Average Annual Total Returns - First Trust Small Cap Core AlphaDEX Fund	First Trust Small Cap Core AlphaDEX Fund 1 Year	First Trust Small Cap Core AlphaDEX Fund 5 Years	First Trust Small Cap Core AlphaDEX Fund 10 Years	First Trust Small Cap Core AlphaDEX Fund Since Inception	First Trust Small Cap Core AlphaDEX Fund Inception Date	First Trust Small Cap Core AlphaDEX Fund After tax on distributions 1 Year	First Trust Small Cap Core AlphaDEX Fund After tax on distributions 5 Years	First Trust Small Cap Core AlphaDEX Fund After tax on distributions 10 Years	First Trust Small Cap Core AlphaDEX Fund After tax on distributions Since Inception	First Trust Small Cap Core AlphaDEX Fund After tax on distributions and sale of fund shares 1 Year	First Trust Small Cap Core AlphaDEX Fund After tax on distributions and sale of fund shares 5 Years	First Trust Small Cap Core AlphaDEX Fund After tax on distributions and sale of fund shares 10 Years	First Trust Small Cap Core AlphaDEX Fund After tax on distributions and sale of fund shares Since Inception	Nasdaq AlphaDEX® Small Cap Core Index (reflects no deduction for fees, expenses or taxes) 1 Year		Nasdaq AlphaDEX® Small Cap Core Index (reflects no deduction for fees, expenses or taxes) 5 Years		Nasdaq AlphaDEX® Small Cap Core Index (reflects no deduction for fees, expenses or taxes) 10 Years		Nasdaq AlphaDEX® Small Cap Core Index (reflects no deduction for fees, expenses or taxes) Since Inception		Nasdaq US 700 Small Cap Index (reflects no deduction for fees, expenses or taxes) 1 Year		Nasdaq US 700 Small Cap Index (reflects no deduction for fees, expenses or taxes) 5 Years		Nasdaq US 700 Small Cap Index (reflects no deduction for fees, expenses or taxes) 10 Years		Nasdaq US 700 Small Cap Index (reflects no deduction for fees, expenses or taxes) Since Inception		S&P SmallCap 600® Index (reflects no deduction for fees, expenses or taxes) 1 Year	S&P SmallCap 600® Index (reflects no deduction for fees, expenses or taxes) 5 Years	S&P SmallCap 600® Index (reflects no deduction for fees, expenses or taxes) 10 Years	S&P SmallCap 600® Index (reflects no deduction for fees, expenses or taxes) Since Inception
Total	21.04%	6.80%	11.42%	7.01%	May 08, 2007	20.46%	6.39%	11.09%	6.72%	12.44%	5.12%	9.33%	5.60%	21.90%	[1]		[1]		[1]		[1]	24.37%	[1]		[1]		[1]		[1]	22.78%	9.56%	13.35%	8.46%

[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.