Label	Element	Value
Risk Return Abstract	rr_RiskReturnAbstract
Prospectus Date	rr_ProspectusDate	Dec. 01, 2020
First Trust Municipal High Income ETF
Risk Return Abstract	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Summary Information
Objective [Heading]	rr_ObjectiveHeading	<span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;margin-left:0%;">Investment Objectives</span>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The First Trust Municipal High Income ETF’s (the "Fund") primary investment objective is to provide federally tax-exempt income, and its secondary objective is long-term capital appreciation.
Expense [Heading]	rr_ExpenseHeading	<span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;margin-left:0%;">Fees and Expenses of the Fund</span>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The following table describes the fees and expenses you may pay if you buy, hold and sell shares of the Fund. Investors may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the table and example below.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<span style="color:#000000;font-family:Arial;font-size:9.00pt;font-weight:bold;margin-left:0%;">Shareholder Fees</span><span style="color:#000000;font-family:Arial;font-size:9.00pt;">(fees paid directly from your investment)</span>
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<span style="color:#000000;font-family:Arial;font-size:9.00pt;font-weight:bold;margin-left:0%;">Annual Fund Operating Expenses</span><span style="color:#000000;font-family:Arial;font-size:9.00pt;">(expenses that you pay each year as a percentage of the value of your investment)</span>
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	<span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;margin-left:0%;">Portfolio Turnover</span>
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 89% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	89.00%
Expense Example [Heading]	rr_ExpenseExampleHeading	<span style="color:#000000;font-family:Arial;font-size:9.00pt;font-style:italic;font-weight:bold;margin-left:0%;">Example</span>
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. The example assumes First Trust's agreement to waive management fees of 0.15% of average daily net assets per year will be terminated following November 30, 2021. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;margin-left:0%;">Principal Investment Strategies</span>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal market conditions, the Fund seeks to achieve its investment objectives by investing at least 80% of its net assets (including investment borrowings) in municipal debt securities that pay interest that is exempt from regular federal income taxes (collectively, “Municipal Securities”). Municipal Securities are generally issued by or on behalf of states, territories or possessions of the U.S. and the District of Columbia and their political subdivisions, agencies, authorities and other instrumentalities. The types of Municipal Securities in which the Fund may invest include municipal lease obligations (and certificates of participation in such obligations), municipal general obligation bonds, municipal revenue bonds, municipal notes, municipal cash equivalents, private activity bonds (including without limitation industrial development bonds), and pre-refunded and escrowed to maturity bonds. In addition, Municipal Securities include securities issued by custodial receipt trusts, which are investment vehicles the underlying assets of which are municipal bonds. The Fund may invest in Municipal Securities of any duration and any maturity.Under normal market conditions, the Fund will invest at least 50% of its net assets in Municipal Securities that are, at the time of investment, rated below investment grade by at least one nationally recognized statistical rating organization rating such securities (or Municipal Securities that are unrated and determined by the Fund’s advisor to be of comparable quality), commonly referred to as “high yield” or “junk” bonds (the “Below Investment Grade Requirement”). The Municipal Securities in which the Fund will invest to satisfy this requirement may include Municipal Securities that are currently in default and not expected to pay the current coupon (“Distressed Municipal Securities”). The Fund may invest up to 10% of its net assets in Distressed Municipal Securities.Under normal market conditions, the Fund may not invest more than 50% of its net assets in Municipal Securities that are, at the time of investment, rated investment grade (i.e., rated Baa3/BBB- or above) by each nationally recognized statistical rating organization rating such securities or, if unrated, determined by the Fund’s advisor to be of comparable quality (the “Investment Grade Limitation”). The Fund will consider pre-refunded or escrowed to maturity bonds, regardless of rating, to be investment grade securities. If, subsequent to purchase by the Fund, a Municipal Security held by the Fund experiences an improvement in credit quality and becomes investment grade, the Fund may continue to hold the Municipal Security and it will not cause the Fund to violate the Investment Grade Limitation; however, the Municipal Security will be taken into account for purposes of determining whether purchases of additional Municipal Securities will cause the Fund to violate such limitation.
