Document and Entity Information - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2022	Jan. 31, 2023	Jun. 30, 2022
Entity Addresses [Line Items]
Document Annual Report	true
Document Period End Date	Dec. 31, 2022
Document Transition Report	false
Entity File Number	1-2328
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 Interactive Data Current	Yes
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
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 Shell Company	false
Documents Incorporated by Reference	GATX’s definitive Proxy Statement to be filed on or about March 17, 2023
ICFR Auditor Attestation Flag	true
Amendment Flag	false
Entity Central Index Key	0000040211
Entity Common Stock, Shares Outstanding		35.3
Entity Well-known Seasoned Issuer	Yes
Entity Public Float			$ 3,300.0
Document Type	10-K
Entity Registrant Name	GATX Corporation
Current Fiscal Year End Date	--12-31
Common Stock [Member] | NEW YORK STOCK EXCHANGE, INC. [Member]
Entity Addresses [Line Items]
Security Exchange Name	NYSE
Title of 12(b) Security	Common Stock
Trading Symbol	GATX
Common Stock [Member] | CHICAGO STOCK EXCHANGE, INC [Member]
Entity Addresses [Line Items]
Security Exchange Name	CHX
Title of 12(b) Security	Common Stock
Trading Symbol	GATX