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. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

Under normal market conditions, the Fund seeks to achieve its investment objective by investing at least 80% of its net assets (including investment borrowings) in a portfolio of closed-end investment companies that are listed and traded in the United States on registered exchanges which invest primarily in municipal debt securities, some or all of which pay interest that is exempt from regular federal income taxes. Closed-end funds issue shares of common stock that are traded on a securities exchange. Because the shares of closed-end funds cannot be redeemed upon demand to the issuer like the shares of an open-end investment company (such as the Fund), investors seek to buy and sell shares of closed-end funds in the secondary market.

Municipal securities are generally issued by or on behalf of states, territories or possessions of the United States and the District of Columbia and their political subdivisions, agencies, authorities and other instrumentalities. The closed-end funds in which the Fund invests may invest in a range of municipal securities, including, but not limited to, 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, the Fund may invest in closed-end funds that hold inverse floating rate securities issued by tender option bond ("TOB") trusts and securities issued by custodial receipt trusts, each of which are investment vehicles the underlying assets of which are municipal bonds. The Fund may invest in closed-end funds that hold municipal securities of any maturity, duration or credit quality, including high yield securities, also known as "junk" bonds.

In selecting closed-end funds for the Fund's portfolio, the Fund's investment advisor utilizes a range of investment approaches. The Fund's investment advisor generally takes a systemic approach to investing, including the utilization of a proprietary model that identifies, sorts and scores closed-end funds based upon various market metrics and economic factors, including, but not limited to, fund size, duration, leverage ratio, average maturity, earnings rate, undistributed net investment income, distribution rate, premium or discount, net asset value and share price returns, sponsor and distribution policies. In addition, the Fund may also invest in exchange-traded funds ("ETFs").

The Fund is classified as "non-diversified" under the Investment Company Act of 1940, as amended (the "1940 Act").

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

You could lose money by investing in the Fund. An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. There can be no assurance that the Fund's investment objective will be achieved. 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 closed-end funds and/or ETFs that hold 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. The Fund invests in closed-end funds and/or ETFs that hold debt securities. 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. An underlying fund is subject to the possibility that during periods of falling interest rates an issuer will call its high yielding debt securities. An underlying fund would then be forced to invest the proceeds at lower interest rates, likely resulting in a decline in the fund's income.

CLOSED-END FUNDS RISK. Shares of closed-end funds trade on exchanges at market prices rather than net asset value and cannot be redeemed on demand. Accordingly, closed-end fund shares may trade at a price greater than net asset value (premium) or less than net asset value (discount). There can be no assurance that a discount on shares of closed-end funds purchased by the Fund will not decrease or that when the Fund seeks to sell shares of a closed-end fund it can receive the net asset value for those shares. As a shareholder in a closed-end fund, the Fund bears its ratable share of the fund's expenses, subjecting Fund shareholders to additional expenses. Additionally, closed-end funds may utilize leverage. As a result, the Fund may be exposed indirectly to leverage through an investment in such securities.

COUNTERPARTY RISK. The Fund may invest in closed-end funds and/or ETFs that are subject to counterparty risk. Fund transactions involving a counterparty are subject to the risk that the counterparty will not fulfill its obligation to the underlying fund. Counterparty risk may arise because of the counterparty's financial condition (i.e., financial difficulties, bankruptcy, or insolvency), market activities and developments, or other reasons, whether foreseen or not. A counterparty's inability to fulfill its obligation may result in significant financial loss to an underlying fund. An underlying fund may be unable to recover its investment from the counterparty or may obtain a limited recovery, and/or recovery may be delayed.

CREDIT RISK. An issuer or other obligated party of a debt security held by an underlying fund 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.

CUSTODIAL RECEIPTS TRUST RISK. The Fund may invest in closed-end funds and/or ETFs that hold custodial receipts. Custodial receipts are financial instruments sold through private placements that represent the right to receive future principal and interest payments on underlying municipal obligations. Custodial receipt trusts may also issue inverse floater securities. If an underlying fund was to hold inverse floaters issued by custodial receipt trusts, the fund would be subject to the risks of inverse floaters described herein. In particular, because the instruments may be leveraged, their market values may be more volatile than other types of debt securities.

