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Label Element Value
Risk Return Abstract rr_RiskReturnAbstract  
Document Type dei_DocumentType 497
Document Period End Date dei_DocumentPeriodEndDate Apr. 08, 2016
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 Apr. 08, 2016
Document Effective Date dei_DocumentEffectiveDate May 05, 2016
Prospectus Date rr_ProspectusDate Apr. 08, 2016
First Trust Mid Cap Growth AlphaDEX(R) Fund  
Risk Return Abstract rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading

SUMMARY INFORMATION
Objective [Heading] rr_ObjectiveHeading

INVESTMENT OBJECTIVE
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The First Trust Mid Cap Growth 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 Nasdaq AlphaDEX(R) Mid Cap Growth Index (the "Index").
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, which are not reflected in the table below.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption

SHAREHOLDER FEES (fees paid directly from your investment)
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 159% of the average value of its portfolio.
Expense Example [Heading] rr_ExpenseExampleHeading

EXAMPLE
Expense Example, No Redemption Narrative [Text Block] rr_ExpenseExampleNoRedemptionNarrativeTextBlock

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 in the secondary market.

The example assumes that you invest $10,000 in the Fund for the time periods indicated. The example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain at current levels. 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 April 7, 2017. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
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 (including investment borrowings) 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. The Fund's investment advisor 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. The Index is owned and is developed, maintained and sponsored by Nasdaq, Inc. (the "Index Provider").

The Index is designed to select growth stocks from the NASDAQ US 600 Mid Cap Index (the "Base Index") that may generate positive alpha, or risk-adjusted returns, relative to traditional indices through the use of the AlphaDEX(R) selection methodology. Alpha is an indication of how much an investment outperforms or underperforms on a risk-adjusted basis relative to its benchmark. The Index attempts to generate positive alpha by identifying stocks based on three factors: value, momentum and size.

The Base Index is a comprehensive, rules-based index designed to measure stock market performance of mid cap US companies, as determined by the Index Provider. The Fund will be concentrated in an industry or a group of industries to the extent that the Index is so concentrated.

Security selection for the Index will be conducted in the following manner:

1.The selection universe for the Index begins with all stocks in the Base Index.
2.The Index Provider then removes any stocks which do not trade on an eligible exchange; duplicate (multiple share classes) stocks; and stocks which do not meet the Index Provider's liquidity screens.
3.The remaining stocks in the universe are then ranked on both growth and value factors. Each stock is classified as either a value stock or a growth stock, as determined by the Index Provider. A stock classified as a value stock will receive its value rank as its "selection score" and a stock classified as a growth stock will receive its growth rank as its "selection score." Only those stocks designated as growth stocks are eligible for the portfolio.
4.The top 225 stocks based on the selection score determined in step 3 comprise the "selected stocks." The selected stocks are then split into quintiles based on their selection score, with higher scoring quintiles receiving a greater weight in the Index.

The Index is reconstituted and rebalanced quarterly and the Fund will make corresponding changes to its portfolio shortly after the Index changes are made public. The inception date of the Index was January 11, 2016. As of February 29, 2016, the Index was composed of 225 securities. See "Index Information" for additional information.

The Fund's strategy includes a quarterly portfolio rebalance and reconstitution which may result in a high rate of turnover. As of February 29, 2016, the Fund had significant investments in financial companies. The securities of companies represented in the Index generally have market capitalizations that are consistent with the name of the Index. To determine the market capitalization range of such securities, the Fund uses the current range of the Index. However, the Fund will not sell a security because the security has exceeded or fallen below the current market capitalization range of the Index. As of February 29, 2016, the market capitalization range of securities in the Index was $1 billion to $8 billion.
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. There can be no assurance that the Fund's investment objective will be achieved.

