v3.25.3
First Trust Multi Cap Growth 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
Nasdaq AlphaDEX Multi Cap Growth(TM) Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent [1]   24.47% 13.55%  
Nasdaq US Multi Cap Growth(TM) Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent [1]   31.11% 16.48%  
S&P Composite 1500® Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent   23.95% 14.13% 12.79%   10.08%
S&P Composite 1500® Growth Index (reflects no deduction for fees, expenses or taxes)[Member]            
Prospectus [Line Items]            
Average Annual Return, Percent   34.15% 16.48% 14.83%   12.03%
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 Multi Cap Growth AlphaDEX Fund            
Prospectus [Line Items]            
Average Annual Return, Percent   23.68% 12.80% 11.47%   9.47%
Performance Inception Date May 08, 2007          
First Trust Multi Cap Growth AlphaDEX Fund | After Taxes on Distributions            
Prospectus [Line Items]            
Average Annual Return, Percent   23.40% 12.59% 11.27%   9.30%
First Trust Multi Cap Growth AlphaDEX Fund | After Taxes on Distributions and Sales            
Prospectus [Line Items]            
Average Annual Return, Percent   14.01% 10.14% 9.44%   8.04%
[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.