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. The Fund invests in closed-end funds and/or ETFs that hold debt securities. 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 an underlying 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.

DERIVATIVES RISK. The Fund may invest in closed-end funds and/or ETFs that utilize derivative instruments. The use of derivative instruments involves risks different from, or possibly greater than, the risks associated with investing directly in securities and other traditional investments. These risks include: (i) the risk that the counterparty to a derivative transaction may not fulfill its contractual obligations; (ii) risk of mispricing or improper valuation; and (iii) the risk that changes in the value of the derivative may not correlate perfectly with the underlying asset. Derivative prices are highly volatile and may fluctuate substantially during a short period of time. Such prices are influenced by numerous factors that affect the markets, including, but not limited to: changing supply and demand relationships; government programs and policies; national and international political and economic events, changes in interest rates, inflation and deflation and changes in supply and demand relationships. Trading derivative instruments involves risks different from, or possibly greater than, the risks associated with investing directly in securities. Derivative contracts ordinarily have leverage inherent in their terms. The low margin deposits normally required in trading derivatives, including futures contracts, permit a high degree of leverage. Accordingly, a relatively small price movement may result in an immediate and substantial loss. The use of leverage may also cause an underlying fund to liquidate portfolio positions when it would not be advantageous to do so in order to satisfy its obligations or to meet collateral segregation requirements. The use of leveraged derivatives can magnify potential for gain or loss and, therefore, amplify the effects of market volatility on share price.

DISTRESSED SECURITIES RISK. The Fund may invest in closed-end funds and/or ETFs that hold distressed debt securities. Distressed debt securities are speculative and involve substantial risks in addition to the risks of investing in high-yield securities that are not in default. Generally, an underlying fund will not receive interest payments from the distressed securities it holds, and there is a substantial risk that the principal will not be repaid. In any reorganization or liquidation proceeding related to a distressed debt security, an underlying fund may lose its entire investment in the security.

ETF RISK. The Fund invests in ETFs, which subjects it to the risks of owning the securities underlying the ETF, as well as the same structural risks faced by an investor purchasing shares of the Fund, including authorized participant concentration risk, market maker risk, premium/discount risk and trading issues risk. As a shareholder in another ETF, the Fund bears its proportionate share of the ETF's expenses, subjecting Fund shareholders to duplicative expenses.

EXTENSION RISK. The Fund invests in closed-end funds and/or ETFs that hold debt securities. 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.

FLOATING RATE SECURITIES RISK. The Fund may invest in closed-end funds and/or ETFs that hold floating rate securities. Floating rate securities are structured so that the security's coupon rate fluctuates based upon the level of a reference rate. As a result, the coupon on floating rate securities will generally decline in a falling interest rate environment, causing an underlying fund to experience a reduction in the income it receives from the security. A floating rate security's coupon rate resets periodically according to the terms of the security. Consequently, in a rising interest rate environment, floating rate securities with coupon rates that reset infrequently may lag behind the changes in market interest rates. Floating rate securities may also contain terms that impose a maximum coupon rate the issuer will pay, regardless of the level of the reference rate which would decrease the value of the security.

HIGH YIELD SECURITIES RISK. The Fund may invest in closed-end funds and/or ETFs that hold high yield securities. 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 an underlying fund.

INCOME RISK. The Fund invests in closed-end funds and/or ETFs that hold debt securities. An underlying fund's income may decline when interest rates fall or if there are defaults in its portfolio. This decline can occur because an underlying 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.

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. The Fund invests in closed-end funds and/or ETFs that hold debt securities. Interest rate risk is the risk that the value of the debt securities in an underlying 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. An underlying 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.

INVERSE FLOATERS RISK. The Fund may invest in closed-end funds and/or ETFs that hold inverse floaters. The use of inverse floaters by an underlying fund creates effective leverage. Due to the leveraged nature of these investments, they will typically be more volatile and involve greater risk than the fixed rate municipal bonds underlying the inverse floaters. An investment in certain inverse floaters will involve the risk that an underlying fund could lose more than its original principal investment. Distributions on inverse floaters bear an inverse relationship to short-term municipal bond interest rates. Thus, distributions paid to an underlying fund on its inverse floaters will be reduced or even eliminated as short-term municipal bond interest rates rise and will increase when short-term municipal bond interest rates fall. Inverse floaters generally will underperform the market for fixed rate municipal bonds in a rising interest rate environment.

