dei_DocumentType

Other

dei_EntityRegistrantName

First Trust Exchange-Traded AlphaDEX Fund

Amendment Flag

dei_AmendmentFlag

false

Document Effective Date

dei_DocumentEffectiveDate

Jul. 03, 2013

Risk/Return [Heading]

rr_RiskReturnHeading

SUMMARY INFORMATION

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The First Trust Consumer Discretionary 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 StrataQuant(R) Consumer Discretionary Index (the "Index").

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.

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 [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 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 98% of the average value of its portfolio.

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 and then you retain the Shares or sell 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 net annual operating expenses remain at current levels until November 30, 2013. 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 November 30, 2013. The example assumes that First Trust's agreement to waive fees and/or pay the Fund's expenses to the extent necessary to prevent the operating expenses of the Fund (excluding interest expense, brokerage commissions and other trading expenses, taxes, and extraordinary expenses) from exceeding 0.70% of average daily net assets per year will be terminated following November 30, 2013. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund will normally invest at least 90% of its net assets 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 "StrataQuant(R) Series," a family of custom enhanced indices developed, maintained and sponsored by NYSE Euronext or its affiliates ("NYSE Euronext" or an "Index Provider"), and licensed to First Trust Portfolios L.P. by NYSE Group, Inc. ("NYSE Group"), an affiliate of NYSE Euronext. Prior to the acquisition of the American Stock Exchange LLC (the "AMEX") by NYSE Euronext, the equity indices in the StrataQuant(R) Series were developed, maintained and sponsored by the AMEX. The Index is a modified equal-dollar weighted index designed by NYSE Euronext to objectively identify and select stocks from the Russell 1000(R) Index in the consumer discretionary sector that may generate positive alpha relative to traditional passive-style 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. As of July 31, 2012, the Index was comprised of 123 stocks. 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 fourth business day of the following month.

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. Companies with smaller market capitalizations are generally subject to additional market risk.

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.

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 that Fund's Index.

CONSUMER DISCRETIONARY SECTOR RISK. The Fund invests in the securities of companies in the consumer discretionary sector to approximately the same extent as the Index. Companies in the consumer discretionary sector manufacture products and provide discretionary services directly to the consumer, and the success of these companies is tied closely to the performance of the overall domestic and international economy, interest rates, competition and consumer confidence. Success depends heavily on disposable household income and consumer spending. Changes in demographics and consumer tastes can also affect the demand for, and success of, consumer discretionary products in the marketplace.

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The bar chart and table below illustrate the annual calendar year returns of the Fund based on NAV for the past four years as well as the average annual Fund and Index returns for the one year and since inception periods ended December 31, 2011. 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 NAV compare to those of the Index, a broad-based market index and two specialized securities market indices. With respect to the Russell 1000(R) Consumer Discretionary Index (formerly the Russell 1000(R) Consumer Discretionary and Services Index), on or about September 18, 2008, Russell Investment Group ("Russell") began calculating its Russell U.S. Indices using an enhanced Russell sector scheme, the Russell Global Sectors (the "RGS"). The RGS and the former U.S. sector scheme ran in tandem until June 30, 2009, when Russell officially transitioned to the RGS and ceased calculating its indices using the U.S. sector scheme. Consequently, performance data is not available for the Russell 1000(R) Consumer Discretionary Index for the entire "Since Inception" period set forth in the table because performance data does not exist using solely the U.S. sector scheme or alternatively, using solely the RGS. 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 indices 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 NAV for the period 12/31/2011 to 9/30/2012 was 11.74%.

