Basis of Presentation (Policies)	6 Months Ended
Basis of Presentation (Policies)	Jun. 30, 2011
Description of Business and Basis of Presentation [Abstract]
New Accounting Pronouncements	New Accounting Pronouncements Presentation of Comprehensive Income — In June 2011, the Financial Accounting Standards Board (“FASB”) issued authoritative accounting guidance that revises the requirements for reporting other comprehensive income and its components. The guidance requires an entity to present net income and other comprehensive income in one continuous statement or in two separate, but consecutive, statements. The guidance becomes effective for periods beginning after December 15, 2011 with early adoption permitted. Upon adoption, GATX may revise the presentation of its financial statements, but application of the new guidance will 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 changes some fair value measurement principles, clarifies application of existing guidance, and enhances fair value disclosure requirements. The guidance requires an entity to disclose gross transfers between Level 1 and Level 2 fair value measurements and the reasons for those transfers. The guidance becomes effective for periods beginning after December 15, 2011. Application of the new guidance is not expected to impact GATX’s financial position, results of operations or cash flows.
Accounting adjustments	Accounting Adjustment In the first quarter of 2010, the Company discovered a clerical error in the preparation of its Consolidated Balance Sheet as of December 31, 2009, and Consolidated Statement of Cash Flows for the quarter and year ended December 31, 2009. The error resulted in a $13.1 million overstatement in each of cash and cash equivalents; accounts payable and accrued expenses; and net cash provided by operating activities. Management has determined that the effect of this error is immaterial and adjusted its Consolidated Balance Sheet and Consolidated Statement of Cash Flows in 2010 to correct this error.

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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