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First Trust Developed Markets ex-US Small Cap AlphaDEX Fund Average Annual Total Returns
 12 Months Ended 60 Months Ended 120 Months Ended 167 Months Ended 176 Months Ended
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Nasdaq AlphaDEX® Developed Markets Ex-US Small Cap™ Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent [1]   50.88% 11.15% 9.57%  
Nasdaq Developed Markets Ex-US Small Cap™ Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent [1]   36.02% 6.90% 7.56%  
MSCI World ex-USA Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent   31.85% 9.46% 8.55% 7.51% 6.53%
First Trust Developed Markets ex-US Small Cap AlphaDEX Fund            
Prospectus [Line Items]            
Average Annual Return, Percent   49.71% 10.13% 8.72% 7.63%  
Performance Inception Date Feb. 15, 2012          
First Trust Developed Markets ex-US Small Cap AlphaDEX Fund | After Taxes on Distributions            
Prospectus [Line Items]            
Average Annual Return, Percent   47.78% 8.64% 7.51% 6.47%  
First Trust Developed Markets ex-US Small Cap AlphaDEX Fund | After Taxes on Distributions and Sales            
Prospectus [Line Items]            
Average Annual Return, Percent   29.32% 7.19% 6.40% 5.59%  
[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.