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Risk/Return: rr_RiskReturnAbstract  
Document Type dei_DocumentType 485BPOS
Document Period End Date dei_DocumentPeriodEndDate Aug. 30, 2011
Registrant Name dei_EntityRegistrantName JANUS ASPEN SERIES
Central Index Key dei_EntityCentralIndexKey 0000906185
Amendment Flag dei_AmendmentFlag false
Document Creation Date dei_DocumentCreationDate Aug. 30, 2011
Document Effective Date dei_DocumentEffectiveDate Aug. 31, 2011
Prospectus Date rr_ProspectusDate Aug. 31, 2011
Moderate Allocation Portfolio
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading PORTFOLIO SUMMARY Moderate Allocation Portfolio
Objective [Heading] rr_ObjectiveHeading INVESTMENT OBJECTIVE
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock Moderate Allocation Portfolio seeks the highest return over time consistent with an emphasis on growth of capital and income.
FEES AND EXPENSES OF THE PORTFOLIO jas906185_FundFeesAndExpenseAbstract  
Expense [Heading] rr_ExpenseHeading FEES AND EXPENSES OF THE PORTFOLIO
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 Portfolio. Owners of variable insurance contracts that invest in the Shares should refer to the variable insurance contract prospectus for a description of fees and expenses, as the following table and examples do not reflect deductions at the separate account level or contract level for any charges that may be incurred under a contract. Inclusion of these charges would increase the fees and expenses described below.
ANNUAL FUND OPERATING EXPENSES rr_OperatingExpensesAbstract  
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)
Other Expenses, New Fund, Based on Estimates [Text] rr_OtherExpensesNewFundBasedOnEstimates Since the Portfolio is new, Other Expenses are based on the estimated expenses that the Portfolio expects to incur in its initial fiscal year.
Acquired Fund Fees and Expenses, Based on Estimates [Text] rr_AcquiredFundFeesAndExpensesBasedOnEstimates Since the Portfolio is new, Acquired Fund Fees and Expenses are estimated based on the Portfolio’s initial allocations to underlying funds.
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination May 1, 2013
EXAMPLE: rr_ExpenseExampleAbstract  
Expense Example [Heading] rr_ExpenseExampleHeading EXAMPLE:
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 Portfolio with the cost of investing in other mutual funds.
Expense Example by Year [Heading] rr_ExpenseExampleByYearHeading The Example assumes that you invest $10,000 in the Portfolio 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 Portfolio’s operating expenses (including the operating expenses of the underlying funds) without waivers or recoupments (if applicable) remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Portfolio Turnover: jas906185_PortfolioTurnoverAltAbstract  
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover:
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Portfolio 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. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Portfolio’s performance.
Principal Investment Strategies jas906185_PrincipalInvestmentStrategiesAbstract  
Strategy [Heading] rr_StrategyHeading PRINCIPAL INVESTMENT STRATEGIES
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

The Portfolio seeks to achieve its investment objective by investing in other Janus mutual funds (“underlying funds”) that represent a variety of asset classes and investment styles. The Portfolio pursues this objective by investing in a diversified portfolio of underlying funds resulting in an allocation of the Portfolio’s investments that normally provides exposure of approximately 60% to stocks and 40% to bonds and money market securities. The target allocation and the allocation of the Portfolio’s assets among underlying funds are based on quantitative and qualitative analysis. Because it invests in other funds, the Portfolio is considered a “fund of funds.”

The Portfolio’s asset allocation is intended to diversify investments among stocks, bonds, and cash equivalents. The portfolio manager regularly reviews the allocation of Portfolio assets in the underlying funds and may modify the underlying funds’ weightings or substitute other underlying funds to emphasize and mitigate risk exposures that may arise as a result of the implementation of the allocations. An independent asset allocation service provides evaluations of asset allocations that the portfolio manager may use in implementing the allocations among the underlying funds. The portfolio manager continually monitors asset class allocations and periodically rebalances the Portfolio’s investments in the underlying funds. The portfolio manager may change the Portfolio’s asset class allocations, the underlying funds, an underlying fund’s asset category, or weightings among asset classes or underlying funds without prior shareholder notice.

The Portfolio will normally allocate approximately 60% of its investments to underlying funds that provide varying exposure to large U.S.-based companies, small- to mid-capitalization companies, and international companies (including emerging markets); and approximately 40% of its investments to underlying bond funds and money market instruments. Refer to Appendix A in this Prospectus for a brief description of the investment strategies of each of the currently available underlying funds.

