Label

Element

Value

Prospectus [Line Items]

rr_ProspectusLineItems

Document Type

dei_DocumentType

497

Document Period End Date

dei_DocumentPeriodEndDate

Jul. 31, 2023

Entity Registrant Name

dei_EntityRegistrantName

First Trust Exchange-Traded Fund III

Entity Central Index Key

dei_EntityCentralIndexKey

0001424212

Entity Inv Company Type

dei_EntityInvCompanyType

N-1A

Amendment Flag

dei_AmendmentFlag

false

Document Creation Date

dei_DocumentCreationDate

Aug. 02, 2024

Document Effective Date

dei_DocumentEffectiveDate

Aug. 02, 2024

First Trust Merger Arbitrage ETF

Prospectus [Line Items]

rr_ProspectusLineItems

Supplement to Prospectus [Text Block]

rr_SupplementToProspectusTextBlock

1. Notwithstanding anything to the contrary in the Fund’s prospectus or summary prospectus, effective as of August 1, 2024, the table entitled “Average Annual Total Returns for the Periods Ended December 31, 2022” set forth in the section entitled “Annual Total Return” set forth in the Fund’s prospectus and summary prospectus is hereby deleted and replaced with the following:

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

	1 Year	Since Inception	Inception Date
Return Before Taxes	3.46%	0.56%	2/4/2020
Return After Taxes on Distributions	3.04%	0.43%
Return After Taxes on Distributions and Sale of Fund Shares	2.05%	0.37%
S&P Merger Arbitrage Total Return Index (reflects no deduction for fees, expenses or taxes)(1)	-4.21%	1.73%
S&P 500® Index (reflects no deduction for fees, expenses or taxes)	-18.11%	7.12%

(1) Effective as of August 1, 2024, the S&P Merger Arbitrage Total Return Index replaced the Credit Suisse Merger Arbitrage Liquid Index, which is no longer available for use by the Fund.