Label

Element

Value

Risk/Return:

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Registrant Name

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First Trust Exchange-Traded AlphaDEX Fund II

Prospectus Date

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May 01, 2017

First Trust Germany AlphaDEX Fund

Risk/Return:

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Risk/Return [Heading]

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Summary Information

Objective [Heading]

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Investment Objective

Objective, Primary [Text Block]

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The First Trust Germany AlphaDEX® 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® Germany Index (the "Index").

Expense [Heading]

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Fees and Expenses of the Fund

Expense Narrative [Text Block]

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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]

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Shareholder Fees
(fees paid directly from your investment)

Operating Expenses Caption [Text]

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Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Portfolio Turnover [Heading]

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Portfolio Turnover

Portfolio Turnover [Text Block]

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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 114% of the average value of its portfolio.

Portfolio Turnover, Rate

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114.00%

Expense Example [Heading]

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Example

Expense Example, No Redemption Narrative [Text Block]

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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 until April 30, 2018, and thereafter at 1.05% to represent the imposition of the 12b-1 fee of 0.25% per annum of the Fund's average daily net assets. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

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Principal Investment Strategies

Strategy Narrative [Text Block]

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The Fund will normally invest at least 90% of its net assets (including investment borrowings) in common stocks and/or depositary receipts 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 Fund invests in securities issued by companies operating in Germany, as classified by the Index Provider. Companies are classified as operating in a country primarily by their country of incorporation, domicile and primary exchange listing.

The Index is designed to select stocks from the NASDAQ Germany Index (the "Base Index") that may generate positive alpha, or risk-adjusted returns, relative to traditional indices through the use of the AlphaDEX^® selection methodology. Alpha is an indication of how much an investment outperforms or underperforms on a risk-adjusted basis relative to its benchmark. The Base Index is a comprehensive, rules-based index designed to measure stock market performance of companies in Germany, 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; stocks which do not meet the Index Provider's liquidity screens; and stocks with a market capitalization less than the midcap breakpoint (50th percentile) as calculated by Nasdaq. As of March 31, 2017, the midcap breakpoint was $3.154 billion.

3.The remaining stocks in the universe are then ranked on both growth and value factors. Each stock receives the best style rank from this step as its selection score.

4.The top 40 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.

5.The Index is subject to sector weighting constraints which are set at 15% above the sector percentages of the Base Index. For example, if financial companies comprise 15% of the Base Index, the Index will be comprised of no more than 30% financials. Stocks will fail the sector constraint if the weight assigned to the stock, when added to the weight assigned to all higher ranking stocks in its sector, is greater than the sector weighting constraint.

The Index is reconstituted and rebalanced on a semi-annual basis 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 December 15, 2014. As of March 31, 2017, the Index was composed of 40 securities. See "Index Information" for additional information.

The Fund may hold investments that are denominated in non-U.S. currencies. The Index includes the securities of small and mid cap companies. As of December 31, 2016, the Fund had significant investments in consumer discretionary companies.

Risk [Heading]

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Principal Risks

Risk Narrative [Text Block]

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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.

AUTHORIZED PARTICIPANT CONCENTRATION RISK. Only an authorized participant (as defined in the "Frequent Purchases and Redemptions" Section) may engage in creation or redemption transactions directly with the Fund. The Fund has a limited number of institutions that act as authorized participants. To the extent that these institutions exit the business or are unable to proceed with creation and/or redemption orders with respect to the Fund and no other authorized participant is able to step forward to create or redeem, in either of these cases, Fund shares may trade at a discount to the Fund's net asset value and possibly face delisting.

CONSUMER DISCRETIONARY COMPANIES RISK. Consumer discretionary companies are companies that provide non-essential goods and services, such as retailers, media companies and consumer services. These companies 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.

CURRENCY EXCHANGE RATE RISK. Changes in currency exchange rates and the relative value of non-U.S. currencies will affect the value of the Fund's investment and the value of Fund shares. Currency exchange rates can be very volatile and can change quickly and unpredictably. As a result, the value of an investment in the Fund may change quickly and without warning and you may lose money.