INVESTMENT RESTRICTION RISK. The Fund's investment in other investment companies is restricted by federal securities laws and the Fund's associated exemptive relief which limit the size of the position the Fund can take in another investment company. These limitations may prevent the Fund from purchasing shares of an investment company that it may have otherwise purchased pursuant to its principal investment strategy.

LEVERAGE RISK. The Fund may invest in closed-end funds and/or ETFs that utilize leverage. Leverage may result in losses that exceed the amount originally invested and may accelerate the rates of losses. Leverage tends to magnify, sometimes significantly, the effect of any increase or decrease in an underlying fund's exposure to an asset or class of assets and may cause the value of the fund's shares to be volatile and sensitive to market swings.

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.

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.

MUNICIPAL LEASE PARTICIPATIONS RISK. The Fund may invest in closed-end funds and/or ETFs that hold municipal lease participations. Participation interests in municipal leases pose special risks because many leases and contracts contain "non-appropriation" clauses that provide that the governmental issuer has no obligation to make future payments under the lease or contract unless money is appropriated for this purpose by the appropriate legislative body.

MUNICIPAL SECURITIES RISK. The Fund invests in closed-end funds and/or ETFs that hold municipal securities. 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 an underlying 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 an underlying 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 an underlying fund to be taxable and may result in a significant decline in the values of such municipal securities.

MUNICIPAL SECURITIES MARKET LIQUIDITY RISK. The Fund invests in closed-end funds and/or ETFs that hold municipal securities. 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 an underlying 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, an underlying 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 VALUATION RISK. The Fund invests in closed-end funds and/or ETFs that hold municipal securities. The municipal securities in which an underlying 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 an underlying 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.

NON-DIVERSIFICATION RISK. The Fund is classified as "non-diversified" under 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.

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.

PREPAYMENT RISK. The Fund invests in closed-end funds and/or ETFs that hold debt securities. 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 an underlying 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 an underlying fund to change.

PRE-REFUNDED BONDS RISK. The Fund may invest in closed-end funds and/or ETFs that hold pre-refunded bonds. Pre-refunded bonds are bonds that have been refunded to a call date prior to the final maturity of principal, or, in the case of pre-refunded bonds commonly referred to as "escrowed-to-maturity bonds," to the final maturity of principal, and remain outstanding in the municipal market. The payment of principal and interest of the pre-refunded bonds held by an underlying fund is funded from securities held in a designated escrow account where such securities are obligations of and carry the full faith and credit of the U.S. Department of the Treasury. The securities held in the escrow fund pledged to pay the principal and interest of the pre-refunded bond do not guarantee the price of the bond. Investment in pre-refunded municipal bonds held by an underlying fund may subject the fund to interest rate risk, market risk and credit risk.

PRIVATE ACTIVITY BONDS RISK. The Fund invests in closed-end funds and/or ETFs that hold private activity bonds. 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 an underlying fund may invest may be negatively impacted by conditions affecting either the general credit of the private enterprise or the project itself. An underlying fund's private activity bond holdings may also pay interest subject to the alternative minimum tax.

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. Although the shares of the Fund are listed for trading on the Exchange, there can be no assurance that an active trading market for such shares will develop or be maintained. Further, secondary markets may be subject to irregular trading activity and wide bid-ask spreads (which may be especially pronounced for smaller funds). 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. 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. In the event market makers cease making a market in the Fund's shares or authorized participants stop submitting purchase or redemption orders for Creation Units, Fund shares may trade at a larger premium or discount to their net asset value. 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 or the Fund does not have enough shareholders.

ZERO COUPON BOND RISK. The Fund may invest in closed-end funds and/or ETFs that hold zero coupon bonds. 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 an underlying 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.

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 two 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.

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 12/31/18 to 09/30/19 was 15.19%.