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Description of Business	12 Months Ended
Description of Business	Dec. 31, 2020
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 market. We report our financial results through three primary business segments: Rail North America, Rail International, and Portfolio Management. Historically, we also reported financial results for American Steamship Company ("ASC") as a fourth segment. On May 14, 2020, we completed the sale of our ASC business, subject to customary post-closing adjustments. As a result, ASC is now reported as discontinued operations, and financial data for the ASC segment has been segregated and presented as discontinued operations for all periods presented. See "Note 27. Discontinued Operations" for additional information. On December 29, 2020, GATX acquired Trifleet Leasing Holding B.V. ("Trifleet"), the fourth largest tank container lessor in the world. Financial results for this business will be reported in the Other segment. See "Note 5. Business Combinations" for additional information.