Average Annual Total Returns - First Trust Multi Cap Growth AlphaDEX Fund	First Trust Multi Cap Growth AlphaDEX Fund 1 Year	First Trust Multi Cap Growth AlphaDEX Fund 5 Years	First Trust Multi Cap Growth AlphaDEX Fund 10 Years	First Trust Multi Cap Growth AlphaDEX Fund Since Inception	First Trust Multi Cap Growth AlphaDEX Fund Inception Date	First Trust Multi Cap Growth AlphaDEX Fund After tax on distributions 1 Year	First Trust Multi Cap Growth AlphaDEX Fund After tax on distributions 5 Years	First Trust Multi Cap Growth AlphaDEX Fund After tax on distributions 10 Years	First Trust Multi Cap Growth AlphaDEX Fund After tax on distributions Since Inception	First Trust Multi Cap Growth AlphaDEX Fund After tax on distributions and sale of fund shares 1 Year	First Trust Multi Cap Growth AlphaDEX Fund After tax on distributions and sale of fund shares 5 Years	First Trust Multi Cap Growth AlphaDEX Fund After tax on distributions and sale of fund shares 10 Years	First Trust Multi Cap Growth AlphaDEX Fund After tax on distributions and sale of fund shares Since Inception	Nasdaq AlphaDEX® Multi Cap Growth Index (reflects no deduction for fees, expenses or taxes) 1 Year		Nasdaq AlphaDEX® Multi Cap Growth Index (reflects no deduction for fees, expenses or taxes) 5 Years		Nasdaq AlphaDEX® Multi Cap Growth Index (reflects no deduction for fees, expenses or taxes) 10 Years		Nasdaq AlphaDEX® Multi Cap Growth Index (reflects no deduction for fees, expenses or taxes) Since Inception		Nasdaq US Multi Cap Growth Index (reflects no deduction for fees, expenses or taxes) 1 Year		Nasdaq US Multi Cap Growth Index (reflects no deduction for fees, expenses or taxes) 5 Years		Nasdaq US Multi Cap Growth Index (reflects no deduction for fees, expenses or taxes) 10 Years		Nasdaq US Multi Cap Growth Index (reflects no deduction for fees, expenses or taxes) Since Inception		S&P Composite 1500® Index (reflects no deduction for fees, expenses or taxes) 1 Year	S&P Composite 1500® Index (reflects no deduction for fees, expenses or taxes) 5 Years	S&P Composite 1500® Index (reflects no deduction for fees, expenses or taxes) 10 Years	S&P Composite 1500® Index (reflects no deduction for fees, expenses or taxes) Since Inception	S&P Composite 1500® Growth Index (reflects no deduction for fees, expenses or taxes) 1 Year	S&P Composite 1500® Growth Index (reflects no deduction for fees, expenses or taxes) 5 Years	S&P Composite 1500® Growth Index (reflects no deduction for fees, expenses or taxes) 10 Years	S&P Composite 1500® Growth Index (reflects no deduction for fees, expenses or taxes) Since Inception
Total	34.59%	16.41%	13.65%	9.92%	May 08, 2007	34.34%	16.22%	13.45%	9.76%	20.43%	13.17%	11.38%	8.29%	35.58%	[1]		[1]		[1]		[1]	34.57%	[1]		[1]		[1]		[1]	17.92%	14.97%	13.67%	9.18%	32.43%	18.54%	16.14%	11.80%

[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.