the Fund’s returns will vary, and you could lose money. The Fund is designed for long-term investors seeking an equity portfolio, including common stocks. Common stocks tend to be more volatile than many other investment choices. Market Risk.  The value of the Fund’s portfolio may decrease if the value of an individual company or security, or multiple companies or securities, in the portfolio decreases or if the portfolio managers’ belief about a company’s intrinsic worth is incorrect. Further, regardless of how well individual companies or securities perform, the value of the Fund’s portfolio could also decrease if there are deteriorating economic or market conditions. It is important to understand that the value of your investment may fall, sometimes sharply, in response to changes in the market, and you could lose money. Market risk may affect a single issuer, industry, economic sector, or the market as a whole. Growth Securities Risk.  The Fund invests in companies that the portfolio managers believe have growth potential. Securities of companies perceived to be “growth” companies may be more volatile than other stocks and may involve special risks. If the portfolio managers’ perception of a company’s growth potential is not realized, the securities purchased may not perform as expected, reducing the Fund’s returns. In addition, because different types of stocks tend to shift in and out of favor depending on market and economic conditions, “growth” stocks may perform differently from the market as a whole and other types of securities. Nondiversification Risk.  The Fund is classified as nondiversified under the Investment Company Act of 1940, as amended. This gives the Fund’s portfolio managers more flexibility to hold larger positions in a smaller number of securities. As a result, an increase or decrease in the value of a single security held by the Fund may have a greater impact on the Fund’s net asset value and total return. Foreign Exposure Risk.  The Fund may have exposure to foreign markets as a result of its investments in foreign securities, including investments in emerging markets, which can be more volatile than the U.S. markets. As a result, its returns and net asset value may be affected to a large degree by fluctuations in currency exchange rates or political or economic conditions in a particular country. In some foreign markets, there may not be protection against failure by other parties to complete transactions. It may not be possible for the Fund to repatriate capital, dividends, interest, and other income from a particular country or governmental entity. In addition, a market swing in one or more countries or regions where the Fund has invested a significant amount of its assets may have a greater effect on the Fund’s performance than it would in a more geographically diversified portfolio. To the extent the Fund invests in foreign debt securities, such investments are sensitive to changes in interest rates. Additionally, investments in securities of foreign governments involve the risk that a foreign government may not be willing or able to pay interest or repay principal when due. The Fund’s investments in emerging market countries may involve risks greater than, or in addition to, the risks of investing in more developed countries. Securities Lending Risk.  The Fund may seek to earn additional income through lending its securities to certain qualified broker-dealers and institutions. There is the risk that when portfolio securities are lent, the securities may not be returned on a timely basis, and the Fund may experience delays and costs in recovering the security or gaining access to the collateral provided to the Fund to collateralize the loan. If the Fund is unable to recover a security on loan, the Fund may use the collateral to purchase replacement securities in the market. There is a risk that the value of the collateral could decrease below the cost of the replacement security by the time the replacement investment is made, resulting in a loss to the Fund. Management Risk.  The Fund is an actively managed investment portfolio and is therefore subject to the risk that the investment strategies employed for the Fund may fail to produce the intended results. The Fund may underperform its benchmark index or other mutual funds with similar investment objectives. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Risk Lose Money [Text]	rr_RiskLoseMoney	The biggest risk is that the Fund’s returns will vary, and you could lose money.