EQUITY SECURITIES RISK. Because the Fund invests in equity securities, the value of the Fund's shares will fluctuate with changes in the value of these equity securities. Equity securities prices fluctuate for several reasons, including changes in investors' perceptions of the financial condition of an issuer or the general condition of the relevant stock market, such as market volatility, or when political or economic events affecting the issuers occur. In addition, common stock prices may be particularly sensitive to rising interest rates, as the cost of capital rises and borrowing costs increase.

FINANCIAL COMPANIES RISK. Financial companies are especially subject to the adverse effects of economic recession, currency exchange rates, government regulation, decreases in the availability of capital, volatile interest rates, portfolio concentrations in geographic markets and in commercial and residential real estate loans, and competition from new entrants in their fields of business.

GROWTH STOCKS INVESTMENT RISK. Growth stocks tend to be more volatile than certain other types of stocks and their prices usually fluctuate more dramatically than the overall stock market. A stock with growth characteristics can have sharp price declines due to decreases in current or expected earnings.

INDEX CONSTITUENT RISK. The Fund may be a constituent of one or more indices. As a result, the Fund may be included in one or more index-tracking ETFs or mutual funds. Being a component security of such a vehicle could greatly affect the trading activity involving the Fund, the size of the Fund and the market volatility of the Fund. Inclusion in an index could significantly increase demand for the Fund and removal from an index could result in outsized selling activity in a relatively short period of time. As a result, the Fund's net asset value could be negatively impacted and the Fund's market price may be significantly below the Fund's net asset value during certain periods. In addition, index rebalances may potentially result in increased trading activity. To the extent buying or selling activity increases, the Fund can be exposed to increased brokerage costs and adverse tax consequences and the market price of the Fund can be negatively affected.

MARKET CAPITALIZATION RISK. 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.

MARKET RISK. Market risk is the risk that a particular security owned by the Fund or shares of the Fund in general may fall in value. Securities are subject to market fluctuations caused by such factors as economic, political, regulatory or market developments, changes in interest rates and perceived trends in securities prices. Shares of the Fund could decline in value or 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.

PORTFOLIO TURNOVER RISK. High portfolio turnover may result in the Fund paying higher levels of transaction costs and generating greater tax liabilities for shareholders. Portfolio turnover risk may cause the Fund's performance to be less than you expect.

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.

SMALLER COMPANIES RISK. Mid-capitalization companies may be more vulnerable to adverse general market or economic developments, and their securities may be less liquid and may experience greater price volatility than larger, more established companies as a result of several factors, including limited trading volumes, products or financial resources, management inexperience and less publicly available information. Accordingly, such companies are generally subject to greater market risk than larger, more established companies.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading

ANNUAL TOTAL RETURN
Bar Chart [Heading] rr_BarChartHeading

FIRST TRUST MID CAP GROWTH ALPHADEX(R) FUND--TOTAL RETURNS[1]
Bar Chart Narrative [Text Block] rr_BarChartNarrativeTextBlock

The bar chart and table below illustrate the annual calendar year returns of the Fund based on net asset value as well as the average annual Fund and Index returns. The bar chart and table provide an indication of the risks of investing in the Fund by showing changes in the Fund's performance from year-to-year and by showing how the Fund's average annual total returns based on net asset value compare to those of the Index and a broad-based market index. On April 8, 2016, the Fund's underlying index changed from the Defined Mid Cap Growth Index to the Nasdaq AlphaDEX(R) Mid Cap Growth Index. Therefore, the Fund's performance and historical returns shown below are not necessarily indicative of the performance that the Fund, based on the Index, would have generated. Returns for an underlying index are only disclosed for those periods in which the index was in existence for the whole period. Because the Fund's new underlying index had an inception date of January 11, 2016, it was not in existence for any of the periods disclosed. The new Index is substantially similar to the Defined Mid Cap Growth Index. See "Total Return Information" for additional performance information regarding the Fund. The Fund's performance information is accessible on the Fund's website at www.ftportfolios.com.

