Document and Entity Information	9 Months Ended
	Sep. 30, 2018 shares
Document and Entity Information [Abstract]
Entity Registrant Name	GATX Corporation
Entity Central Index Key	0000040211
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Amendment Flag	false
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding	37,632,377