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Risk/Return: rr_RiskReturnAbstract  
Document Type dei_DocumentType 485BPOS
Document Period End Date dei_DocumentPeriodEndDate May 10, 2011
Registrant Name dei_EntityRegistrantName FIRST TRUST EXCHANGE-TRADED ALPHADEX FUND
Central Index Key dei_EntityCentralIndexKey 0001383496
Amendment Flag dei_AmendmentFlag false
Document Creation Date dei_DocumentCreationDate May 10, 2011
Document Effective Date dei_DocumentEffectiveDate May 10, 2011
Prospectus Date rr_ProspectusDate May 09, 2011
First Trust Mega Cap AlphaDEX Fund
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading SUMMARY INFORMATION
Objective [Heading] rr_ObjectiveHeading INVESTMENT OBJECTIVE
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The First Trust Mega Cap AlphaDEX(R) Fund (the "Fund") seeks investment results that correspond generally to the price and yield (before the Fund's fees and expenses) of an equity index called the Defined Mega Cap Index (the "Index").
FEES AND EXPENSES OF THE FUND ftetaf_FundFeesAndExpensesAbstract  
Expense [Heading] rr_ExpenseHeading FEES AND EXPENSES OF THE FUND
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock The following table describes the fees and expenses you may pay if you buy and hold Shares of the Fund. Investors purchasing and selling Shares may be subject to costs (including customary brokerage commissions) charged by their broker.
SHAREHOLDER FEES rr_ShareholderFeesAbstract  
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption SHAREHOLDER FEES (fees paid directly from your investment)
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
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)
Management Fees rr_ManagementFeesOverAssets 0.70%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none [1]
Other Expenses rr_OtherExpensesOverAssets none
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.70%
EXAMPLE rr_ExpenseExampleAbstract  
Expense Example [Heading] rr_ExpenseExampleHeading EXAMPLE
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

The example below is intended to help you compare the cost of investing in the Fund with the cost of investing in other funds. This example does not take into account customary brokerage commissions that you pay when purchasing or selling Shares of the Fund on NYSE Arca.

The example assumes that you invest $10,000 in the Fund for the time periods indicated and then you retain the Shares or redeem all of your Shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund's annual operating expenses remain at current levels until May 6, 2012. Additionally, the example assumes that the Fund imposes a 12b-1 fee of 0.25% per annum of the Fund's average daily net assets following May 6, 2012. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
1 YEAR rr_ExpenseExampleYear01 72
3 YEARS rr_ExpenseExampleYear03 257
PORTFOLIO TURNOVER ftetaf_PortfolioTurnoverAltAbstract  
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.
Strategy [Heading] rr_StrategyHeading PRINCIPAL INVESTMENT STRATEGIES
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

The Fund will normally invest at least 90% of its net assets plus the amount of any borrowings for investment purposes in common stocks that comprise the Index. The Fund, using an "indexing" investment approach, attempts to replicate, before fees and expenses, the performance of the Index. First Trust seeks a correlation of 0.95 or better (before fees and expenses) between the Fund's performance and the performance of the Index; a figure of 1.00 would represent perfect correlation. First Trust will regularly monitor the Fund's tracking accuracy and will seek to maintain an appropriate correlation.

The Index is in the "Defined Index Series," a family of custom enhanced indices developed, maintained and sponsored by Standard & Poor's Financial Services LLC ("S&P" or the "Index Provider"). The Index is a modified equal-dollar weighted index designed by S&P to objectively identify and select stocks from the S&P BMI US Index that may generate positive alpha relative to traditional passive-style indices through the use of the AlphaDEX(R) screening methodology. Alpha is an indication of how much an investment outperforms or underperforms on a risk-adjusted basis relative to its benchmark. The securities of companies represented in the Index have market capitalizations that are consistent with the name of the Index. As of April 29, 2011, the Index was comprised of 50 securities and the market capitalization range represented in the index was approximately $21.7 billion to $433.4 billion. The Index is rebalanced and reconstituted as of the last business day of each calendar quarter. Changes to the Index will be effective at the open of trading on the sixth business day of the following month.

The Fund intends to invest entirely in the Index, however, there may also be instances in which the Fund may be underweighted or overweighted in certain securities in the Index, not invest in certain securities included in the Index, purchase securities not in the Index that are appropriate to substitute for certain securities in the Index or utilize various combinations of the above techniques in seeking to track the Index
Strategy Portfolio Concentration [Text] rr_StrategyPortfolioConcentration The Fund will normally invest at least 90% of its net assets plus the amount of any borrowings for investment purposes in common stocks that comprise the Index.
Risk [Heading] rr_RiskHeading PRINCIPAL RISKS
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

You could lose money by investing in the Fund. An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.

