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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName JANUS INVESTMENT FUND
Prospectus Date rr_ProspectusDate Oct. 29, 2018
Janus Henderson Government Money Market Fund | Class T Shares  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading <b>FUND SUMMARY</b><br/><br/><b>Janus Henderson Government Money Market Fund</b>
Objective [Heading] rr_ObjectiveHeading <b>INVESTMENT OBJECTIVES </b>
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock Janus Henderson Government Money Market Fund seeks capital preservation and liquidity
Objective, Secondary [Text Block] rr_ObjectiveSecondaryTextBlock with current income as a secondary objective.
Expense [Heading] rr_ExpenseHeading <b>FEES AND EXPENSES OF THE FUND </b>
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 <b>ANNUAL FUND OPERATING EXPENSES</b><br/>(expenses that you pay each year as a percentage of the value of your investment)
Expense Example [Heading] rr_ExpenseExampleHeading <b>EXAMPLE:</b>
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 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 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 <b>PRINCIPAL INVESTMENT STRATEGIES </b>
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The Fund pursues its investment objectives by primarily investing, under normal circumstances, at least 80% of its net assets (plus any borrowings for investment purposes) in U.S. Government securities and repurchase agreements that are collateralized by U.S. Government securities.

The Fund operates as a “government money market fund” as such term is defined in or interpreted under Rule 2a-7 under the Investment Company Act of 1940, as amended. As a government money market fund, the Fund pursues its investment objectives by normally investing at least 99.5% of its total assets in cash, U.S. Government securities, and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash and/or government securities). The Fund’s investments in U.S. Government securities may include obligations issued and/or guaranteed as to principal and interest by the United States Government or by its agencies and instrumentalities, and repurchase agreements secured by such obligations. Although U.S. Government agencies and instrumentalities may be chartered or sponsored by Acts of Congress, their securities are not issued by, and may not be guaranteed by (i.e., backed by the full faith and credit of), the U.S. Treasury. Some government agency and instrumentality securities not backed by the full faith and credit of the United States are supported by the issuer’s ability to borrow from the U.S. Treasury, some are supported only by the credit of the issuer, and some are supported by the United States in some other way. The Fund’s investments in securities issued by U.S. Government agencies and instrumentalities may be significant.

The Fund will:
  • invest in high-quality, short-term money market instruments that present minimal credit risks, as determined by Janus Capital
  • invest only in U.S. dollar-denominated instruments that have a remaining maturity of 397 days or less (as calculated pursuant to Rule 2a-7 under the Investment Company Act of 1940, as amended)
  • maintain a dollar-weighted average portfolio maturity of 60 days or less and maintain a dollar-weighted average portfolio life (portfolio maturity measured without reference to any maturity shortening provisions of adjustable rate securities by reference to their interest rate reset dates) of 120 days or less
As a government money market fund, the Board of Trustees has determined not to subject the Fund to a liquidity fee and/or redemption gate on Fund redemptions. Please note that the Board of Trustees has reserved the ability to change this determination with respect to liquidity fees and/or redemption gates, but only after providing appropriate prior notice to shareholders.
Risk [Heading] rr_RiskHeading <b>PRINCIPAL INVESTMENT RISKS </b>
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock Money Market Risk.  You could lose money by investing in the Fund. Although the Fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the Fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Factors such as, but not limited to, an increase in interest rates, a decline in the credit quality of one or more issuers, large redemptions of the Fund’s Shares, or adverse market conditions impacting the trading or the value of money market instruments could cause the Fund’s share price to decrease below $1.00. Janus Capital has no legal obligation to provide financial support to the Fund, and you should not expect that Janus Capital will provide financial support to the Fund at any time. Accordingly, if the value of Fund Shares were to fall below $1.00 per share, there is no guarantee that Janus Capital or its affiliates would protect the Fund or redeeming shareholders against any loss of principal.

