Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	JANUS INVESTMENT FUND
Prospectus Date	rr_ProspectusDate	Jan. 28, 2015
Janus Contrarian Fund | Class D Shares
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	FUND SUMMARY Janus Contrarian Fund
Objective [Heading]	rr_ObjectiveHeading	INVESTMENT OBJECTIVE
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Janus Contrarian 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 61% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	61.00%
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, under normal circumstances, at least 80% of its net assets in equity securities with the potential for long-term growth of capital. The portfolio manager emphasizes investments in companies with attractive price/free cash flow, which is the relationship between the price of a stock and the company’s available cash from operations, minus capital expenditures. The portfolio manager will typically seek attractively valued companies that are improving their free cash flow and returns on invested capital. Such companies may also include special situations companies that are experiencing management changes and/or are currently out of favor. The Fund may also invest in foreign securities, which may include investments in emerging markets. The portfolio manager applies a “bottom up” approach in choosing investments. In other words, the portfolio manager looks 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 invest in equity and debt securities of real estate-related companies. Such companies may include those in the real estate industry or real estate-related industries. These securities may include common stocks, preferred stocks, and other equity securities, including, but not limited to, mortgage-backed securities, real estate-backed securities, securities of real estate investment trusts (“REITs”) and similar REIT-like entities. A REIT is a trust that invests in real estate-related projects, such as properties, mortgage loans, and construction loans. REITs are generally categorized as equity, mortgage, or hybrid REITs. A REIT may be listed on an exchange or traded over-the-counter. The Fund may also take long or short positions in derivatives, which are instruments that have a value derived from, or directly linked to, an underlying asset, such as equity securities, fixed-income securities, commodities, currencies, interest rates, or market indices. For purposes of meeting its 80% investment policy, the Fund may include derivatives that have characteristics similar to the securities in which the Fund may directly invest. The types of derivatives in which the Fund may invest include options, futures, swaps, warrants, and forward exchange contracts. The Fund may use derivatives to hedge, to earn income or enhance returns, as a substitute for securities in which the Fund invests, to increase or decrease the Fund’s exposure to a particular market, to adjust the Fund’s currency exposure relative to its benchmark index, to gain access to foreign markets where direct investment may be restricted or unavailable, or to manage the Fund’s risk profile. 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. Value Investing Risk. Because different types of stocks tend to shift in and out of favor depending on market and economic conditions, “value” stocks may perform differently than other types of stocks and from the market as a whole, and can continue to be undervalued by the market for long periods of time. It is also possible that a value stock will never appreciate to the extent expected by the portfolio manager. 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 manager’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. Market risk may affect a single issuer, industry, economic sector, or the market as a whole. 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 portfolio manager’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. Special Situations Risk. The Fund may invest in companies that demonstrate special situations or turnarounds, meaning companies that have experienced significant business problems but are believed to have favorable prospects for recovery. Special situations may include significant changes in a company’s allocation of its existing capital, a restructuring of assets, or a redirection of free cash flow. Special situations may also result from: (i) significant changes in industry structure through regulatory developments or shifts in competition; (ii) a new or improved product, service, operation, or technological advance; (iii) changes in senior management or other extraordinary corporate event; (iv) differences in market supply of and demand for the security; or (v) significant changes in cost structure. Investments in “special situations” companies can present greater risks than investments in companies not experiencing special situations, and the Fund’s performance could be adversely impacted if the securities selected decline in value or fail to appreciate in value. Nondiversification Risk. The Fund is classified as nondiversified under the Investment Company Act of 1940, as amended. This gives the Fund’s portfolio manager 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. 