Average Annual Total Returns

12 Months Ended

60 Months Ended

99 Months Ended

Aug. 31, 2025

Dec. 31, 2024

Dec. 31, 2024

Dec. 31, 2024

Bloomberg Municipal Long Bond (22+) Index (reflects no deduction for fees, expenses or taxes)[Member]

Prospectus [Line Items]

Average Annual Return, Percent

1.40%

0.52%

1.86%

MFLX Blended Benchmark (reflects no deduction for fees, expenses or taxes)[Member]

Prospectus [Line Items]

Average Annual Return, Percent

^[1]

3.66%

1.85%

2.85%

Bloomberg Municipal Bond Index (reflects no deduction for fees, expenses or taxes)[Member]

Prospectus [Line Items]

Average Annual Return, Percent

1.05%

0.99%

1.83%

First Trust Flexible Municipal High Income ETF

Prospectus [Line Items]

Average Annual Return, Percent

4.65%

1.06%

1.73%

Performance Inception Date

Sep. 27, 2016

First Trust Flexible Municipal High Income ETF | After Taxes on Distributions

Prospectus [Line Items]

Average Annual Return, Percent

4.65%

1.06%

1.72%

First Trust Flexible Municipal High Income ETF | After Taxes on Distributions and Sales

Prospectus [Line Items]

Average Annual Return, Percent

4.34%

1.67%

2.14%

[1]	The Blended Benchmark returns are a 50/50 split between the Bloomberg Municipal High Yield Bond Index and the Bloomberg Municipal Bond Index.