Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2020	Jan. 31, 2021
Entity Addresses [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 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 Interactive Data Current	Yes
Amendment Flag	false
Document Fiscal Year Focus	2020
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 Public Float	$ 2.1
Entity Common Stock, Shares Outstanding		35,100,000
Entity Shell Company	false
Documents Incorporated by Reference	GATX’s definitive Proxy Statement to be filed on or about March 12, 2021
ICFR Auditor Attestation Flag	true
Common Stock [Member] | NEW YORK STOCK EXCHANGE, INC. [Member]
Entity Addresses [Line Items]
Title of 12(b) Security	Common Stock
Trading Symbol	GATX
Security Exchange Name	NYSE
Common Stock [Member] | CHICAGO STOCK EXCHANGE, INC [Member]
Entity Addresses [Line Items]
Title of 12(b) Security	Common Stock
Trading Symbol	GATX
Security Exchange Name	CHX
Senior Notes [Member] | NEW YORK STOCK EXCHANGE, INC. [Member]
Entity Addresses [Line Items]
Title of 12(b) Security	5.625% Senior Notes due 2066
Trading Symbol	GMTA
Security Exchange Name	NYSE