CYBER SECURITY RISK. As the use of Internet technology has become more prevalent in the course of business, the Fund has become more susceptible to potential operational risks through breaches in cyber security. A breach in cyber security refers to both intentional and unintentional events that may cause the Fund to lose proprietary information, suffer data corruption or lose operational capacity. Such events could cause the Fund to incur regulatory penalties, reputational damage, additional compliance costs associated with corrective measures and/or financial loss. Cyber security breaches may involve unauthorized access to the Fund's digital information systems through "hacking" or malicious software coding, but may also result from outside attacks such as denial-of-service attacks through efforts to make network services unavailable to intended users. In addition, cyber security breaches of the Fund's third party service providers, such as its administrator, transfer agent, custodian, or sub-advisor, as applicable, or issuers in which the Fund invests, can also subject the Fund to many of the same risks associated with direct cyber security breaches. The Fund has established risk management systems designed to reduce the risks associated with cyber security. However, there is no guarantee that such efforts will succeed, especially because the Fund does not directly control the cyber security systems of issuers or third party service providers.

DEPOSITARY RECEIPTS RISK. Depositary receipts may be less liquid than the underlying shares in their primary trading market. Any distributions paid to the holders of depositary receipts are usually subject to a fee charged by the depositary. Holders of depositary receipts may have limited voting rights, and investment restrictions in certain countries may adversely impact the value of depositary receipts because such restrictions may limit the ability to convert the equity shares into depositary receipts and vice versa. Such restrictions may cause the equity shares of the underlying issuer to trade at a discount or premium to the market price of the depositary receipts.

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.

EUROPEAN MARKETS RISK. The Fund invests in securities issued by companies operating in Europe. Investing in Europe involves risks not typically associated with investments in the United States. While many countries in Europe are considered to have developed markets, investing in the developed countries of Europe imposes different risks than those associated with investing in other developed markets. Many countries in Europe are members of the European Union ("EU"), which faces major issues involving its membership, structure, procedures and policies. Efforts of the member states to continue to unify their economic and monetary policies may increase the potential for similarities in the movements of European markets and may reduce any diversification benefit a Fund may seek by investing in multiple countries within Europe. European countries that are members of, or candidates to join, the Economic and Monetary Union ("EMU") (which is comprised of EU members that have adopted the Euro currency) are subject to restrictions on inflation rates, interest rates, deficits and debt levels, as well as fiscal and monetary controls. By adopting the Euro as its currency, a member state relinquishes control over its own monetary policies. As a result, European countries are significantly affected by fiscal and monetary controls implemented by the EMU, and it is possible that the timing and substance of these controls may not address the needs of all EMU member countries. In addition, the fiscal policies of a single member state can impact and pose economic risks to the EU as a whole. Investing in Euro-denominated securities also creates exposure to a currency that may not fully reflect the strengths and weaknesses of the disparate economies that comprise Europe. There is continued concern over national-level support for the Euro, which could lead to certain countries leaving the EMU, the implementation of currency controls, or potentially the dissolution of the Euro. The dissolution of the Euro would have significant negative effects on European economies and would cause funds with holdings denominated in Euros to face substantial challenges, including difficulties relating to settlement of trades and valuation of holdings, diminished liquidity, and the redenomination of holdings into other currencies.

EUROPEAN UNION RISK. Continuing uncertainty as to the status of the Euro and the European Monetary Union and the potential for certain countries to withdraw from the institution has created significant volatility in currency and financial markets generally. Any partial or complete dissolution of the European Union ("EU") could have significant adverse effects on currency and financial markets, and on the values of a Fund's portfolio investments. The United Kingdom's referendum on June 23, 2016 to leave the European Union (known as "Brexit") sparked depreciation in the value of the British pound, short-term declines in the stock markets and heightened risk of continued economic volatility worldwide. Although the long-term effects of Brexit are difficult to gauge and cannot be fully known, they could have wide ranging implications for the United Kingdom's economy, including: possible inflation or recession, continued depreciation of the pound, or disruption to Britain's trading arrangements with the rest of Europe. The United Kingdom is one of the EU's largest economies; its departure also may negatively impact the EU and Europe as a whole, such as by causing volatility within the union, trigging prolonged economic downturns in certain European countries or sparking additional member states to contemplate departing the EU (thereby perpetuating political instability in the region).

