v3.24.3
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Risk Return Abstract rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName First Trust Exchange-Traded Fund III
Prospectus Date rr_ProspectusDate Dec. 02, 2024
First Trust Ultra Short Duration Municipal 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;">Investment Objective</span>
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The First Trust Ultra Short Duration Municipal ETF (the "Fund") seeks to provide federally tax-exempt income consistent with capital preservation.
Expense [Heading] rr_ExpenseHeading <span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;">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;">Shareholder Fees</span> <br/><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;">Annual Fund Operating Expenses</span> <br/><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;">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 112% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 112.00%
Expense Example [Heading] rr_ExpenseExampleHeading <span style="color:#000000;font-family:Arial;font-size:9.00pt;font-style:italic;font-weight:bold;">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.The example assumes that you invest $10,000 in the Fund for the time periods indicated and then hold or sell all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain at current levels. 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;">Principal Investment Strategies</span>
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 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. Under normal market conditions, the weighted average duration of the Fund’s portfolio is expected to be less than one year. In selecting the Fund’s portfolio, the portfolio managers will focus on Municipal Securities that meet basic infrastructure needs or provide critical services within their communities. The Fund will invest in Municipal Securities issued throughout the United States, including in Municipal Securities issued by entities in at least 10 different states, with an emphasis on Municipal Securities issued in states with growing populations and healthy employment trends.Duration is a mathematical calculation of the average life of a debt security (or portfolio of debt securities) that serves as a measure of its price risk. In general, each year of duration represents an expected 1% change in the value of a security for every 1% immediate change in interest rates. For example, if a portfolio of Municipal Securities has an average duration of three years, its value can be expected to fall about 3% if interest rates rise by 1%. Conversely, the portfolio’s value can be expected to rise about 3% if interest rates fall by 1%. As a result, prices of instruments with shorter durations tend to be less sensitive to interest rate changes than instruments with longer durations. As the value of a security changes over time, so will its duration. Under normal market conditions, the Fund will invest at least 80% of its net assets in Municipal Securities that are, at the time of investment, rated as investment grade (i.e., rated Baa3/BBB- or above) by at least one nationally recognized statistical rating organization rating such securities or, if unrated, Municipal Securities determined by the Fund’s investment advisor to be of comparable quality. The types of Municipal Securities in which the Fund may invest as a part of its principal investment strategy include municipal general obligation bonds, municipal revenue bonds, municipal notes, alternative minimum tax bonds, private activity bonds (including without limitation industrial development bonds), and pre-refunded and escrowed to maturity bonds. Interest on alternative minimum tax bonds and private activity bonds may be subject to the federal alternative minimum tax on individuals. For tax years beginning after December 31, 2022, interest on the bonds may affect the corporate alternative minimum tax for certain corporations. The Fund may also purchase new issues of Municipal Securities on a when-issued or forward commitment basis. The Municipal Securities in which the Fund invests may be fixed, variable or floating-rate securities.