Interest Rate Risk.  Although the Fund seeks to invest only in high-quality, short-term money market instruments, there is a risk that the value of the securities it holds will fall as a result of the changes in interest rates, which are near historically low levels. Rising interest rates could cause the value of the Fund’s investments to decline, and a decline in interest rates is likely to cause the Fund’s yield to decline. During periods of unusually low interest rates, the Fund’s yield may approach zero. Such low interest rate environments may prevent the Fund from providing a positive yield or could impair the Fund’s ability to maintain the value of your investment at $1.00 per share. Over time, the real value of the Fund’s yield may be eroded by inflation.

Credit Quality Risk.  The value of the securities which the Fund holds may fall based on an issuer’s actual or perceived creditworthiness, or an issuer’s ability to meet its obligations (potentially even the U.S. Government). The credit quality of the Fund’s holdings can change rapidly in certain market environments and any downgrade or default of a portfolio security could cause the Fund’s yield to fall or cause the value of the Fund’s Shares to fall below $1.00 per share.

Liquidity Risk.  The liquidity of the securities which the Fund holds may deteriorate rapidly due to credit events affecting one or more issuers or guarantors or due to general market conditions and a lack of willing buyers. In such cases, the Fund may have to accept a lower price or may be unable to sell a security. An inability to sell one or more securities may negatively affect the Fund’s ability to maintain a $1.00 share price or prevent the Fund from being able to take advantage of other investment opportunities. Additionally, adverse market conditions impacting the trading of the value of money market instruments and/or an unusually high volume of redemption requests could cause the Fund to be unable to pay redemption proceeds within a short period of time. If the Fund is forced to sell securities at an unfavorable time and/or under unfavorable conditions, such sales may adversely affect the Fund’s ability to maintain the value of your investment at $1.00 per share.

Market Risk.  The Fund’s yield will vary as the short-term securities in its portfolio mature and the proceeds are reinvested in securities with different interest rates. For capital preservation and liquidity, the Fund may have a greater concentration in short-term securities including, but not limited to, investing up to all of its assets in overnight securities, which may result in a reduction of the Fund’s yield.

Collateral Risk.  With respect to collateral received in repurchase transactions or other investments, the Fund may have significant exposure to government agencies not secured by the full faith and credit of the United States. Such exposure, depending on market conditions, could have a negative impact on the Fund, including minimizing the value of any collateral.

Counterparty Risk.  Fund transactions involving a counterparty are subject to the risk that the counterparty or a third party will not fulfill its obligation to the Fund (“counterparty risk”). Counterparty risk may arise because of the counterparty’s financial condition (i.e., financial difficulties, bankruptcy, or insolvency), market activities and developments, or other reasons, whether foreseen or not. A counterparty’s inability to fulfill its obligation may result in significant financial loss to the Fund. The Fund may be unable to recover its investment from the counterparty or may obtain a limited recovery, and/or recovery may be delayed. The Fund may be exposed to counterparty risk through its investments in certain securities, including, but not limited to, repurchase agreements and debt securities. The Fund intends to enter into financial transactions with counterparties that Janus Capital believes to be creditworthy at the time of the transaction. There is always the risk that Janus Capital’s analysis of a counterparty’s creditworthiness is incorrect or may change due to market conditions. To the extent that the Fund focuses its transactions with a limited number of counterparties, it will have greater exposure to the risks associated with one or more counterparties.

Variable and Floating Rate Demand Notes Risk.  Variable and floating rate securities are subject to changes in value based on changes in market interest rates or changes in the issuer’s or guarantor’s creditworthiness. In addition, the absence of an active market for these securities could make it difficult for the Fund to dispose of them if the issuer defaults.

U.S. Government Securities Risk.  Certain U.S. Government securities are not guaranteed or backed by the full faith and credit of the United States. For these securities, the Fund must look principally to the agency or instrumentality issuing or guaranteeing the securities for repayment and may not be able to assert a claim against the United States if the agency or instrumentality does not meet its commitment. Such securities may involve increased risk of loss of principal and interest compared to government debt securities that are backed by the full faith and credit of the United States.