FLUCTUATION OF NET ASSET VALUE RISK. The net asset value of shares of the Fund will generally fluctuate with changes in the market value of the Fund's holdings. The market prices of shares will generally fluctuate in accordance with changes in net asset value as well as the relative supply of and demand for shares on Nasdaq. The Fund's investment advisor cannot predict whether shares will trade below, at or above their net asset value because the shares trade on Nasdaq at market prices and not at net asset value. Price differences may be due, in large part, to the fact that supply and demand forces at work in the secondary trading market for shares will be closely related to, but not identical to, the same forces influencing the prices of the holdings of the Fund trading individually or in the aggregate at any point in time. However, given that shares can only be purchased and redeemed either in-kind or for cash in Creation Units, and only to and from broker-dealers and large institutional investors that have entered into participation agreements (unlike shares of closed-end funds, which frequently trade at appreciable discounts from, and sometimes at premiums to, their net asset value), the Fund's investment advisor believes that large discounts or premiums to the net asset value of shares should not be sustained.

GERMANY RISK. Investing in German companies involves additional risks, including, but not limited to: significant demographic challenges to sustained long-term growth; low fertility rates and declining net immigration putting pressure on the country's social welfare system; and the costly and time-consuming modernization and integration of the eastern German economy. Additionally, the European sovereign-debt crisis has resulted in a weakened Euro and has put into question the future financial prospects of Germany and the surrounding region.

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 exchange-traded funds 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 MAKER RISK. If the Fund has lower average daily trading volumes, it may rely on a small number of third-party market makers to provide a market for the purchase and sale of shares. Any trading halt or other problem relating to the trading activity of these market makers could result in a dramatic change in the spread between the Fund's net asset value and the price at which the Fund's shares are trading on Nasdaq, which could result in a decrease in value of the Fund's shares. In addition, decisions by market makers or authorized participants to reduce their role or step away from these activities in times of market stress could inhibit the effectiveness of the arbitrage process in maintaining the relationship between the underlying values of the Fund's portfolio securities and the Fund's market price. This reduced effectiveness could result in Fund shares trading at a discount to net asset value and also in greater than normal intraday bid-ask spreads for Fund shares.

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.

To the extent the Fund calculates its net asset value based on fair value prices and the value of its Index is based on securities closing prices on the applicable foreign exchange (i.e., the value of its Index is not based on fair value prices), the Fund's ability to track its Index may be adversely affected.

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 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 invested in certain issuers.

NON-U.S. SECURITIES RISK. Non-U.S. securities are subject to higher volatility than securities of domestic issuers due to possible adverse political, social or economic developments, restrictions on foreign investment or exchange of securities, lack of liquidity, currency exchange rates, excessive taxation, government seizure of assets, different legal or accounting standards, and less government supervision and regulation of exchanges in foreign countries.

PORTFOLIO TURNOVER RISK. The Fund's strategy may frequently involve buying and selling portfolio securities to rebalance the Fund's exposure to various market sectors. 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. Small and/or 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.

TRADING ISSUES RISK. Although the shares of the Fund are listed for trading on Nasdaq, there can be no assurance that an active trading market for such shares will develop or be maintained. Trading in shares on Nasdaq may be halted due to market conditions or for reasons that, in the view of Nasdaq, make trading in shares inadvisable. In addition, trading in shares on Nasdaq is subject to trading halts caused by extraordinary market volatility pursuant to Nasdaq "circuit breaker" rules. Market makers are under no obligation to make a market in the Fund's shares, and authorized participants are not obligated to submit purchase or redemption orders for Creation Units. There can be no assurance that the requirements of Nasdaq necessary to maintain the listing of the Fund will continue to be met or will remain unchanged. The Fund may have difficulty maintaining its listing on Nasdaq in the event the Fund's assets are small or the Fund does not have enough shareholders.

