Document and Entity Information In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2012
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	Q3
Trading Symbol	GMT
Entity Registrant Name	GATX Corporation
Entity Central Index Key	0000040211
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding	46.9