Risk [Heading]	rr_RiskHeading	<span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;margin-left:0%;">Principal Risks</span>
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 objectives will be achieved. The order of the below risk factors does not indicate the significance of any particular risk factor.ALTERNATIVE MINIMUM TAX RISK. Although the interest received from municipal securities is generally exempt from federal income tax, the Fund may invest in municipal securities subject to the federal alternative minimum tax. Accordingly, investment in the Fund could cause shareholders to be subject to the federal alternative minimum tax.AUTHORIZED PARTICIPANT CONCENTRATION RISK. Only an authorized participant may engage in creation or redemption transactions directly with the Fund. A limited number of institutions act as authorized participants for the Fund. To the extent that these institutions exit the business or are unable to proceed with creation and/or redemption orders and no other authorized participant steps forward to create or redeem, the Fund’s shares may trade at a premium or discount to the Fund’s net asset value and possibly face delisting.CALL RISK. Some debt securities may be redeemed, or “called,” at the option of the issuer before their stated maturity date. In general, an issuer will call its debt securities if they can be refinanced by issuing new debt securities which bear a lower interest rate. The Fund is subject to the possibility that during periods of falling interest rates an issuer will call its high yielding debt securities. The Fund would then be forced to invest the proceeds at lower interest rates, likely resulting in a decline in the Fund’s income.CASH TRANSACTIONS RISK. The Fund will, under most circumstances, effect a significant portion of creations and redemptions for cash rather than in-kind. As a result, an investment in the Fund may be less tax-efficient than an investment in an ETF that effects all of its creations and redemptions in-kind. Because the Fund may effect redemptions for cash, it may be required to sell portfolio securities in order to obtain the cash needed to distribute redemption proceeds. A sale of shares may result in capital gains or losses and may also result in higher brokerage costs.CREDIT RISK. An issuer or other obligated party of a debt security may be unable or unwilling to make dividend, interest and/or principal payments when due. In addition, the value of a debt security may decline because of concerns about the issuer’s ability or unwillingness to make such payments.CREDIT SPREAD RISK. From time to time, spreads (i.e., the difference in yield between debt securities that have different credit qualities or other differences) may increase, which may reduce the market value of some of the Fund’s debt securities. While the Fund may employ strategies to mitigate credit spread risk, these strategies may not be successful.CYBER SECURITY RISK. The Fund is susceptible to 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 issuers of securities in which the Fund invests or the Fund’s third-party service providers, such as its administrator, transfer agent, custodian, or sub-advisor, as applicable, can also subject the Fund to many of the same risks associated with direct cyber security breaches. Although the Fund has established risk management systems designed to reduce the risks associated with cyber security, 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.DEBT SECURITIES RISK. Investments in debt securities subject the holder to the credit risk of the issuer. Credit risk refers to the possibility that the issuer or other obligor of a security will not be able or willing to make payments of interest and principal when due. Generally, the value of debt securities will change inversely with changes in interest rates. To the extent that interest rates rise, certain underlying obligations may be paid off substantially slower than originally anticipated and the value of those securities may fall sharply. During periods of falling interest rates, the income received by the Fund may decline. If the principal on a debt security is prepaid before expected, the prepayments of principal may have to be reinvested in obligations paying interest at lower rates. Debt securities generally do not trade on a securities exchange making them generally less liquid and more difficult to value than common stock.EXTENSION RISK. Extension risk is the risk that, when interest rates rise, certain obligations will be paid off by the issuer (or other obligated party) more slowly than anticipated, causing the value of these debt securities to fall. Rising interest rates tend to extend the duration of debt securities, making their market value more sensitive to changes in interest rates. The value of longer-term debt securities generally changes more in response to changes in interest rates than shorter-term debt securities. As a result, in a period of rising interest rates, securities may exhibit additional volatility and may lose value.HIGH YIELD SECURITIES RISK. High yield securities, or “junk” bonds, are subject to greater market fluctuations, are less liquid and provide a greater risk of loss than investment grade securities, and therefore, are considered to be highly speculative. In general, high yield securities may have a greater risk of default than other types of securities and could cause income and principal losses for the Fund.INCOME RISK. The Fund’s income may decline when interest rates fall or if there are defaults in its portfolio. This decline can occur because the Fund may subsequently invest in lower-yielding securities as debt securities in its portfolio mature, are near maturity or are called, or the Fund otherwise needs to purchase additional debt securities.