Performance Table Heading

rr_PerformanceTableHeading

AVERAGE ANNUAL TOTAL RETURNS FOR THE PERIODS ENDED DECEMBER 31, 2011

Risk/Return:

rr_RiskReturnAbstract

Management Fees

rr_ManagementFeesOverAssets

0.50%

Other Expenses

rr_OtherExpensesOverAssets

0.22%

Fee Waiver and Expense Reimbursement

rr_FeeWaiverOrReimbursementOverAssets

(0.02%)

^[1]

Expense Example, No Redemption, 1 Year

rr_ExpenseExampleNoRedemptionYear01

$ 72

Expense Example, No Redemption, 5 Years

rr_ExpenseExampleNoRedemptionYear05

501

Annual Return 2008

rr_AnnualReturn2008

(42.87%)

Annual Return 2010

rr_AnnualReturn2010

31.98%

1 Year

rr_AverageAnnualReturnYear01

0.98%

Inception Date

rr_AverageAnnualReturnInceptionDate

May 08, 2007

Risk/Return:

rr_RiskReturnAbstract

Since Inception

rr_AverageAnnualReturnSinceInception

0.20%

1 Year

rr_AverageAnnualReturnYear01

0.64%

Inception Date

rr_AverageAnnualReturnInceptionDate

May 08, 2007

Risk/Return:

rr_RiskReturnAbstract

Since Inception

rr_AverageAnnualReturnSinceInception

1.23%

1 Year

rr_AverageAnnualReturnYear01

1.50%

Inception Date

rr_AverageAnnualReturnInceptionDate

May 08, 2007

Risk/Return:

rr_RiskReturnAbstract

Since Inception

rr_AverageAnnualReturnSinceInception

1.54%

1 Year

rr_AverageAnnualReturnYear01

5.39%

Inception Date

rr_AverageAnnualReturnInceptionDate

May 08, 2007

Risk/Return:

rr_RiskReturnAbstract

Objective [Heading]

rr_ObjectiveHeading

INVESTMENT OBJECTIVE

Expense [Heading]

rr_ExpenseHeading

FEES AND EXPENSES OF THE FUND

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

SHAREHOLDER FEES (fees paid directly from your investment)

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

PORTFOLIO TURNOVER

Expense Example [Heading]

rr_ExpenseExampleHeading

EXAMPLE

Strategy [Heading]

rr_StrategyHeading

PRINCIPAL INVESTMENT STRATEGIES

Risk [Heading]

rr_RiskHeading

PRINCIPAL RISKS

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

ANNUAL TOTAL RETURN

Bar Chart [Heading]

rr_BarChartHeading

FIRST TRUST ENERGY ALPHADEX(R) FUND--TOTAL RETURNS(1)

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the four-year period ended December 31, 2011, the Fund's highest and lowest calendar quarter returns were 29.92% and -38.75%, respectively, for the quarters ended June 30, 2008 and December 31, 2008. The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.76%

Total Net Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursement

rr_NetExpensesOverAssets

0.70%

3 Years

rr_ExpenseExampleNoRedemptionYear03

280

Expense Example, No Redemption, 10 Years

rr_ExpenseExampleNoRedemptionYear10

1,198

Annual Return 2009

rr_AnnualReturn2009

47.52%

Annual Return 2011

rr_AnnualReturn2011

(8.16%)

Since Inception

rr_AverageAnnualReturnSinceInception

(0.31%)

1 Year

rr_AverageAnnualReturnYear01

(8.30%)

Inception Date

rr_AverageAnnualReturnInceptionDate

May 08, 2007

Risk/Return:

rr_RiskReturnAbstract

Since Inception

rr_AverageAnnualReturnSinceInception

(0.35%)

1 Year

rr_AverageAnnualReturnYear01

1.50%

Inception Date

rr_AverageAnnualReturnInceptionDate

May 08, 2007

Risk/Return:

rr_RiskReturnAbstract

Since Inception

rr_AverageAnnualReturnSinceInception

0.38%

1 Year

rr_AverageAnnualReturnYear01

4.72%

Inception Date

rr_AverageAnnualReturnInceptionDate

May 08, 2007

Risk/Return:

rr_RiskReturnAbstract

Since Inception

rr_AverageAnnualReturnSinceInception

Risk/Return [Heading]

rr_RiskReturnHeading

SUMMARY INFORMATION

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The First Trust Mid Cap Core 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 Mid Cap Core Index (the "Index").

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.

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 [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 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 94% of the average value of its portfolio.

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 and then you retain the Shares or sell 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. 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 November 30, 2013. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund will normally invest at least 90% of its net assets 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 S&P Dow Jones Indices LLC ("S&P" or an "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 MidCap 400(R) Index that may generate positive alpha relative to traditional passive-style 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. As of July 31, 2012, the Index was comprised of 299 stocks and the market capitalization range represented in the Index was $289 million to $12.8 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.

