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Description of Business	6 Months Ended
Description of Business	Jun. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	Description of Business As used herein, "GATX," "we," "us," "our," and similar terms refer to GATX Corporation and its subsidiaries, unless indicated otherwise. We lease, operate, manage, and remarket long-lived, widely-used assets, primarily in the rail and marine markets. We also invest in joint ventures that complement our existing business activities. We report our financial results through four primary business segments: Rail North America, Rail International, American Steamship Company (“ASC”), and Portfolio Management.