Returns before taxes do not reflect the effects of any income or capital gains taxes. All after-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of any state or local tax. Returns after taxes on distributions reflect the taxed return on the payment of dividends and capital gains. Returns after taxes on distributions and sale of shares assume you sold your shares at period end, and, therefore, are also adjusted for any capital gains or losses incurred. Returns for the market index do not include expenses, which are deducted from Fund returns, or taxes.

Your own actual after-tax returns will depend on your specific tax situation and may differ from what is shown here. After-tax returns are not relevant to investors who hold Fund shares in tax-deferred accounts such as individual retirement accounts (IRAs) or employee-sponsored retirement plans.
Bar Chart Footnotes [Text Block] rr_BarChartFootnotesTextBlock
[1]The Fund's year-to-date return based on net asset value for the period 12/31/2015 to 3/31/2016 was 0.16%.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock

During the period shown in the chart above:

BEST QUARTER WORST QUARTER
13.55% (March 31, 2013) -8.68% (September 30, 2015)

The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Performance Table Heading rr_PerformanceTableHeading

AVERAGE ANNUAL TOTAL RETURNS FOR THE PERIODS ENDED DECEMBER 31, 2015
Performance Table Footnotes rr_PerformanceTableFootnotesTextBlock
*On April 8, 2016, the Fund's underlying index changed from the Defined Mid Cap Growth Index to the Nasdaq AlphaDEX(R) Mid Cap Growth Index. Because the Fund's new underlying index had an inception date of January 11, 2016, performance information is not included above. The new Index is substantially similar to the Defined Mid Cap Growth Index.
First Trust Mid Cap Growth AlphaDEX(R) Fund | First Trust Mid Cap Growth AlphaDEX(R) Fund  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Management Fees rr_ManagementFeesOverAssets 0.70%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none [1]
Other Expenses rr_Component1OtherExpensesOverAssets none
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.70%
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 $ 72
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 278
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 501
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,144
Annual Return 2012 rr_AnnualReturn2012 9.90%
Annual Return 2013 rr_AnnualReturn2013 36.64%
Annual Return 2014 rr_AnnualReturn2014 4.98%
Annual Return 2015 rr_AnnualReturn2015 (1.20%)
1 Year rr_AverageAnnualReturnYear01 (1.20%)
Since Inception rr_AverageAnnualReturnSinceInception 8.29%
Inception Date rr_AverageAnnualReturnInceptionDate Apr. 19, 2011
First Trust Mid Cap Growth AlphaDEX(R) Fund | After Taxes on Distributions | First Trust Mid Cap Growth AlphaDEX(R) Fund  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 (1.39%)
Since Inception rr_AverageAnnualReturnSinceInception 8.14%
First Trust Mid Cap Growth AlphaDEX(R) Fund | After Taxes on Distributions and Sales | First Trust Mid Cap Growth AlphaDEX(R) Fund  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 (0.67%)
Since Inception rr_AverageAnnualReturnSinceInception 6.45%
First Trust Mid Cap Growth AlphaDEX(R) Fund | S&P MidCap 400(R) Growth Index  
Risk Return Abstract rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel (reflects no deduction for fees, expenses or taxes)
1 Year rr_AverageAnnualReturnYear01 2.02%
Since Inception rr_AverageAnnualReturnSinceInception 9.98%
First Trust Mid Cap Growth AlphaDEX(R) Fund | Nasdaq AlphaDEX(R) Mid Cap Growth Index  
Risk Return Abstract rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel (reflects no deduction for fees, expenses or taxes)
1 Year rr_AverageAnnualReturnYear01
Since Inception rr_AverageAnnualReturnSinceInception
First Trust Mid Cap Growth AlphaDEX(R) Fund | Defined Mid Cap Growth Index  
Risk Return Abstract rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel (reflects no deduction for fees, expenses or taxes)
1 Year rr_AverageAnnualReturnYear01 (0.42%)
Since Inception rr_AverageAnnualReturnSinceInception 9.12%
[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 any time before April 7, 2017.