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 investment can be substantially greater than the derivative’s original cost, and can therefore involve leverage. Leverage may cause the Fund to be more volatile than if it had not used leverage. Derivatives can be complex instruments and may involve analysis that differs from that required for other investment types used by the Fund. If the value of a derivative does not correlate well with the particular market or other asset class to which the derivative is intended to provide exposure, the derivative may not produce the anticipated result. Derivatives can also reduce the opportunity for gain or result in losses by offsetting positive returns in other investments. Derivatives can be less liquid than other types of investments and entail the risk that the counterparty will default on its payment obligations. If the counterparty to a derivative transaction defaults, the Fund would risk the loss of the net amount of the payments that it contractually is entitled to receive. To the extent the Fund enters into short derivative positions, the Fund may be exposed to risks similar to those associated with short sales, including the risk that the Fund’s losses are theoretically unlimited. Real Estate Securities Risk. The Fund’s performance may be affected by the risks associated with investments in real estate-related companies. The value of real estate-related companies’ securities is sensitive to changes in real estate values and rental income, property taxes, interest rates, tax and regulatory requirements, supply and demand, and the management skill and creditworthiness of the company. Investments in REITs involve the same risks as other real estate investments. In addition, a REIT could fail to qualify for tax-free pass-through of its income under the Internal Revenue Code or fail to maintain its exemption from registration under the Investment Company Act of 1940, as amended, which could produce adverse economic consequences for the REIT and its investors, including the Fund. 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 manager 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 February 16, 2010, as a result of the restructuring of Class J Shares, the predecessor share class. The performance shown for Class D Shares for periods prior to February 16, 2010, reflects the performance of the Fund’s former Class J Shares, calculated using the fees and expenses in effect during the periods shown, net of any applicable fee and expense limitations or waivers. If Class D Shares of the Fund had been available during periods prior to February 16, 2010, 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 a broad-based securities market index. The index is not actively managed and is not available for direct investment. 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 janus.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 a broad-based securities market index. The index is not actively managed and is not available for direct investment. 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	janus.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  30.97%          Worst Quarter:  4th Quarter 2008  −27.53%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns (periods ended 12/31/14)
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 Narrative	rr_PerformanceTableNarrativeTextBlock	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. This index is used to calculate the performance fee adjustment. After-tax returns are calculated using distributions for the Fund’s Class D Shares for the periods following February 16, 2010; and for the Fund’s Class J Shares, the predecessor share class, for the periods prior to February 16, 2010. 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 janus.com/allfunds or by calling 1-800-525-3713.
Janus Contrarian 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	0.61%
Other Expenses	rr_OtherExpensesOverAssets	0.19%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.80%
Expense Example, By Year, Column [Text]	rr_ExpenseExampleByYearColumnName	Class D Shares
1 Year	rr_ExpenseExampleYear01	82
3 Years	rr_ExpenseExampleYear03	255
5 Years	rr_ExpenseExampleYear05	444
10 Years	rr_ExpenseExampleYear10	990
2005	rr_AnnualReturn2005	16.02%
2006	rr_AnnualReturn2006	24.58%
2007	rr_AnnualReturn2007	21.22%
2008	rr_AnnualReturn2008	(48.11%)
2009	rr_AnnualReturn2009	37.02%
2010	rr_AnnualReturn2010	11.25%
2011	rr_AnnualReturn2011	(15.78%)
2012	rr_AnnualReturn2012	23.97%
2013	rr_AnnualReturn2013	38.56%
2014	rr_AnnualReturn2014	17.46%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	30.97%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(27.53%)
Janus Contrarian 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	17.46%
5 Years	rr_AverageAnnualReturnYear05	13.58%
10 Years	rr_AverageAnnualReturnYear10	8.94%
Since Inception	rr_AverageAnnualReturnSinceInception	8.18%
Inception Date	rr_AverageAnnualReturnInceptionDate	Feb. 29, 2000
Janus Contrarian 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	14.53%
5 Years	rr_AverageAnnualReturnYear05	12.95%
10 Years	rr_AverageAnnualReturnYear10	8.28%
Since Inception	rr_AverageAnnualReturnSinceInception	7.65%
Inception Date	rr_AverageAnnualReturnInceptionDate	Feb. 29, 2000
Janus Contrarian 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
1 Year	rr_AverageAnnualReturnYear01	11.89%
5 Years	rr_AverageAnnualReturnYear05	10.85%
10 Years	rr_AverageAnnualReturnYear10	7.31%
Since Inception	rr_AverageAnnualReturnSinceInception	6.80%
Inception Date	rr_AverageAnnualReturnInceptionDate	Feb. 29, 2000
Janus Contrarian 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	13.69%
5 Years	rr_AverageAnnualReturnYear05	15.45%
10 Years	rr_AverageAnnualReturnYear10	7.67%
Since Inception	rr_AverageAnnualReturnSinceInception	4.79%
Inception Date	rr_AverageAnnualReturnInceptionDate	Feb. 29, 2000