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Shareholders' Equity (Details) - USD ($)
shares in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Jan. 31, 2016
Jan. 31, 2014
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 $ 250,000,000
Common stock reserved for conversion and incentive plans          
Common stock reserved for conversion and incentive plans 5.6        
Stock Repurchased During Period, Shares (2.7) (2.4) (1.9)    
Share-based compensation award plans [Member]          
Common stock reserved for conversion and incentive plans          
Common stock reserved for conversion and incentive plans 2.2        
GATX Corporation 2004 Equity Incentive Compensation Plan [Member]          
Common stock reserved for conversion and incentive plans          
Common stock reserved for conversion and incentive plans 3.4        
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