Strategy Portfolio Concentration [Text] rr_StrategyPortfolioConcentration <span style="color:#000000;font-family:Arial;font-size:9.00pt;">In selecting the Fund’s portfolio, the portfolio managers will focus on Municipal Securities that meet basic </span><span style="color:#000000;font-family:Arial;font-size:9.00pt;margin-left:0%;">infrastructure needs or provide critical services within their communities. The Fund will invest in Municipal Securities issued throughout the United States, including in Municipal Securities issued by entities in at least 10 different states, with an emphasis </span><span style="color:#000000;font-family:Arial;font-size:9.00pt;">on Municipal Securities issued in states with growing populations and healthy employment trends.</span>
Risk [Heading] rr_RiskHeading <span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;">Principal Risks</span>
Risk Caption rr_RiskCaption <span style="color:#000000;font-family:Arial;font-size:9.00pt;margin-left:0%;">You could lose money by investing in the Fund.</span><span style="color:#000000;font-family:Arial;font-size:9.00pt;">An investment in the Fund is not a deposit of a bank and is not insured or </span><span style="color:#000000;font-family:Arial;font-size:9.00pt;margin-left:0%;">guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.</span><span style="color:#000000;font-family:Arial;font-size:9.00pt;"> There can be no assurance that </span><span style="color:#000000;font-family:Arial;font-size:9.00pt;margin-left:0%;">the Fund’s investment objective will be achieved. The order of the below risk factors does not indicate the significance of any </span><span style="color:#000000;font-family:Arial;font-size:9.00pt;">particular risk factor.</span>
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading <span style="color:#000000;font-family:Arial;font-size:9.90pt;font-weight:bold;">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 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 market index and a broad-based market index. The Fund’s performance information is accessible on the Fund’s website at http://www.ftportfolios.com.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns <span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">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 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 market index</span><span style="font-family:Arial;font-size:9.00pt;line-height:10.80pt;"> </span><span style="font-family:Arial;font-size:9.00pt;">and a broad-based market index. </span>
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress <span style="font-family:Arial;font-size:9.00pt;">http://www.ftportfolios.com</span>
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture <span style="color:#000000;font-family:Arial;font-size:9.00pt;">The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.</span>
Bar Chart [Heading] rr_BarChartHeading <span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">First Trust Ultra Short Duration Municipal ETF</span> <br/><span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">Calendar Year Total Returns as of 12/31 </span><span style="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 12/31/23 to 09/30/24 was 2.26%.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock During the periods shown in the chart above:ReturnPeriod EndedBest Quarter1.50%December 31, 2023Worst Quarter-0.80%March 31, 2022Year-to-Date2.26%September 30, 2024
Performance Table Heading rr_PerformanceTableHeading <span style="font-family:Arial;font-size:9.90pt;font-weight:bold;">Average Annual Total Returns for the Periods Ended December 31, 2023</span>
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate <span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">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.</span>
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred <span style="color:#000000;font-family:Arial;font-size:9.00pt;margin-left:0%;">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 </span><span style="color:#000000;font-family:Arial;font-size:9.00pt;">(</span><span style="color:#000000;font-family:Arial;font-size:9.00pt;font-style:italic;">IRAs</span><span style="color:#000000;font-family:Arial;font-size:9.00pt;">) or employee-sponsored retirement plans.</span>
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher <span style="font-family:Arial;font-size:9.00pt;">Returns after taxes on distributions reflect the taxed return on the payment of dividends </span><span style="font-family:Arial;font-size:9.00pt;margin-left:0%;">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 </span><span style="font-family:Arial;font-size:9.00pt;">an index do not include expenses, which are deducted from Fund returns, or taxes.</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.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 an index do not include expenses, which are deducted from Fund returns, or taxes.Your own actual after-tax returns will depend on your specific tax situation and may differ from what is shown here. After-tax returns are not relevant to investors who hold Fund shares in tax-deferred accounts such as individual retirement accounts (IRAs) or employee-sponsored retirement plans.
First Trust Ultra Short Duration Municipal ETF | Risk Lose Money [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock You could lose money by investing in the Fund.
First Trust Ultra Short Duration Municipal ETF | Risk Not Insured Depository Institution [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Alternative Minimum Tax Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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 on individuals. For tax years beginning after December 31, 2022, interest on the bonds may affect the corporate alternative minimum tax for certain corporations. Accordingly, investment in the Fund could cause shareholders to be subject to the federal alternative minimum tax.
First Trust Ultra Short Duration Municipal ETF | Authorized Participant Concentration Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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 (the difference between the market price of the Fund's shares and the Fund's net asset value) and possibly face delisting and the bid/ask spread (the difference between the price that someone is willing to pay for shares of the Fund at a specific point in time versus the price at which someone is willing to sell) on the Fund’s shares may widen.
