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Shareholders' Equity (Details) - USD ($)
shares in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jan. 25, 2019
Class of Stock [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years      
Stock Repurchase Program, Authorized Amount       $ 300,000,000
Common stock reserved for conversion and incentive plans        
Common stock reserved for conversion and incentive plans 8.3      
Stock Repurchased During Period, Shares 0.0 (2.0) (1.5)  
Share-based compensation award plans [Member]        
Common stock reserved for conversion and incentive plans        
Common stock reserved for conversion and incentive plans 2.1      
GATX Corporation 2004 Equity Incentive Compensation Plan [Member]        
Common stock reserved for conversion and incentive plans        
Common stock reserved for conversion and incentive plans 6.2      
Restricted Stock and Performance Share Awards [Member]        
Class of Stock [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years