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’s shares, the size of the Fund and the market volatility of the Fund. Inclusion in an index could 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 below the Fund’s net asset value during certain periods. In addition, index rebalances may potentially result in increased trading activity in the Fund’s shares.INDUSTRIAL DEVELOPMENT BOND RISK. Industrial development bonds are revenue bonds issued by or on behalf of public authorities to obtain funds to finance various public and/or privately operated facilities, including those for business and manufacturing, housing, sports, pollution control, airport, mass transit, port and parking facilities. To the extent that investments in the industrial development sector represent a significant portion of the Fund’s portfolio, the Fund will be sensitive to changes in, and its performance may depend to a greater extent on, the overall condition of the industrial development sector. These bonds are normally secured only by the revenues from the project and are not general obligations of the issuer or otherwise secured by state or local government tax receipts. Generally, the value and credit quality of these bonds are sensitive to the risks related to an economic slowdown.INFLATION RISK. Inflation risk is the risk that the value of assets or income from investments will be less in the future as inflation decreases the value of money. As inflation increases, the present value of the Fund’s assets and distributions may decline.INTEREST RATE RISK. Interest rate risk is the risk that the value of the debt securities in the Fund’s portfolio will decline because of rising market interest rates. Interest rate risk is generally lower for shorter term debt securities and higher for longer-term debt securities. The Fund may be subject to a greater risk of rising interest rates than would normally be the case due to the current period of historically low rates and the effect of potential government fiscal policy initiatives and resulting market reaction to those initiatives. Duration is a reasonably accurate measure of a debt security’s price sensitivity to changes in interest rates and a common measure of interest rate risk. Duration measures a debt security’s expected life on a present value basis, taking into account the debt security’s yield, interest payments and final maturity. In general, duration represents the expected percentage change in the value of a security for an immediate 1% change in interest rates. For example, the price of a debt security with a three-year duration would be expected to drop by approximately 3% in response to a 1% increase in interest rates. Therefore, prices of debt securities with shorter durations tend to be less sensitive to interest rate changes than debt securities with longer durations. As the value of a debt security changes over time, so will its duration.MANAGEMENT RISK. The Fund is subject to management risk because it is an actively managed portfolio. In managing the Fund’s investment portfolio, the portfolio managers will apply investment techniques and risk analyses that may not produce the desired result. There can be no guarantee that the Fund will meet its investment objective(s).MARKET MAKER RISK. The Fund faces numerous market trading risks, including the potential lack of an active market for Fund shares due to a limited number of market markers. 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. The Fund 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 the Exchange, which could result in a decrease in value of the Fund’s shares. 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, 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. In addition, local, regional or global events such as war, acts of terrorism, spread of infectious diseases or other public health issues, recessions, or other events could have a significant negative impact on the Fund and its investments. For example, the coronavirus disease 2019 (COVID-19) global pandemic and the aggressive responses taken by many governments, including closing borders, restricting international and domestic travel, and the imposition of prolonged quarantines or similar restrictions, has had negative impacts, and in many cases severe impacts, on markets worldwide. Additionally, the COVID-19 pandemic has caused prolonged disruptions to the normal business operations of companies around the world and the impact of such disruptions is hard to predict. Such events may affect certain geographic regions, countries, sectors and industries more significantly than others. Such events could adversely affect the prices and liquidity of the Fund’s portfolio securities or other instruments and could result in disruptions in the trading markets. Any of such circumstances could have a materially negative impact on the value of the Fund’s shares and result in increased market volatility. During any such events, the Fund’s shares may trade at increased premiums or discounts to their net asset value.MUNICIPAL SECURITIES MARKET LIQUIDITY RISK. From time to time, inventories of municipal securities held by brokers and dealers may decrease, lessening their ability to make a market in these securities. Any reduction in market making capacity has the potential to decrease the Fund’s ability to buy or sell municipal securities, and increase price volatility and trading costs, particularly during periods of economic or market stress. As a result, the Fund may be forced to accept a lower price to sell a municipal security, to sell other securities to raise cash, or to give up an investment opportunity, any of which could have a negative effect on performance.MUNICIPAL SECURITIES RISK. Issuers, including governmental issuers, may be unable to pay their