VALUE INVESTMENT RISK. The intrinsic value of a stock with value characteristics may not be fully recognized by the market for a long time or a stock judged to be undervalued may actually be appropriately priced at a low level.

Bar Chart and Performance Table [Heading]

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Annual Total Return

Performance Narrative [Text Block]

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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, the Fund's previous Index and three broad based securities market indices. 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.

On July 14, 2015 the Fund's underlying index changed from the Defined Germany Index to the NASDAQ AlphaDEX® Germany 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 December 15, 2014, it was not in existence for all of the periods disclosed. The new Index is substantially similar to the Defined Germany Index.

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 [Heading]

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First Trust Germany AlphaDEX® Fund
Calendar Year Total Returns as of 12/31

Bar Chart Closing [Text Block]

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During the periods shown in the chart above:

Best Quarter		Worst Quarter
12.51%	September 30, 2013	-15.56%	September 30, 2014

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

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Average Annual Total Returns for the Periods Ended December 31, 2016

First Trust Germany AlphaDEX Fund | First Trust Germany AlphaDEX Fund

Risk/Return:

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

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none

Management Fees

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0.80%

Distribution and Service (12b-1) Fees

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none

^[1]

Other Expenses

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none

Total Annual Fund Operating Expenses

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0.80%

1 Year

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$ 82

3 Years

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309

5 Years

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555

10 Years

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$ 1,260

2013

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26.61%

2014

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(11.60%)

2015

rr_AnnualReturn2015

1.94%

2016

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1.67%

Highest Quarterly Return, Label

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Best Quarter

Highest Quarterly Return, Date

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Sep. 30, 2013

Highest Quarterly Return

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12.51%

Lowest Quarterly Return, Label

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Worst Quarter

Lowest Quarterly Return, Date

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Sep. 30, 2014

Lowest Quarterly Return

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(15.56%)

1 Year

rr_AverageAnnualReturnYear01

1.67%

Since Inception

rr_AverageAnnualReturnSinceInception

5.72%

Inception Date

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Feb. 14, 2012

First Trust Germany AlphaDEX Fund | After tax on distributions | First Trust Germany AlphaDEX Fund

Risk/Return:

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1 Year

rr_AverageAnnualReturnYear01

1.06%

Since Inception

rr_AverageAnnualReturnSinceInception

5.04%

First Trust Germany AlphaDEX Fund | After tax on distributions and sale of fund shares | First Trust Germany AlphaDEX Fund

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

0.93%

Since Inception

rr_AverageAnnualReturnSinceInception

4.14%

First Trust Germany AlphaDEX Fund | Defined Germany Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

^[2]

Since Inception

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^[2]

First Trust Germany AlphaDEX Fund | NASDAQ AlphaDEX® Germany Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

1.85%

^[3]

Since Inception

rr_AverageAnnualReturnSinceInception

^[3]

First Trust Germany AlphaDEX Fund | NASDAQ Germany Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

2.38%

^[3]

Since Inception

rr_AverageAnnualReturnSinceInception

^[3]

First Trust Germany AlphaDEX Fund | S&P Germany BMI Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

2.80%

Since Inception

rr_AverageAnnualReturnSinceInception

6.62%

First Trust Germany AlphaDEX Fund | MSCI Germany Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

2.75%

Since Inception

rr_AverageAnnualReturnSinceInception

6.29%

[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 April 30, 2018.

[2] The Defined Germany Index was replaced with the NASDAQ AlphaDEX® Germany Index effective July 14, 2015. The new Index is substantially similar to the old index. The old index was terminated September 30, 2016, so performance data does not exist for these time periods.

[3] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.