Description of Business	3 Months Ended
Mar. 31, 2012
Description of Business [Abstract]
Description of Business	NOTE 1. Description of Business GATX Corporation (“GATX” or the “Company”) leases, operates, manages and remarkets long-lived, widely-used assets primarily in the rail and marine markets. GATX also invests in joint ventures that complement existing business activities. Headquartered in Chicago, Illinois, GATX has three financial reporting segments: Rail, American Steamship Company (“ASC”) and Portfolio Management.

The entire disclosure for the nature of an entity's business, the major products or services it sells or provides and its principal markets, including the locations of those markets. If the entity operates in more than one business, the disclosure also indicates the relative importance of its operations in each business and the basis for the determination (for example, assets, revenues, or earnings).

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

-Name Statement of Position (SOP)

-Publisher AICPA

-Number 94-6

-Paragraph 10

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 275

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6003-108592

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