obligations as they come due. The values of municipal securities that depend on a specific revenue source to fund their payment obligations may fluctuate as a result of actual or anticipated changes in the cash flows generated by the revenue source or changes in the priority of the municipal obligation to receive the cash flows generated by the revenue source. The values of municipal securities held by the Fund may be adversely affected by local political and economic conditions and developments. Adverse conditions in an industry significant to a local economy could have a correspondingly adverse effect on the financial condition of local issuers. In addition, income from municipal securities held by the Fund could be declared taxable because of, among other things, unfavorable changes in tax laws, adverse interpretations by the Internal Revenue Service or state tax authorities, or noncompliant conduct of an issuer or other obligated party. Loss of tax-exempt status may cause interest received and distributed to shareholders by the Fund to be taxable and may result in a significant decline in the values of such municipal securities.MUNICIPAL SECURITIES VALUATION RISK. The municipal securities in which the Fund invests are typically valued by a pricing service utilizing a range of market-based inputs and assumptions, including readily available market quotations obtained from broker-dealers making markets in such securities, cash flows and transactions for comparable instruments. There is no assurance that the Fund will be able to sell a portfolio security at the price established by the pricing service, which could result in a loss to the Fund.PREMIUM/DISCOUNT RISK. The market price of the Fund’s shares will generally fluctuate in accordance with changes in the Fund’s net asset value as well as the relative supply of and demand for shares on the Exchange. The Fund’s investment advisor cannot predict whether shares will trade below, at or above their net asset value because the shares trade on the Exchange 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, 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 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. During stressed market conditions, the market for the Fund’s shares may become less liquid in response to deteriorating liquidity in the market for the Fund’s underlying portfolio holdings, which could in turn lead to differences between the market price of the Fund’s shares and their net asset value.PREPAYMENT RISK. Prepayment risk is the risk that the issuer of a debt security will repay principal prior to the scheduled maturity date. Debt securities allowing prepayment may offer less potential for gains during a period of declining interest rates, as the Fund may be required to reinvest the proceeds of any prepayment at lower interest rates. These factors may cause the value of an investment in the Fund to change.PRIVATE ACTIVITY BONDS RISK. Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise is responsible for the payment of principal and interest on the bond and the issuer ordinarily does not pledge its full faith, credit and taxing power for repayment. The private enterprise can have a substantially different credit profile than the issuer. The private activity bonds in which the Fund may invest may be negatively impacted by conditions affecting either the general credit of the private enterprise or the project itself. The Fund’s private activity bond holdings may also pay interest subject to the alternative minimum tax.RESTRICTED SECURITIES RISK. Restricted securities are securities that cannot be offered for public resale unless registered under the applicable securities laws or that have a contractual restriction that prohibits or limits their resale. The Fund may be unable to sell a restricted security on short notice or may be able to sell them only at a price below current value.SIGNIFICANT EXPOSURE RISK. To the extent that the Fund invests a large percentage of its assets in a single asset class or the securities of issuers within the same country, state, region, industry or sector, an adverse economic, business or political development may affect the value of the Fund’s investments more than if the Fund were more broadly diversified. A significant exposure makes the Fund more susceptible to any single occurrence and may subject the Fund to greater market risk than a fund that is more broadly diversified.TRADING ISSUES RISK. Trading in shares on the Exchange may be halted due to market conditions or for reasons that, in the view of the Exchange, make trading in shares inadvisable. In addition, trading in shares on the Exchange is subject to trading halts caused by extraordinary market volatility pursuant to the Exchange’s “circuit breaker” rules. There can be no assurance that the requirements of the Exchange 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 the Exchange in the event the Fund’s assets are small, the Fund does not have enough shareholders, or if the Fund is unable to proceed with creation and/or redemption orders.ZERO COUPON BOND RISK. Zero coupon bonds do not pay interest on a current basis and may be highly volatile as interest rates rise or fall. Although zero coupon bonds generate income for accounting purposes, they do not produce cash flow, and thus the Fund could be forced to liquidate securities at an inopportune time in order to generate cash to distribute to shareholders as required by tax laws.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;margin-left:0%;">Annual Total Return</span>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	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 compared to those of a broad-based market index and a blended benchmark. 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.
