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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName JANUS INVESTMENT FUND
Prospectus Date rr_ProspectusDate May 31, 2012
Janus Research Fund | Class N Shares
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Fund summary

Janus Research Fund
Objective [Heading] rr_ObjectiveHeading INVESTMENT OBJECTIVE
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock Janus Research 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)
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 88% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 88.00%
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 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 PRINCIPAL INVESTMENT STRATEGIES
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The Fund pursues its investment objective by investing primarily in 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 equity and debt securities, which may include investments in emerging markets.

Janus Capital’s equity research analysts (the “Research Team”) select investments for the Fund. The Research Team, comprised of sector specialists, conducts fundamental analysis with a focus on “bottom up” research, quantitative modeling, and valuation analysis. Using this research process, analysts rate their stocks based upon attractiveness. Analysts bring their high-conviction ideas to their respective sector teams. Sector teams compare the appreciation and risk potential of each of the team’s high-conviction ideas and construct a sector portfolio that is intended to maximize the best risk-reward opportunities.
Although the Research Team may find high-conviction investment ideas anywhere in the world, the Research Team may emphasize investments in securities of U.S.-based issuers.

Positions may be sold when, among other things, there is no longer high conviction in the return potential of the investment or if the risk characteristics have caused a re-evaluation of the opportunity. This may occur if the stock has appreciated and reflects the anticipated value, if another company represents a better risk-reward opportunity, or if the investment’s fundamental characteristics deteriorate. Securities may also be sold from the portfolio to rebalance sector weightings.

Janus Capital’s Director of Equity Research oversees the investment process and is responsible for the day-to-day management of the Fund. It is expected that the Fund will be broadly diversified among a variety of industry sectors. The Fund intends to be fully invested under normal circumstances. However, under unusual circumstances, if the Research Team does not have high conviction in enough investment opportunities, the Fund’s uninvested assets may be held in cash or similar instruments.

The Fund may invest its assets in derivatives, which are instruments that have a value derived from or directly linked to an underlying asset, such as equity securities, bonds, commodities, currencies, interest rates, or market indices, as substitutes for securities in which the Fund invests. The Fund may invest in derivative instruments (by taking long and/or short positions) for different purposes, including hedging (to offset risks associated with an investment, currency exposure, or market conditions) and to earn income and enhance returns. For more information on the Fund’s use of derivatives, refer to the Fund’s shareholder reports and Form N-Q reports, which are filed with the Securities and Exchange Commission.

The Fund may lend portfolio securities on a short-term or long-term basis, in an amount equal to up to 1/3 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 investment personnel’s 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.

Growth Securities Risk. The Fund invests in companies after assessing their growth potential. Securities of companies perceived to be “growth” companies may be more volatile than other stocks and may involve special risks. If the investment personnel’s 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. The Fund compares and broadly matches its sector weights to those of a growth-based index. If growth stocks are out of favor, sectors that are larger in a growth index may underperform, leading to Fund underperformance relative to indices less biased toward growth stocks.

Foreign Exposure Risk. The Fund may have significant exposure to foreign markets, including 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. 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.

Derivatives Risk. Derivatives can be highly volatile and involve risks in addition to the risks of the underlying referenced securities. Gains or losses from a derivative can be substantially greater than the derivative’s original cost, and can therefore involve leverage. Derivatives can be less liquid than other types of investments and entail the risk that the counterparty will default on its payment obligations.

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.

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 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 N Shares of the Fund commenced operations on May 31, 2012. The performance shown for Class N Shares reflects the performance of the Fund’s Class T Shares, calculated using the fees and expenses of Class T Shares, net of any applicable fee and expense limitations or waivers. If Class N Shares of the Fund had been available during periods prior to May 31, 2012, the performance shown may have been different.

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. The indices are not actively managed and are not available for direct investment. All figures assume reinvestment of dividends and distributions.

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 janus.com/advisor/mutual-funds 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.