First Trust Ultra Short Duration Municipal ETF | Call Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Cash Transactions Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock CASH TRANSACTIONS RISK. The Fund will, under most circumstances, effect a significant portion of its 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 portfolio securities may result in capital gains or losses and may also result in higher brokerage costs.
First Trust Ultra Short Duration Municipal ETF | Credit Rating Agency Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock CREDIT RATING AGENCY RISK. Credit ratings are determined by credit rating agencies such as S&P Global Ratings, Moody’s Investors Services, Inc. and Fitch Inc., and are only the opinions of such entities. Ratings assigned by a rating agency are not absolute standards of credit quality and do not evaluate market risk or the liquidity of securities. First Trust makes no warranty whatsoever regarding the ability of such ratings to accurately reflect the creditworthiness of an issuer. Any shortcomings, changes to or inefficiencies in credit rating agencies’ processes for determining credit ratings may adversely affect the credit ratings of securities held by the Fund or securities in which the Fund would otherwise invest and, as a result, may adversely affect those securities’ perceived or actual credit risk, as well as the Fund’s performance.
First Trust Ultra Short Duration Municipal ETF | Credit Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Credit Spread Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Current Market Conditions Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock CURRENT MARKET CONDITIONS RISK. Current market conditions risk is the risk that a particular investment, or shares of the Fund in general, may fall in value due to current market conditions. As a means to fight inflation, which remains at elevated levels, the Federal Reserve and certain foreign central banks have raised interest rates; however, the Federal Reserve has recently lowered interest rates and may continue to do so. U.S. regulators have proposed several changes to market and issuer regulations which would directly impact the Fund, and any regulatory changes could adversely impact the Fund’s ability to achieve its investment strategies or make certain investments. Recent and potential future bank failures could result in disruption to the broader banking industry or markets generally and reduce confidence in financial institutions and the economy as a whole, which may also heighten market volatility and reduce liquidity. Additionally, challenges in commercial real estate markets, including rising interest rates, declining valuations and increasing vacancies, could have a broader impact on financial markets. The ongoing adversarial political climate in the United States, as well as political and diplomatic events both domestic and abroad, have and may continue to have an adverse impact the U.S. regulatory landscape, markets and investor behavior, which could have a negative impact on the Fund’s investments and operations. The change in administration resulting from the 2024 United States national elections could result in significant impacts to international trade relations, tax and immigration policies, and other aspects of the national and international political and financial landscape, which could affect, among other things, inflation and the securities markets generally. Other unexpected political, regulatory and diplomatic events within the U.S. and abroad may affect investor and consumer confidence and may adversely impact financial markets and the broader economy. For example, ongoing armed conflicts between Russia and Ukraine in Europe and among Israel, Iran, Hamas and other militant groups in the Middle East, have caused and could continue to cause significant market disruptions and volatility within the markets in Russia, Europe, the Middle East and the United States. The hostilities and sanctions resulting from those hostilities have and could continue to have a significant impact on certain Fund investments as well as Fund performance and liquidity. The economies of the United States and its trading partners, as well as the financial markets generally, may be adversely impacted by trade disputes and other matters. For example, the United States has imposed trade barriers and restrictions on China. In addition, the Chinese government is engaged in a longstanding dispute with Taiwan, continually threatening an invasion. If the political climate between the United States and China does not improve or continues to deteriorate, if China were to attempt invading Taiwan, or if other geopolitical conflicts develop or worsen, economies, markets and individual securities may be adversely affected, and the value of the Fund’s assets may go down. A public health crisis and the ensuing policies enacted by governments and central banks may cause significant volatility and uncertainty in global financial markets, negatively impacting global growth prospects. As the COVID-19 global pandemic illustrated, such events may affect certain geographic regions, countries, sectors and industries more significantly than others. Advancements in technology may also adversely impact markets and the overall performance of the Fund. For instance, the economy may be significantly impacted by the advanced development and increased regulation of artificial intelligence. Additionally, cyber security breaches of both government and non-government entities could have negative impacts on infrastructure and the ability of such entities, including the Fund, to operate properly. These events, and any other future events, may adversely affect the prices and liquidity of the Fund’s portfolio investments and could result in disruptions in the trading markets.