Bar Chart [Heading]	rr_BarChartHeading	<span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;margin-left:0%;">First Trust Municipal High Income ETF</span><span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;">Calendar Year Total Returns as of 12/31 </span><span style="color:#000000;font-family:Arial;font-size:7pt;font-weight:bold;position:relative;top:-3.25pt;">(1)</span>
Bar Chart Footnotes [Text Block]	rr_BarChartFootnotesTextBlock	(1)The Fund's calendar year-to-date total return based on net asset value for the period 12/31/19 to 09/30/20 was -0.30%.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the periods shown in the chart above:Best Quarter
Performance Table Heading	rr_PerformanceTableHeading	<span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;margin-left:0%;">Average Annual Total Returns for the Periods Ended December 31, 2019</span>
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.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.
First Trust Municipal High Income ETF | First Trust Municipal High Income ETF
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
Other Expenses	rr_OtherExpensesOverAssets	none
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.70%
Fee Waiver	rr_FeeWaiverOrReimbursementOverAssets	0.15%	[1]
Total Annual Fund Operating Expenses After Fee Waiver	rr_NetExpensesOverAssets	0.55%
1 Year	rr_ExpenseExampleNoRedemptionYear01	$ 56
3 Years	rr_ExpenseExampleNoRedemptionYear03	209
5 Years	rr_ExpenseExampleNoRedemptionYear05	375
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 856
2018	rr_AnnualReturn2018	2.04%
2019	rr_AnnualReturn2019	10.58%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	<span style="color:#000000;font-family:Arial;font-size:9.00pt;font-weight:bold;">Best Quarter</span>
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2019
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	3.62%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	<span style="color:#000000;font-family:Arial;font-size:9.00pt;font-weight:bold;">Worst Quarter</span>
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2018
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(0.35%)
1 Year	rr_AverageAnnualReturnYear01	10.58%
Since Inception	rr_AverageAnnualReturnSinceInception	6.45%
Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 01, 2017
First Trust Municipal High Income ETF | After tax on distributions | First Trust Municipal High Income ETF
Risk Return Abstract	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	9.05%
Since Inception	rr_AverageAnnualReturnSinceInception	5.02%
First Trust Municipal High Income ETF | After tax on distributions and sale of fund shares | First Trust Municipal High Income ETF
Risk Return Abstract	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	6.23%
Since Inception	rr_AverageAnnualReturnSinceInception	4.31%
First Trust Municipal High Income ETF | Bloomberg Barclays Municipal Bond Index (reflects no deduction for fees, expenses or taxes)
Risk Return Abstract	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	7.54%
Since Inception	rr_AverageAnnualReturnSinceInception	4.28%
First Trust Municipal High Income ETF | Blended Benchmark (reflects no deduction for fees, expenses or taxes)
Risk Return Abstract	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	8.26%	[2]
Since Inception	rr_AverageAnnualReturnSinceInception	5.70%	[2]

[1] Pursuant to a contractual agreement, First Trust Advisors L.P., the Fund's investment advisor, has agreed to waive management fees of 0.15% of average daily net assets until November 30, 2021. The waiver agreement may be terminated by action of the Trust's Board of Trustees at any time upon 60 days' written notice by the Trust, on behalf of the Fund, or by the Fund's investment advisor only after November 30, 2021.

[2] The Blended Benchmark consists of the following two indexes: 50% of the Bloomberg Barclays High Yield 10-Year Municipal Index (8-12 years), an index comprised of bonds with a final maturity between 8 and 12 years that are part of the Bloomberg Barclays Municipal Bond High Yield Index; and 50% of the Bloomberg Barclays Revenue 10-Year Municipal Index (8-12 years), which is comprised of revenue bonds that have a final maturity between 8 and 12 years that are part of the Bloomberg Barclays Municipal Bond Index.The Blended Index returns are calculated by using the monthly return of the two indices during each period shown above. At the beginning of each month the two indices are rebalanced to a 50-50 ratio to account for divergence from that ratio that occurred during the course of each month. The monthly returns are then compounded for each period shown above, giving the performance for the Blended Index for each period shown above.