Document and Entity Information shares in Millions	9 Months Ended
	Sep. 30, 2015 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, 2015
Amendment Flag	false
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q3
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 Common Stock, Shares Outstanding	42.4