Label

Element

Value

Prospectus [Line Items]

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Document Type

dei_DocumentType

497

Document Period End Date

dei_DocumentPeriodEndDate

Jul. 31, 2023

Entity Registrant Name

dei_EntityRegistrantName

FIRST TRUST EXCHANGE-TRADED ALPHADEX® FUND

Entity Central Index Key

dei_EntityCentralIndexKey

0001383496

Entity Inv Company Type

dei_EntityInvCompanyType

N-1A

Amendment Flag

dei_AmendmentFlag

false

Document Creation Date

dei_DocumentCreationDate

Jun. 18, 2024

Document Effective Date

dei_DocumentEffectiveDate

Jun. 18, 2024

First Trust Multi Cap Growth AlphaDEX Fund

Prospectus [Line Items]

rr_ProspectusLineItems

Supplement to Prospectus [Text Block]

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Notwithstanding anything to the contrary in the Fund’s Prospectus or Summary Prospectus, the Average Annual Total Return table in the section entitled “Annual Total Return” is deleted in its entirety and replaced with the following:

Average Annual Total Returns for the Periods Ended December 31, 2022

	1 Year	5 Years	10 Years	Since Inception	Inception Date
Return Before Taxes	-23.77%	8.11%	11.65%	8.03%	5/8/2007
Return After Taxes on Distributions	-23.95%	7.95%	11.47%	7.87%
Return After Taxes on Distributions and Sale of Fund Shares	-14.07%	6.32%	9.62%	6.67%
Nasdaq AlphaDEX® Multi Cap GrowthTM Index(1) (reflects no deduction for fees, expenses or taxes)	-23.30%	8.84%	N/A	N/A
Nasdaq US Multi Cap GrowthTM Index(1) (reflects no deduction for fees, expenses or taxes)	-25.47%	10.29%	N/A	N/A
S&P Composite 1500® Index (reflects no deduction for fees, expenses or taxes)	-28.66%	9.92%	13.33%	9.75%
S&P Composite 1500® Growth Index (reflects no deduction for fees, expenses or taxes)	-17.78%	9.15%	12.40%	8.34%
Russell 3000® Index (reflects no deduction for fees, expenses or taxes)	-19.21%	8.79%	12.13%	8.15%

(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.