v3.26.1
First Trust United Kingdom AlphaDEX Fund Average Annual Total Returns
 12 Months Ended 60 Months Ended 120 Months Ended 167 Months Ended
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Nasdaq AlphaDEX® United Kingdom™ Index (reflects no deduction for fees, expenses or taxes)[Member]          
Prospectus [Line Items]          
Average Annual Return, Percent [1]   39.19% 11.66% 7.30%
Nasdaq United Kingdom™ Index (reflects no deduction for fees, expenses or taxes)[Member]          
Prospectus [Line Items]          
Average Annual Return, Percent [1]   35.13% 12.12% 7.47%
MSCI United Kingdom Index (reflects no deduction for fees, expenses or taxes)[Member]          
Prospectus [Line Items]          
Average Annual Return, Percent   35.11% 13.33% 7.83% 6.58%
First Trust United Kingdom AlphaDEX Fund          
Prospectus [Line Items]          
Average Annual Return, Percent   37.60% 10.30% 6.10% 7.40%
Performance Inception Date Feb. 14, 2012        
First Trust United Kingdom AlphaDEX Fund | After Taxes on Distributions          
Prospectus [Line Items]          
Average Annual Return, Percent   35.87% 8.52% 4.57% 5.93%
First Trust United Kingdom AlphaDEX Fund | After Taxes on Distributions and Sales          
Prospectus [Line Items]          
Average Annual Return, Percent   22.17% 7.17% 3.99% 5.21%
[1] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.