First Trust Ultra Short Duration Municipal ETF | Cyber Security Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Debt Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Extension Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Floating Rate Debt Instruments Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock FLOATING RATE DEBT INSTRUMENTS RISK. Investments in floating rate debt instruments are subject to the same risks as investments in other types of debt securities, including credit risk, interest rate risk, liquidity risk and valuation risk. Floating rate debt instruments include debt securities issued by corporate and governmental entities, as well as bank loans, mortgage-backed securities and asset-backed securities. Floating rate debt instruments are structured so that the security’s coupon rate fluctuates based upon the level of a reference rate. Most commonly, the coupon rate of a floating rate debt instrument is set at the level of a widely followed interest rate, plus a fixed spread. As a result, the coupon on floating rate debt instrument will generally decline in a falling interest rate environment, causing the Fund to experience a reduction in the income it receives from the instrument. A floating rate debt instrument’s coupon rate resets periodically according to its terms. Consequently, in a rising interest rate environment, floating rate debt instruments with coupon rates that reset infrequently may lag behind the changes in market interest rates. Floating rate debt instruments may also contain terms that impose a maximum coupon rate the issuer will pay, regardless of the level of the reference rate. To the extent the Fund invests in floating rate loans, such instruments may be subject to legal or contractual restrictions on resale, may trade infrequently, and their value may be impaired when the Fund needs to liquidate such securities. It is possible that the collateral securing a floating rate loan may be insufficient or unavailable to the Fund, and that the Fund’s rights to collateral may be limited by bankruptcy or insolvency laws. Additionally, floating rate loans may not be considered “securities” under federal securities laws, and purchasers, such as the Fund, therefore may not be entitled to rely on the anti-fraud protections of the federal securities laws.
First Trust Ultra Short Duration Municipal ETF | Income Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Index Or Model Constituent Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock INDEX OR MODEL CONSTITUENT RISK. The Fund may be a constituent of one or more indices or ETF models. 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.
First Trust Ultra Short Duration Municipal ETF | Industrial Development Bond Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Inflation Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Interest Rate Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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 during periods of low interest rates. 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. Higher sensitivity to interest rates is generally correlated with higher levels of volatility and, therefore, greater risk. As the value of a debt security changes over time, so will its duration.
First Trust Ultra Short Duration Municipal ETF | Management Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Market Maker Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Market Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock MARKET RISK. Market risk is the risk that a particular investment, or shares of the Fund in general, may fall in value. Securities are subject to market fluctuations caused by real or perceived adverse economic, political, and regulatory factors 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, market manipulation, government defaults, government shutdowns, regulatory actions, political changes, diplomatic developments, the imposition of sanctions and other similar measures, spread of infectious diseases or other public health issues, recessions, natural disasters, or other events could have a significant negative impact on the Fund and its investments. Any of such circumstances could have a materially negative impact on the value of the Fund’s shares, the liquidity of an investment, and may 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, the bid/ask spread on the Fund’s shares may widen and the returns on investment may fluctuate.
First Trust Ultra Short Duration Municipal ETF | Municipal Securities Market Liquidity Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Municipal Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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. Due to the COVID-19 pandemic, the risks of the municipal securities market have been magnified. The costs associated with combating the pandemic and the negative impact on tax revenues has adversely affected the financial condition of many states and political subdivisions. These risks may also adversely affect several sectors of the municipal bond market, such as airports, toll roads, hospitals and colleges, among many others. The full impact of the COVID-19 pandemic on state and political subdivisions’ ability to make payments on debt obligations is impossible to predict, but could negatively impact the value of bonds, the ability of state and political subdivisions to make payments when due and the performance of the Fund. Municipal securities issued by U.S. territories, including, but not limited to, Puerto Rico, the U.S. Virgin Islands and Guam, may face a heightened risk of default due to adverse economic conditions in such territories, which could negatively impact the value of any Fund investments in U.S. territorial issuers and the Fund’s investment performance.
