Shareholders' Equity (Details) In Millions, unless otherwise specified	Dec. 31, 2012
Common stock reserved for conversion and incentive plans
Common stock reserved for conversion and incentive plans	6.6
Conversion of outstanding preferred stock [Member]
Common stock reserved for conversion and incentive plans
Common stock reserved for conversion and incentive plans	0.1
GATX Corporation 2004 Equity Incentive Compensation Plan [Member]
Common stock reserved for conversion and incentive plans
Common stock reserved for conversion and incentive plans	6.5

Aggregate number of common shares reserved for future issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.29)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant