Cover	12 Months Ended
Dec. 31, 2020
Cover [Abstract]
Documents Incorporated by Reference	GATX’s definitive Proxy Statement filed on March 12, 2021

Documents incorporated by reference.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-23

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