Average Annual Total Returns - First Trust Mid Cap Growth AlphaDEX Fund	First Trust Mid Cap Growth AlphaDEX Fund 1 Year	First Trust Mid Cap Growth AlphaDEX Fund 5 Years	First Trust Mid Cap Growth AlphaDEX Fund Since Inception	First Trust Mid Cap Growth AlphaDEX Fund Inception Date	First Trust Mid Cap Growth AlphaDEX Fund After tax on distributions 1 Year	First Trust Mid Cap Growth AlphaDEX Fund After tax on distributions 5 Years	First Trust Mid Cap Growth AlphaDEX Fund After tax on distributions Since Inception	First Trust Mid Cap Growth AlphaDEX Fund After tax on distributions and sale of fund shares 1 Year	First Trust Mid Cap Growth AlphaDEX Fund After tax on distributions and sale of fund shares 5 Years	First Trust Mid Cap Growth AlphaDEX Fund After tax on distributions and sale of fund shares Since Inception	Nasdaq AlphaDEX® Mid Cap Growth Index (reflects no deduction for fees, expenses or taxes) 1 Year		Nasdaq AlphaDEX® Mid Cap Growth Index (reflects no deduction for fees, expenses or taxes) 5 Years		Nasdaq AlphaDEX® Mid Cap Growth Index (reflects no deduction for fees, expenses or taxes) Since Inception		Nasdaq US 600 Mid Cap Growth Index (reflects no deduction for fees, expenses or taxes) 1 Year		Nasdaq US 600 Mid Cap Growth Index (reflects no deduction for fees, expenses or taxes) 5 Years		Nasdaq US 600 Mid Cap Growth Index (reflects no deduction for fees, expenses or taxes) Since Inception		S&P MidCap 400® Growth Index (reflects no deduction for fees, expenses or taxes) 1 Year	S&P MidCap 400® Growth Index (reflects no deduction for fees, expenses or taxes) 5 Years	S&P MidCap 400® Growth Index (reflects no deduction for fees, expenses or taxes) Since Inception
Total	36.90%	18.11%	13.25%	Apr. 19, 2011	36.74%	17.96%	13.10%	21.83%	14.65%	11.03%	37.95%	[1]		[1]		[1]	36.60%	[1]		[1]		[1]	22.77%	13.86%	11.96%

[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.