MARKET RISK. Market risk is the risk that a particular stock owned by the Fund, Shares of the Fund or stocks in general may fall in value. Shares are subject to market fluctuations caused by such factors as economic, political, regulatory or market developments, changes in interest rates and perceived trends in stock prices. Overall stock values could decline generally or could underperform other investments.

NON-CORRELATION RISK. The Fund's return may not match the return of the Index for a number of reasons. For example, the Fund incurs operating expenses not applicable to the Index, and may incur costs in buying and selling securities, especially when rebalancing the Fund's portfolio holdings to reflect changes in the composition of the Index. In addition, the Fund's portfolio holdings may not exactly replicate the securities included in the Index or the ratios between the securities included in the Index.

REPLICATION MANAGEMENT RISK. The Fund is exposed to additional market risk due to its policy of investing principally in the securities included in the Index. As a result of this policy, securities held by the Fund will generally not be bought or sold in response to market fluctuations and the securities may be issued by companies concentrated in a particular industry. Therefore, the Fund will generally not sell a stock because the stock's issuer is in financial trouble, unless that stock is removed or is anticipated to be removed from the Index.

NON-DIVERSIFICATION RISK. The Fund is classified as "non-diversified" under the Investment Company Act of 1940, as amended (the "1940 Act"). As a result, the Fund is only limited as to the percentage of its assets which may be invested in the securities of any one issuer by the diversification requirements imposed by the Internal Revenue Code of 1986, as amended (the "Code"). The Fund may invest a relatively high percentage of its assets in a limited number of issuers. As a result, the Fund may be more susceptible to a single adverse economic or regulatory occurrence affecting one or more of these issuers, experience increased volatility and be highly concentrated in certain issues.

MARKET CAPITALIZATION RISK. The securities of companies represented in the Index have market capitalizations that are consistent with the name of the Index. For purposes of determining the market capitalization range of such securities, the Fund will use the current range of the Index. However, the Fund will not be forced to sell a stock because the stock has exceeded or fallen below the current market capitalization range of the Index. Because of market movement, there can be no assurance that the securities in the Fund will stay within a given market capitalization range. As a result, the Fund may be exposed to additional risk or may not give investors the opportunity to invest fully in a given market capitalization range.

ENERGY COMPANIES RISK. The Fund invests in energy companies. Energy companies include integrated oil companies that are involved in the exploration, production and refining process, gas distributors and pipeline-related companies and other energy companies involved with mining, producing and delivering energy-related services and drilling. General problems of energy companies include volatile fluctuations in price and supply of energy fuels, international politics, terrorist attacks, reduced demand as a result of increases in energy efficiency and energy conservation, the success of exploration projects, clean-up and litigation costs relating to oil spills and environmental damage, and tax and other regulatory policies of various governments. Natural disasters such as hurricanes in the Gulf of Mexico will also impact the petroleum industry. Oil production and refining companies are subject to extensive federal, state and local environmental laws and regulations regarding air emissions and the disposal of hazardous materials. In addition, recently oil prices have been extremely volatile.

INDEX TRACKING RISK. You should anticipate that the value of Fund Shares will decline, more or less, in correlation with any decline in the value of the Fund's Index.
Risk Lose Money [Text] rr_RiskLoseMoney You could lose money by investing in the Fund.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.
Risk Nondiversified Status [Text] rr_RiskNondiversifiedStatus NON-DIVERSIFICATION RISK. The Fund is classified as "non-diversified" under the Investment Company Act of 1940, as amended (the "1940 Act"). As a result, the Fund is only limited as to the percentage of its assets which may be invested in the securities of any one issuer by the diversification requirements imposed by the Internal Revenue Code of 1986, as amended (the "Code"). The Fund may invest a relatively high percentage of its assets in a limited number of issuers. As a result, the Fund may be more susceptible to a single adverse economic or regulatory occurrence affecting one or more of these issuers, experience increased volatility and be highly concentrated in certain issues.
PERFORMANCE ftetaf_FundPastPerformanceAbstract  
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading PERFORMANCE
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The Fund has not yet commenced operations and, therefore, does not have a performance history. Once available, the Fund's performance information and information that gives some indication of the risks of an investment in the Fund by comparing the Fund's performance with a broad measure of market performance will be available on the Fund's website at www.ftportfolios.com.
Performance One Year or Less [Text] rr_PerformanceOneYearOrLess The Fund has not yet commenced operations and, therefore, does not have a performance history.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.ftportfolios.com
[1] Although the Fund has adopted a 12b-1 plan that permits it to pay up to 0.25% per annum, it will not pay 12b-1 fees at any time before May 6, 2012.