Repurchase Agreement Risk.  Repurchase agreements involve the risk that the seller will fail to repurchase the security, as agreed. In that case, the Fund will bear the risk of market value fluctuations until the security can be sold and may encounter delays and incur costs in liquidating the security.
Risk Lose Money [Text] rr_RiskLoseMoney You could lose money by investing in the Fund.
Risk Money Market Fund May Not Preserve Dollar [Text] rr_RiskMoneyMarketFundMayNotPreserveDollar Although the Fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.
RIsk Not Insured [Text] rr_RiskNotInsured An investment in the Fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Risk Money Market Fund Sponsor May Not Provide Support [Text] rr_RiskMoneyMarketFundSponsorMayNotProvideSupport Janus Capital has no legal obligation to provide financial support to the Fund, and you should not expect that Janus Capital will provide financial support to the Fund at any time.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading <b>PERFORMANCE INFORMATION </b>
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 T Shares (formerly named Class J Shares, the initial share class) of the Fund commenced operations with the Fund’s inception. The performance shown for Class T Shares is calculated using the fees and expenses of Class T Shares in effect during the periods shown, 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 shows how the Fund’s returns over different periods average out. 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 does not necessarily indicate how it will perform in the future. Updated performance information is available at janushenderson.com/performance or by calling 1-877-335-2687.
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.<br/><br/>The bar chart depicts the change in performance from year to year during the periods indicated.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone <i>1-877-335-2687</i>
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress <i>janushenderson.com/performance</i>
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture <i>The Fund’s past performance does not necessarily indicate how it will perform in the future.</i>
Bar Chart [Heading] rr_BarChartHeading <b>Annual Total Returns for Class T Shares</b> (calendar year-end)
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock Best Quarter:  1st Quarter 2008  0.74%          Worst Quarter:  4th Quarter 2014  0.00%
Year to Date Return, Label rr_YearToDateReturnLabel The Fund’s year-to-date return as of the calendar quarter ended September 30, 2018 was 0.86%.
Performance Table Heading rr_PerformanceTableHeading <b>Average Annual Total Returns </b>(periods ended 12/31/17)
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock Updated performance information is available at janushenderson.com/performance or by calling 1-877-335-2687.
Money Market Seven Day Yield, Caption [Text] rr_MoneyMarketSevenDayYieldCaption The 7-day yield on December 31, 2017 was 0.59% for Janus Henderson Government Money Market Fund.
Money Market Seven Day Yield rr_MoneyMarketSevenDayYield 0.59%
Janus Henderson Government Money Market Fund | Class T Shares | Class T  
Risk/Return: rr_RiskReturnAbstract  
Operating Expenses Column [Text] rr_OperatingExpensesColumnName Class T
Management Fees rr_ManagementFeesOverAssets 0.20%
Other Expenses rr_OtherExpensesOverAssets 0.51%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.71%
Expense Example, By Year, Column [Text] rr_ExpenseExampleByYearColumnName Class T Shares
1 Year rr_ExpenseExampleYear01 $ 73
3 Years rr_ExpenseExampleYear03 227
5 Years rr_ExpenseExampleYear05 395
10 Years rr_ExpenseExampleYear10 $ 883
2008 rr_AnnualReturn2008 1.83%
2009 rr_AnnualReturn2009 0.01%
2010 rr_AnnualReturn2010 0.02%
2011 rr_AnnualReturn2011 none
2012 rr_AnnualReturn2012 none
2013 rr_AnnualReturn2013 none
2014 rr_AnnualReturn2014 none
2015 rr_AnnualReturn2015 none
2016 rr_AnnualReturn2016 none
2017 rr_AnnualReturn2017 0.30%
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Sep. 30, 2018
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 0.86%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel <b>Best Quarter:</b>
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2008
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 0.74%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel <b>Worst Quarter:</b>
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2014
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn none
Janus Henderson Government Money Market Fund | Class T Shares | Return Before Taxes | Class T  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Return, Column Name rr_AverageAnnualReturnColumnName Class T Shares
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 0.30%
5 Years rr_AverageAnnualReturnYear05 0.06%
10 Years rr_AverageAnnualReturnYear10 0.22%
Since Inception rr_AverageAnnualReturnSinceInception 2.21%
Inception Date rr_AverageAnnualReturnInceptionDate Feb. 14, 1995