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. The indices are not actively managed and are not available for direct investment. All figures assume reinvestment of dividends and distributions.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 1-877-335-2687
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress janus.com/advisor/mutual-funds
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 N Shares (calendar year-end)
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
Best Quarter: Second Quarter 2009 22.23%        Worst Quarter: Fourth Quarter 2008 −25.30%
Year to Date Return, Label rr_YearToDateReturnLabel The Fund’s year-to-date return as of the calendar quarter ended March 31, 2012 was 15.27%.
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns (periods ended 12/31/11)
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 After-tax returns are calculated using distributions for the Fund’s Class T Shares for the periods prior to December 31, 2011. If Class N 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 janus.com/advisor/mutual-funds or by calling 1-877-335-2687.
Janus Research Fund | Class N Shares | Class N
  
Risk/Return: rr_RiskReturnAbstract  
Operating Expenses Column [Text] rr_OperatingExpensesColumnName Class N
Management Fees (may adjust up or down) rr_ManagementFeesOverAssets 0.57%
Other Expenses rr_OtherExpensesOverAssets 0.05% [1]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.62%
Expense Example, By Year, Column [Text] rr_ExpenseExampleByYearColumnName Class N Shares
1 Year rr_ExpenseExampleYear01 63
3 Years rr_ExpenseExampleYear03 199
5 Years rr_ExpenseExampleYear05 346
10 Years rr_ExpenseExampleYear10 774
2002 rr_AnnualReturn2002 (29.00%)
2003 rr_AnnualReturn2003 32.11%
2004 rr_AnnualReturn2004 10.77%
2005 rr_AnnualReturn2005 6.82%
2006 rr_AnnualReturn2006 8.65%
2007 rr_AnnualReturn2007 24.52%
2008 rr_AnnualReturn2008 (44.36%)
2009 rr_AnnualReturn2009 43.01%
2010 rr_AnnualReturn2010 20.97%
2011 rr_AnnualReturn2011 (3.93%)
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Mar. 31, 2012
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 15.27%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 22.23%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (25.30%)
Janus Research Fund | Class N Shares | Return Before Taxes | Class N
  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName
Class N Shares
Label rr_AverageAnnualReturnLabel
Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 (3.93%)
5 Years rr_AverageAnnualReturnYear05 2.86%
10 Years rr_AverageAnnualReturnYear10 3.34%
Since Inception rr_AverageAnnualReturnSinceInception 9.76%
Inception Date rr_AverageAnnualReturnInceptionDate May 03, 1993
Janus Research Fund | Class N Shares | Return After Taxes on Distributions | Class N
  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName
Class N Shares
Label rr_AverageAnnualReturnLabel
Return After Taxes on Distributions
1 Year rr_AverageAnnualReturnYear01 (4.01%)
5 Years rr_AverageAnnualReturnYear05 2.79%
10 Years rr_AverageAnnualReturnYear10 3.29%
Since Inception rr_AverageAnnualReturnSinceInception 8.49%
Inception Date rr_AverageAnnualReturnInceptionDate May 03, 1993
Janus Research Fund | Class N Shares | Return After Taxes on Distributions and Sale of Fund Shares | Class N
  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName
Class N Shares
Label rr_AverageAnnualReturnLabel
Return After Taxes on Distributions and Sale of Fund Shares
 [2]
1 Year rr_AverageAnnualReturnYear01 (2.44%)
5 Years rr_AverageAnnualReturnYear05 2.44%
10 Years rr_AverageAnnualReturnYear10 2.89%
Since Inception rr_AverageAnnualReturnSinceInception 8.09%
Inception Date rr_AverageAnnualReturnInceptionDate May 03, 1993
Janus Research Fund | Class N 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 2.64%
5 Years rr_AverageAnnualReturnYear05 2.50%
10 Years rr_AverageAnnualReturnYear10 2.60%
Since Inception rr_AverageAnnualReturnSinceInception 7.06%
Inception Date rr_AverageAnnualReturnInceptionDate May 03, 1993
Janus Research Fund | Class N 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 2.11%
5 Years rr_AverageAnnualReturnYear05 (0.25%)
10 Years rr_AverageAnnualReturnYear10 2.92%
Since Inception rr_AverageAnnualReturnSinceInception 7.83%
Inception Date rr_AverageAnnualReturnInceptionDate May 03, 1993
[1] Other Expenses are based on the estimated expenses that the Shares expect to incur during their initial fiscal period.
[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.