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First Trust Utilities AlphaDEX Fund Average Annual Total Returns
 12 Months Ended 60 Months Ended 120 Months Ended 164 Months Ended 212 Months Ended
Jul. 31, 2025
Dec. 31, 2024
Dec. 31, 2024
Dec. 31, 2024
Dec. 31, 2024
Dec. 31, 2024
StrataQuant® Utilities Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent   23.67% 8.92% 8.03%   7.62%
Russell 1000® Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent   24.51% 14.28% 12.87%   10.12%
S&P 500® Utilities Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent   23.43% 6.61% 8.43%   7.25%
Russell 1000® Utilities Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent [1]   23.73% 7.08% 8.33%  
Russell 3000® Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent   23.81% 13.86% 12.55% 13.21% 9.93%
First Trust Utilities AlphaDEX Fund            
Prospectus [Line Items]            
Average Annual Return, Percent   22.71% 8.21% 7.31%   6.80%
Performance Inception Date May 08, 2007          
First Trust Utilities AlphaDEX Fund | After Taxes on Distributions            
Prospectus [Line Items]            
Average Annual Return, Percent   21.44% 7.02% 6.04%   5.55%
First Trust Utilities AlphaDEX Fund | After Taxes on Distributions and Sales            
Prospectus [Line Items]            
Average Annual Return, Percent   13.39% 5.84% 5.18%   4.88%
[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.