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Description of Business	12 Months Ended
Description of Business	Dec. 31, 2012
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	Description of Business Headquartered in Chicago, Illinois, 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. In 2012, the Company modified the composition of its reportable segments to reflect an increasing focus on international growth and related changes in its management structure. As a result, the Company now reports financial results through four primary business segments: Rail North America, Rail International, American Steamship Company (“ASC”) and Portfolio Management. All segment information is presented on this basis.