First Trust Ultra Short Duration Municipal ETF | Operational Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock OPERATIONAL RISK. The Fund is subject to risks arising from various operational factors, including, but not limited to, human error, processing and communication errors, errors of the Fund’s service providers, counterparties or other third-parties, failed or inadequate processes and technology or systems failures. The Fund relies on third-parties for a range of services, including custody. Any delay or failure relating to engaging or maintaining such service providers may affect the Fund’s ability to meet its investment objective. Although the Fund and the Fund's investment advisor seek to reduce these operational risks through controls and procedures, there is no way to completely protect against such risks.
First Trust Ultra Short Duration Municipal ETF | Portfolio Turnover Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock PORTFOLIO TURNOVER RISK. High portfolio turnover may result in the Fund paying higher levels of transaction costs and may generate greater tax liabilities for shareholders. Portfolio turnover risk may cause the Fund’s performance to be less than expected.
First Trust Ultra Short Duration Municipal ETF | Premium Discount Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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 and the bid/ask spread on the Fund’s shares may widen.
First Trust Ultra Short Duration Municipal ETF | Prepayment Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
First Trust Ultra Short Duration Municipal ETF | Pre Refunded Bonds Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock PRE-REFUNDED BONDS RISK. 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 the 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 the Fund may subject the Fund to interest rate risk, market risk and credit risk.
First Trust Ultra Short Duration Municipal ETF | Private Activity Bonds Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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 federal alternative minimum tax on individuals. For tax years beginning after December 31, 2022, interest on the bonds may affect the corporate alternative minimum tax for certain corporations.
First Trust Ultra Short Duration Municipal ETF | Significant Exposure Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock SIGNIFICANT EXPOSURE RISK. To the extent that the Fund invests a significant 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.
First Trust Ultra Short Duration Municipal ETF | Trading Issues Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock TRADING ISSUES RISK. Trading in Fund 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 Fund 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.
First Trust Ultra Short Duration Municipal ETF | Valuation Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock VALUATION RISK. The sale price the Fund could receive for a security may differ from the Fund’s valuation of the security, particularly for securities that trade in low volume or volatile markets or that are valued using a fair value methodology. Unlike publicly traded securities that trade on national securities exchanges, there is no central place or exchange for trading most debt securities. Debt securities generally trade on an “over-the-counter” market. Due to the lack of centralized information and trading, and variations in lot sizes of certain debt securities, the valuation of debt securities may carry more uncertainty and risk than that of publicly traded securities. Debt securities are commonly valued by third-party pricing services that utilize 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. However, because the available information is less reliable and more subjective, elements of judgment may play a greater role in valuation of debt securities than for other types of securities. Different pricing services may incorporate different assumptions and inputs into their valuation methodologies, potentially resulting in different values for the same securities. As a result, if the Advisor were to change its valuation policies, or if the Fund were to change pricing services, or if a pricing service were to change its valuation methodology, there could be a material impact, either positive or negative, on the Fund’s net asset value. Additionally, pricing services generally price debt securities assuming orderly transactions of an institutional “round lot” size, but some trades may occur in smaller, “odd lot” sizes, often at lower prices than institutional round lot trades. 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. In addition, the value of the debt securities in the Fund's portfolio may change on days or during time periods when shareholders will not be able to purchase or sell the Fund's shares. Authorized Participants who purchase or redeem Fund shares on days when the Fund is holding fair-valued securities may receive fewer or more shares, or lower or higher redemption proceeds, than they would have received had the Fund not fair-valued securities or used a different valuation methodology. Net asset value calculation may also be impacted by operational risks arising from factors such as failures in systems and technology.
