First Trust Flexible Municipal High Income ETF S000053970 [Member] C000169822 [Member] Bloomberg Municipal Long Bond (22+) Index (reflects no deduction for fees, expenses or taxes)[Member] MFLX Blended Benchmark (reflects no deduction for fees, expenses or taxes)[Member] Bloomberg Municipal Bond Index (reflects no deduction for fees, expenses or taxes)[Member] Alternative Minimum Tax Risk [Member] Authorized Participant Concentration Risk [Member] Call Risk [Member] Cash Transactions Risk [Member] Counterparty Risk [Member] Credit Rating Agency Risk [Member] Credit Risk [Member] Credit Spread Risk [Member] Current Market Conditions Risk [Member] Custodial Receipts Trust Risk [Member] Cyber Security Risk [Member] Debt Securities Risk [Member] Derivatives Risk [Member] Distressed Securities Risk [Member] Extension Risk [Member] Futures Contracts Risk [Member] High Yield Securities Risk [Member] Income Risk [Member] Industrial Development Bond Risk [Member] Inflation Risk [Member] Interest Rate Risk [Member] Inverse Floaters Risk [Member] Management Risk [Member] Market Maker Risk [Member] Market Risk [Member] Municipal Lease Participations Risk [Member] Municipal Securities Market Liquidity Risk [Member] Municipal Securities Risk [Member] Operational Risk [Member] Premium Discount Risk [Member] Prepayment Risk [Member] Pre Refunded Bonds Risk [Member] Private Activity Bonds Risk [Member] Significant Exposure Risk [Member] Tender Option Bond Risk [Member] Trading Issues Risk [Member] Valuation Risk [Member]