v3.26.1
First Trust Japan AlphaDEX Fund Average Annual Total Returns
 12 Months Ended 60 Months Ended 120 Months Ended 176 Months Ended
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Nasdaq AlphaDEX® Japan™ Index (reflects no deduction for fees, expenses or taxes)[Member]          
Prospectus [Line Items]          
Average Annual Return, Percent [1]   32.90% 8.99% 6.42%
Nasdaq Japan™ Index (reflects no deduction for fees, expenses or taxes)[Member]          
Prospectus [Line Items]          
Average Annual Return, Percent [1]   24.78% 6.47% 7.39%
MSCI Japan Index (reflects no deduction for fees, expenses or taxes)[Member]          
Prospectus [Line Items]          
Average Annual Return, Percent   24.60% 6.60% 7.62% 7.25%
First Trust Japan AlphaDEX Fund          
Prospectus [Line Items]          
Average Annual Return, Percent   32.14% 8.38% 5.87% 5.49%
Performance Inception Date Apr. 18, 2011        
First Trust Japan AlphaDEX Fund | After Taxes on Distributions          
Prospectus [Line Items]          
Average Annual Return, Percent   30.68% 7.16% 4.91% 4.71%
First Trust Japan AlphaDEX Fund | After Taxes on Distributions and Sales          
Prospectus [Line Items]          
Average Annual Return, Percent   19.00% 5.95% 4.17% 4.03%
[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.