Risk Nondiversified Status [Text]	rr_RiskNondiversifiedStatus	Nondiversification Risk.  The Fund is classified as nondiversified under the Investment Company Act of 1940, as amended. This gives the Fund’s portfolio managers more flexibility to hold larger positions in a smaller number of securities. As a result, an increase or decrease in the value of a single security held by the Fund may have a greater impact on the Fund’s net asset value and total return.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	PERFORMANCE INFORMATION
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following information provides some indication of the risks of investing in the Fund by showing how the Fund’s performance has varied over time. Class D Shares of the Fund commenced operations on January 27, 2017. The performance shown for Class D Shares reflects the performance of the Fund’s Class S Shares from July 6, 2009 to January 27, 2017, calculated using the fees and expenses of Class S Shares, net of any applicable fee and expense limitations or waivers. The performance shown for Class D Shares reflects the performance of Janus Adviser Forty Fund’s (the “JAD predecessor fund”) Class S Shares (formerly named Class I Shares) from August 1, 2000 to July 6, 2009 (prior to the reorganization), calculated using the fees and expenses of Class S Shares of the JAD predecessor fund, net of any applicable fee and expense limitations or waivers. For the periods prior to August 1, 2000, the performance shown for Class D Shares reflects the historical performance of the Retirement Shares of Janus Aspen Series – Forty Portfolio (as a result of a separate prior reorganization of those Retirement Shares into the JAD predecessor fund). The performance shown for certain periods prior to August 1, 2000 was calculated using the fees and expenses of Class S Shares of the JAD predecessor fund, without the effect of any fee and expense limitations or waivers. If Class D Shares of the Fund had been available during periods prior to January 27, 2017, the performance shown may have been different. The performance shown for periods following the Fund’s commencement of Class D Shares reflects the fees and expenses of Class D Shares, net of any applicable fee and expense limitations or waivers. The bar chart depicts the change in performance from year to year during the periods indicated. The table compares the Fund’s average annual returns for the periods indicated to broad-based securities market indices. All figures assume reinvestment of dividends and distributions. For certain periods, the Fund’s performance reflects the effect of expense waivers. Without the effect of these expense waivers, the performance shown would have been lower. The Fund’s past performance (before and after taxes) does not necessarily indicate how it will perform in the future. Updated performance information is available at janushenderson.com/allfunds or by calling 1-800-525-3713.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following information provides some indication of the risks of investing in the Fund by showing how the Fund’s performance has varied over time. The bar chart depicts the change in performance from year to year during the periods indicated.
Performance Additional Market Index [Text]	rr_PerformanceAdditionalMarketIndex	The table compares the Fund’s average annual returns for the periods indicated to broad-based securities market indices. All figures assume reinvestment of dividends and distributions.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-800-525-3713
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	janushenderson.com/allfunds
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past performance (before and after taxes) does not necessarily indicate how it will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Annual Total Returns for Class D Shares (calendar year-end)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best Quarter:  2nd Quarter 2009  20.90%          Worst Quarter:  4th Quarter 2008  – 25.11%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns (periods ended 12/31/17)
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historically 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 your individual tax situation and may differ from those shown in the preceding table. The after-tax return information shown above does not apply to Fund shares held through a tax-deferred account, such as a 401(k) plan or an IRA.
Performance Table Explanation after Tax Higher	rr_PerformanceTableExplanationAfterTaxHigher	If the Fund incurs a loss, which generates a tax benefit, the Return After Taxes on Distributions and Sale of Fund Shares may exceed the Fund’s other return figures.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	The Fund’s primary benchmark index is the Russell 1000® Growth Index. The Fund also compares its performance to the S&P 500® Index. The Russell 1000® Growth Index is used to calculate the Fund’s performance fee adjustment. The indices are described below. The Russell 1000® Growth Index measures the performance of those Russell 1000® companies with higher price-to-book ratios and higher forecasted growth values. The S&P 500® Index is a commonly recognized, market-capitalization weighted index of 500 widely held equity securities, designed to measure broad U.S. equity performance. After-tax returns are calculated using distributions for the Fund’s Class D Shares for the periods following January 27, 2017; for the Fund’s Class S Shares for periods January 27, 2017 to July 6, 2009; for the JAD predecessor fund’s Class S Shares (formerly named Class I Shares) for the periods August 1, 2000 to July 6, 2009; and actual distributions for other classes of shares for periods prior to August 1, 2000. If Class D Shares of the Fund had been available during these earlier periods, distributions may have been different and thus, after-tax returns may have been different from those shown. After-tax returns are calculated using the historically highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on your individual tax situation and may differ from those shown in the preceding table. The after-tax return information shown above does not apply to Fund shares held through a tax-deferred account, such as a 401(k) plan or an IRA.
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	Updated performance information is available at janushenderson.com/allfunds or by calling 1-800-525-3713.
