Description of Business	3 Months Ended
Mar. 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 four primary business segments: Rail North America, Rail International, Portfolio Management, and American Steamship Company ("ASC"). On February 7, 2020, we entered into an agreement to sell ASC. The sale is subject to customary closing conditions. See "Note 25. Subsequent Events" in Part II, Item 8 in our Annual Report on Form 10-K for the year ended December 31, 2019 and "Note 16. Subsequent Events" of this Form 10-Q for additional information.

The entire disclosure for the nature of an entity's business, major products or services, principal markets including location, and the relative importance of its operations in each business and the basis for the determination, including but not limited to, assets, revenues, or earnings. For an entity that has not commenced principal operations, disclosures about the risks and uncertainties related to the activities in which the entity is currently engaged and an understanding of what those activities are being directed toward.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e5967-108592

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-URI http://asc.fasb.org/topic&trid=2134479

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