First Trust Ultra Short Duration Municipal ETF | When Issued and Delayed Delivery Transactions Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock WHEN-ISSUED AND DELAYED DELIVERY TRANSACTIONS RISK. The Fund may purchase securities on a when-issued, delayed delivery or forward commitment basis. In such a transaction, the purchase price of the securities is typically fixed at the time of the commitment, but delivery and payment can take place some time after the date of the commitment. At the time of delivery of the securities, the value may be more or less than the purchase or sale price. Purchasing securities on a when-issued, delayed delivery or forward commitment basis may give rise to investment leverage and may increase the Fund’s volatility. Default by, or bankruptcy of, a counterparty to a when-issued, delayed delivery or forward commitment transaction would expose the Fund to possible losses because of an adverse market action, expenses or delays in connection with the purchase or sale of the pools specified in such transaction.
First Trust Ultra Short Duration Municipal ETF | First Trust Ultra Short Duration Municipal ETF  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)[ctag:span_t-rule1] rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Management Fees[ctag:span_t-indent1] rr_ManagementFeesOverAssets 0.45%
Distribution and Service (12b-1) Fees[ctag:span_t-indent1] rr_DistributionAndService12b1FeesOverAssets none
Other Expenses[ctag:span_t-indent1-rule] rr_OtherExpensesOverAssets none
Total Annual Fund Operating Expenses[ctag:span_t-indent1-rule3] rr_ExpensesOverAssets 0.45%
1 Year rr_ExpenseExampleNoRedemptionYear01 $ 46
3 Years rr_ExpenseExampleNoRedemptionYear03 144
5 Years rr_ExpenseExampleNoRedemptionYear05 252
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 567
2019 rr_AnnualReturn2019 1.81%
2020 rr_AnnualReturn2020 1.40%
2021 rr_AnnualReturn2021 0.33%
2022 rr_AnnualReturn2022 (0.08%)
2023 rr_AnnualReturn2023 3.20%
Year to Date Return, Label rr_YearToDateReturnLabel <span style="font-family:Arial;font-size:9.00pt;margin-left:0.0pt;">Year-to-Date</span>
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Sep. 30, 2024
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 2.26%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel <span style="font-family:Arial;font-size:9.00pt;margin-left:0.0pt;">Best Quarter</span>
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2023
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 1.50%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel <span style="font-family:Arial;font-size:9.00pt;margin-left:0.0pt;">Worst Quarter</span>
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2022
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (0.80%)
1 Year rr_AverageAnnualReturnYear01 3.20%
5 Years rr_AverageAnnualReturnYear05 1.32%
Since Inception rr_AverageAnnualReturnSinceInception 1.37%
Inception Date rr_AverageAnnualReturnInceptionDate Nov. 01, 2018
First Trust Ultra Short Duration Municipal ETF | After tax on distributions | First Trust Ultra Short Duration Municipal ETF  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 2.26%
5 Years rr_AverageAnnualReturnYear05 0.80%
Since Inception rr_AverageAnnualReturnSinceInception 0.85%
First Trust Ultra Short Duration Municipal ETF | After tax on distributions and sale of fund shares | First Trust Ultra Short Duration Municipal ETF  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.89%
5 Years rr_AverageAnnualReturnYear05 0.79%
Since Inception rr_AverageAnnualReturnSinceInception 0.83%
First Trust Ultra Short Duration Municipal ETF | Bloomberg Municipal Bond Index (reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 6.40%
5 Years rr_AverageAnnualReturnYear05 2.25%
Since Inception rr_AverageAnnualReturnSinceInception 2.65%
First Trust Ultra Short Duration Municipal ETF | Bloomberg Municipal Short-Term Index (reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 3.45%
5 Years rr_AverageAnnualReturnYear05 1.43%
Since Inception rr_AverageAnnualReturnSinceInception 1.47%