Janus Henderson Forty Fund | Class D Shares | Class D
Risk/Return:	rr_RiskReturnAbstract
Operating Expenses Column [Text]	rr_OperatingExpensesColumnName	Class D
Management Fees (may adjust up or down)	rr_ManagementFeesOverAssets	0.53%
Other Expenses	rr_OtherExpensesOverAssets	0.29%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.82%	[1]
Fee Waiver	rr_FeeWaiverOrReimbursementOverAssets	0.03%	[1]
Total Annual Fund Operating Expenses After Fee Waiver	rr_NetExpensesOverAssets	0.79%	[1]
Expense Example, By Year, Column [Text]	rr_ExpenseExampleByYearColumnName	Class D Shares
1 Year	rr_ExpenseExampleYear01	$ 84
3 Years	rr_ExpenseExampleYear03	262
5 Years	rr_ExpenseExampleYear05	455
10 Years	rr_ExpenseExampleYear10	$ 1,014
2008	rr_AnnualReturn2008	(44.02%)
2009	rr_AnnualReturn2009	43.53%
2010	rr_AnnualReturn2010	5.62%
2011	rr_AnnualReturn2011	(7.32%)
2012	rr_AnnualReturn2012	23.65%
2013	rr_AnnualReturn2013	31.73%
2014	rr_AnnualReturn2014	8.57%
2015	rr_AnnualReturn2015	11.75%
2016	rr_AnnualReturn2016	1.92%
2017	rr_AnnualReturn2017	29.10%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	20.90%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(25.11%)
Janus Henderson Forty Fund | Class D Shares | Return Before Taxes | Class D
Risk/Return:	rr_RiskReturnAbstract
Column	rr_AverageAnnualReturnColumnName	Class D Shares
Label	rr_AverageAnnualReturnLabel	Return Before Taxes
1 Year	rr_AverageAnnualReturnYear01	29.10%
5 Years	rr_AverageAnnualReturnYear05	16.03%
10 Years	rr_AverageAnnualReturnYear10	7.42%
Since Inception of Predecessor Fund	rr_AverageAnnualReturnSinceInception	11.15%
Inception Date	rr_AverageAnnualReturnInceptionDate	May 01, 1997
Janus Henderson Forty Fund | Class D Shares | Return After Taxes on Distributions | Class D
Risk/Return:	rr_RiskReturnAbstract
Column	rr_AverageAnnualReturnColumnName	Class D Shares
Label	rr_AverageAnnualReturnLabel	Return After Taxes on Distributions
1 Year	rr_AverageAnnualReturnYear01	24.87%
5 Years	rr_AverageAnnualReturnYear05	11.21%
10 Years	rr_AverageAnnualReturnYear10	5.07%
Since Inception of Predecessor Fund	rr_AverageAnnualReturnSinceInception	9.92%
Inception Date	rr_AverageAnnualReturnInceptionDate	May 01, 1997
Janus Henderson Forty Fund | Class D Shares | Return After Taxes on Distributions and Sale of Fund Shares | Class D
Risk/Return:	rr_RiskReturnAbstract
Column	rr_AverageAnnualReturnColumnName	Class D Shares
Label	rr_AverageAnnualReturnLabel	Return After Taxes on Distributions and Sale of Fund Shares	[2]
1 Year	rr_AverageAnnualReturnYear01	19.55%
5 Years	rr_AverageAnnualReturnYear05	11.82%
10 Years	rr_AverageAnnualReturnYear10	5.46%
Since Inception of Predecessor Fund	rr_AverageAnnualReturnSinceInception	9.53%
Inception Date	rr_AverageAnnualReturnInceptionDate	May 01, 1997
Janus Henderson Forty Fund | Class D Shares | Russell 1000® Growth Index (reflects no deduction for expenses, fees, or taxes)
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Russell 1000® Growth Index
1 Year	rr_AverageAnnualReturnYear01	30.21%
5 Years	rr_AverageAnnualReturnYear05	17.33%
10 Years	rr_AverageAnnualReturnYear10	10.00%
Since Inception of Predecessor Fund	rr_AverageAnnualReturnSinceInception	7.67%
Inception Date	rr_AverageAnnualReturnInceptionDate	May 01, 1997
Janus Henderson Forty Fund | Class D Shares | S&P 500® Index (reflects no deduction for expenses, fees, or taxes)
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	S&P 500® Index
1 Year	rr_AverageAnnualReturnYear01	21.83%
5 Years	rr_AverageAnnualReturnYear05	15.79%
10 Years	rr_AverageAnnualReturnYear10	8.50%
Since Inception of Predecessor Fund	rr_AverageAnnualReturnSinceInception	8.03%
Inception Date	rr_AverageAnnualReturnInceptionDate	May 01, 1997

[1] Janus Capital has contractually agreed to waive its investment advisory fee and/or reimburse operating expenses to the extent that the Fund’s total annual fund operating expenses (excluding any performance adjustments to management fees, administrative services fees (including out-of-pocket costs), brokerage commissions, interest, dividends, taxes, acquired fund fees and expenses, and extraordinary expenses) exceed 0.77% until at least February 1, 2019. The contractual waiver may be terminated or modified prior to this date only at the discretion of the Board of Trustees.

[2] If the Fund incurs a loss, which generates a tax benefit, the Return After Taxes on Distributions and Sale of Fund Shares may exceed the Fund’s other return figures.