Document and Entity Information shares in Thousands	6 Months Ended
	Jun. 30, 2023 shares
Document Information [Line Items]
Document Transition Report	false
Entity Registrant Name	GATX Corporation
Document Period End Date	Jun. 30, 2023
City Area Code	312
Local Phone Number	621-6200
Title of 12(b) Security	Common Stock
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding	35,400
Entity Shell Company	false
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
Amendment Flag	false
Entity Central Index Key	0000040211
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q2
Current Fiscal Year End Date	--12-31
Document Quarterly Report	true
Entity Incorporation, State or Country Code	NY
Entity Tax Identification Number	36-1124040
Document Type	10-Q
Entity File Number	1-2328
NEW YORK STOCK EXCHANGE, INC. [Member]
Document Information [Line Items]
Trading Symbol	GATX
Security Exchange Name	NYSE
CHICAGO STOCK EXCHANGE, INC [Member]
Document Information [Line Items]
Trading Symbol	GATX
Security Exchange Name	CHX