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. Companies with smaller market capitalizations are generally subject to additional market risk.

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.

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 that Fund's Index.

MARKET CAPITALIZATION RISK. The Fund normally invests at least 90% of its assets in common stocks that comprise the Index. The securities of companies represented in the Index generally 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.

SMALLER COMPANY RISK. The Fund invests in mid capitalization companies. Such 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.

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The bar chart and table below illustrate the annual calendar year returns of the Fund based on NAV for the past four years as well as the average annual Fund and Index returns for the one year and since inception periods ended December 31, 2011. 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 NAV compare to those of the Index and a broad-based market 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 NAV for the period 12/31/2011 to 9/30/2012 was 10.04%.

Performance Table Heading

rr_PerformanceTableHeading

AVERAGE ANNUAL TOTAL RETURNS FOR THE PERIODS ENDED DECEMBER 31, 2011

Risk/Return:

rr_RiskReturnAbstract

Management Fees

rr_ManagementFeesOverAssets

0.50%

Fees Previously Waived or Expenses Reimbursed by First Trust

rr_Component1OtherExpensesOverAssets

0.03%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.70%

Total Net Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursement

rr_NetExpensesOverAssets

0.70%

3 Years

rr_ExpenseExampleNoRedemptionYear03

269

Expense Example, No Redemption, 10 Years

rr_ExpenseExampleNoRedemptionYear10

1,135

Annual Return 2009

rr_AnnualReturn2009

47.38%

Annual Return 2011

rr_AnnualReturn2011

0.83%

Since Inception

rr_AverageAnnualReturnSinceInception

2.47%

1 Year

rr_AverageAnnualReturnYear01

0.66%

Inception Date

rr_AverageAnnualReturnInceptionDate

May 08, 2007

Risk/Return:

rr_RiskReturnAbstract

Since Inception

rr_AverageAnnualReturnSinceInception

1.97%

1 Year

rr_AverageAnnualReturnYear01

1.63%

Inception Date

rr_AverageAnnualReturnInceptionDate

May 08, 2007

Risk/Return:

rr_RiskReturnAbstract

Since Inception

rr_AverageAnnualReturnSinceInception

1.19%

Risk/Return [Heading]

rr_RiskReturnHeading

SUMMARY INFORMATION

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The First Trust Small Cap Core 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 Small Cap Core Index (the "Index").

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.

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 [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 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 101% of the average value of its portfolio.

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 and then you retain the Shares or sell 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. 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 November 30, 2013. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund will normally invest at least 90% of its net assets 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 S&P Dow Jones Indices LLC ("S&P" or an "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 SmallCap 600(R) Index that may generate positive alpha relative to traditional passive-style 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. As of July 31, 2012, the Index was comprised of 450 stocks and the market capitalization range represented in the Index was $80 million to $3.2 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.

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. Companies with smaller market capitalizations are generally subject to additional market risk.

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.

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 that Fund's Index.

MARKET CAPITALIZATION RISK. The Fund normally invests at least 90% of its assets in common stocks that comprise the Index. The securities of companies represented in the Index generally 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. Because the Fund invests in small capitalization companies, the Fund is more vulnerable to adverse general market or economic developments, may be less liquid, and may experience greater price volatility than larger, more established companies.

SMALLER COMPANY RISK. The Fund invests in small capitalization companies. Such 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.

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The bar chart and table below illustrate the annual calendar year returns of the Fund based on NAV for the past four years as well as the average annual Fund and Index returns for the one year and since inception periods ended December 31, 2011. 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 NAV compare to those of the Index and a broad-based market 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]

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(1) The Fund's year-to-date return based on NAV for the period 12/31/2011 to 9/30/2012 was 12.75%.

Performance Table Heading

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AVERAGE ANNUAL TOTAL RETURNS FOR THE PERIODS ENDED DECEMBER 31, 2011

Risk/Return:

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Management Fees

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0.50%

Fees Previously Waived or Expenses Reimbursed by First Trust