When market conditions dictate a more defensive strategy, the Portfolio or an underlying fund may temporarily hold cash or invest its assets in temporary investments. In that case, the Portfolio may take positions that are inconsistent with its investment objective. As a result, the Portfolio may not achieve its investment objective.
Principal Investment Risks jas906185_PrincipalInvestmentRisksAbstract  
Risk [Heading] rr_RiskHeading PRINCIPAL INVESTMENT RISKS
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

The biggest risk is that the Portfolio’s returns will vary, and you could lose money. The Portfolio is designed for long-term investors seeking growth of capital and income. Investments in a portfolio with common stock exposure tend to be more volatile than many other investment choices.

Main Risks Associated with the Portfolio

Allocation Risk. The Portfolio’s ability to achieve its investment objective depends largely upon Janus Capital’s allocation of assets among the underlying funds and other securities, using the optimization process and the judgment of the portfolio manager. You could lose money on your investment in the Portfolio as a result of these allocations. The Portfolio will typically invest in a number of different underlying funds; however, to the extent that the Portfolio invests a significant portion of its assets in a single underlying fund, it will be more sensitive to the risks associated with that fund and any investments in which that fund concentrates.

Affiliated Fund Risk. Janus Capital has the authority to select and substitute underlying affiliated mutual funds. The fees paid to Janus Capital by some Janus funds are generally higher than the fees paid to Janus Capital by the Portfolio or by other funds and share classes available for investment by the Portfolio. These conditions may create a conflict of interest when selecting underlying affiliated mutual funds and share classes for investment. Janus Capital, however, is a fiduciary to the Portfolio and its shareholders and is legally obligated to act in their best interest when selecting underlying affiliated mutual funds.

Main Risks Associated with the Underlying Funds and Securities

The biggest risk is that the underlying funds’ returns will vary, and you could lose money.

Market Risk. Approximately 60% of the Portfolio’s assets are allocated to equity investments through investments in underlying funds. The value of an underlying fund’s portfolio may decrease if the value of an individual company or multiple companies in the portfolio decreases or if a portfolio manager’s belief about a company’s intrinsic worth is incorrect. Further, regardless of how well individual companies perform, the value of an underlying 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.

Fixed-Income Securities Risk. Approximately 40% of the Portfolio’s assets are allocated to fixed-income securities through investments in underlying funds. Typically, the values of fixed-income securities change inversely with prevailing interest rates. Therefore, a fundamental risk of fixed-income securities is interest rate risk, which is the risk that their value will generally decline as prevailing interest rates rise, which may cause an underlying fund’s net asset value to likewise decrease. How specific fixed-income securities may react to changes in interest rates will depend on the specific characteristics of each security. Fixed-income securities are also subject to credit risk, prepayment risk, valuation risk, and liquidity risk.

Foreign Exposure Risk. Certain underlying funds may have significant exposure to foreign markets, including emerging markets, which can be more volatile than the U.S. markets. As a result, the underlying fund’s returns and net asset value, as well as the Portfolio’s 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. A market swing in one or more countries or regions where an underlying fund has invested a significant amount of its assets may have a greater effect on the Portfolio’s performance than it would in a more geographically diversified portfolio. Investments in emerging market countries may involve risks greater than, or in addition to, the risks of investing in more developed countries.

Emerging Markets Risk. The risks of foreign investing mentioned above are heightened when investing in emerging markets. Emerging markets securities are exposed to a number of additional risks, which may result from less government supervision and regulation of business and industry practices (including the potential lack of strict finance and accounting controls and standards), stock exchanges, brokers, and listed companies, making these investments potentially more volatile in price and less liquid than investments in developed securities markets, resulting in greater risk to investors. In addition, an underlying fund’s investments may be denominated in foreign currencies and therefore, changes in the value of a country’s currency compared to the U.S. dollar may affect the value of an underlying fund’s investments. To the extent that an underlying fund invests a significant portion of its assets in the securities of issuers in or companies of a single country or region, it is more likely to be impacted by events or conditions affecting that country or region which could have a negative impact on the Portfolio’s performance. Some of the risks of investing directly in foreign and emerging market securities may be reduced when an underlying fund invests indirectly in foreign securities through various other investment vehicles including derivatives, which also involve specialized risks.

Mortgage-Backed Securities Risk. Mortgage-backed securities tend to be more sensitive to changes in interest rates than other types of securities. Investments in mortgage-backed securities are subject to both extension risk, where borrowers extend the duration of their mortgages in times of rising interest rates, and prepayment risk, where borrowers pay off their mortgages sooner than expected in times of declining interest rates. These risks may reduce the Portfolio’s returns. In addition, investments in mortgage-backed securities, including those comprised of subprime mortgages, may be subject to a higher degree of credit risk, valuation risk, and liquidity risk than various other types of fixed-income securities.

