Accounting Changes	12 Months Ended
Accounting Changes	Dec. 31, 2012
Accounting Changes and Error Corrections [Abstract]
Accounting Changes	Accounting Changes Goodwill — In September 2011, the Financial Accounting Standards Board (FASB) issued authoritative accounting guidance that allows an entity to assess qualitative factors to determine if performing the two-step goodwill impairment test is necessary. The assessment of qualitative factors requires an entity to evaluate all events or circumstances that could impact the likelihood that the fair value of a reporting unit is less than its carrying amount. The guidance is effective for periods beginning after December 15, 2011. Application of the new guidance did not impact GATX’s financial position, results of operations or cash flows. Fair Value Measurement — In May 2011, the FASB issued authoritative accounting guidance that changed some fair value measurement principles, clarified the application of existing guidance, and enhanced fair value disclosure requirements. The guidance requires an entity to disclose transfers between Level 1 and Level 2 fair value measurements and the reasons for those transfers. The guidance is effective for periods beginning after December 15, 2011. Application of the new guidance did not impact GATX’s financial position, results of operations or cash flows.

The entire disclosure for reporting accounting changes and error corrections. It includes the conveyance of information necessary for a user of the Company's financial information to understand all aspects and required disclosure information concerning all changes and error corrections reported in the Company's financial statements for the period.

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