Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2021	Jan. 31, 2022
Entity Addresses [Line Items]
Document Type	10-K/A
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Document Transition Report	false
Entity File Number	1-2328
Entity Registrant Name	GATX Corporation
Entity Incorporation, State or Country Code	NY
Entity Tax Identification Number	36-1124040
Entity Address, Address Line One	233 South Wacker Drive
Entity Address, City or Town	Chicago
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60606
City Area Code	(312)
Local Phone Number	621-6200
Entity Central Index Key	0000040211
Entity Interactive Data Current	Yes
Amendment Flag	true
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		35,400,000
Entity Shell Company	false
Documents Incorporated by Reference	GATX’s definitive Proxy Statement filed on March 11, 2022
ICFR Auditor Attestation Flag	true
Entity Public Float	$ 3.1
Amendment Description	Explanatory NoteOn February 17, 2022, GATX Corporation (the "Company") filed with the Securities and Exchange Commission its Annual Report on Form 10-K for the year ended December 31, 2021. This amendment No. 1 to Form 10-K ("Amendment No. 1") of the Company is being filed solely to amend Item 15(c) to include the separate financial statements of Alpha Partners Leasing Limited ("APLL") as required under Rule 3-09 of Regulation S-X. The financial statements of APLL for its fiscal year ended December 31, 2021 were not available at the time the Company filed its Annual Report on Form 10-K. The required financial statements are now provided as Exhibit 99.1 to this Amendment No. 1.Item 15 is the only portion of the Company's Annual Report on Form 10-K being supplemented or amended by this Amendment No. 1. This Amendment No. 1 does not change any other information set forth in the original filing of the Company's Annual Report on Form 10-K for the year ended December 31, 2021. This Amendment No. 1 consists solely of the preceding cover page, this explanatory note, the information required by Item 15(c) of Form 10-K as provided in Exhibit 99.1, a signature page, the consent of the independent auditors for APLL, and certifications required to be filed as exhibits hereto.
Auditor Name	Ernst & Young LLP
Auditor Location	Chicago, IL
Auditor Firm ID	42
NEW YORK STOCK EXCHANGE, INC. [Member] | Common Stock [Member]
Entity Addresses [Line Items]
Security Exchange Name	NYSE
Title of 12(b) Security	Common Stock
Trading Symbol	GATX
CHICAGO STOCK EXCHANGE, INC [Member] | Common Stock [Member]
Entity Addresses [Line Items]
Security Exchange Name	CHX
Title of 12(b) Security	Common Stock
Trading Symbol	GATX