Exchange-Traded Funds Risk. Certain underlying funds may purchase shares of exchange-traded funds (“ETFs”) to gain exposure to a particular portion of the market. ETFs are pooled investment vehicles, which may be managed or unmanaged, that generally seek to track the performance of a specific index. ETFs are traded on an exchange at market prices that may vary from the net asset value of their underlying investments. When an underlying fund invests in an ETF, in addition to directly bearing the expenses associated with its own operations, it will bear a pro rata portion of the ETF’s expenses. ETFs have certain inherent risks generally associated with investments in a portfolio of common stocks, including the risk that the general level of stock prices may decline, thereby adversely affecting the value of each unit of the ETF. ETFs also involve the risk that an active trading market for an ETF’s shares may not develop or be maintained.

Exchange-Traded Notes Risk. Certain underlying funds may invest in exchange-traded notes (“ETNs”), which are debt securities whose returns are linked to a particular index. ETNs are typically linked to the performance of a commodities index that reflects the potential return on unleveraged investments in futures contracts of physical commodities, plus a specified rate of interest that could be earned on cash collateral. ETNs are subject to credit risk. The value of an ETN may vary and may be influenced by time to maturity, level of supply and demand for the ETN, volatility and lack of liquidity in underlying commodities markets, changes in the applicable interest rates, changes in the issuer’s credit rating, and economic, legal, political, or geographic events that affect the referenced commodity. When an underlying fund invests in ETNs it will bear its proportionate share of any fees and expenses borne by the ETN. There may be restrictions on an underlying fund’s right to redeem its investment in an ETN, which is meant to be held until maturity. An underlying fund’s decision to sell its ETN holdings may be limited by the availability of a secondary market.

Derivatives Risk. Certain underlying funds may invest in derivatives. 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. Certain underlying funds may seek to earn additional income through lending their 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 underlying fund may experience delays and costs in recovering the security or gaining access to the collateral provided to collateralize the loan. If the underlying fund is unable to recover a security on loan, the underlying 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 underlying fund.

An investment in the Portfolio 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 Portfolio’s returns will vary, and you could lose money.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the Portfolio is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
PERFORMANCE INFORMATION jas906185_FundPastPerformanceAbstract  
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading PERFORMANCE INFORMATION
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The Portfolio does not have a full calendar year of operations. Performance information for certain periods will be included in the Portfolio’s first annual and/or semiannual report. Performance information for each underlying fund is available in its prospectus.
Performance One Year or Less [Text] rr_PerformanceOneYearOrLess The Portfolio does not have a full calendar year of operations.
Moderate Allocation Portfolio | Institutional Shares
  
ANNUAL FUND OPERATING EXPENSES rr_OperatingExpensesAbstract  
Management Fees rr_ManagementFeesOverAssets 0.05%
Other Expenses rr_OtherExpensesOverAssets 0.77% [1]
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.73% [2]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.55% [3]
Fee Waiver rr_FeeWaiverOrReimbursementOverAssets 0.43% [3]
Net Annual Fund Operating Expenses After Fee Waiver rr_NetExpensesOverAssets 1.12% [3]
EXAMPLE: rr_ExpenseExampleAbstract  
Expense Example, By Year, Column [Text] rr_ExpenseExampleByYearColumnName Institutional Shares
1 Year rr_ExpenseExampleYear01 158
3 Years rr_ExpenseExampleYear03 490
[1] Since the Portfolio is new, Other Expenses are based on the estimated expenses that the Portfolio expects to incur in its initial fiscal year.
[2] Since the Portfolio is new, Acquired Fund Fees and Expenses are estimated based on the Portfolio's initial allocations to underlying funds.
[3] Janus Capital has contractually agreed to waive the Portfolio's total annual fund operating expenses (excluding any expenses of an underlying fund (acquired fund fees and expenses), brokerage commissions, interest, dividends, taxes, and extraordinary expenses) to 0.39% until at least May 1, 2013. The contractual waiver may be terminated or modified prior to this date only at the discretion of the Board of Trustees. For a period of three years subsequent to the Portfolio's commencement of operations, Janus Capital may recover from the Portfolio fees and expenses previously waived or reimbursed if the Portfolio's expense ratio, including recovered expenses, falls below the expense limit.