Document and Entity Information shares in Millions	6 Months Ended
	Jun. 30, 2020 shares
Entity Addresses [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2020
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 Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding	35.0
Entity Shell Company	false
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Current Fiscal Year End Date	--12-31
CHICAGO STOCK EXCHANGE, INC [Member]
Entity Addresses [Line Items]
Security Exchange Name	CHX
Common Stock [Member] | NEW YORK STOCK EXCHANGE, INC. [Member]
Entity Addresses [Line Items]
Trading Symbol	GATX
Security Exchange Name	NYSE
Title of 12(b) Security	Common Stock
Senior Notes [Member] | NEW YORK STOCK EXCHANGE, INC. [Member]
Entity Addresses [Line Items]
Trading Symbol	GMTA
Security Exchange Name	NYSE
Title of 12(b) Security	5.625% Senior Notes due 2066