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Shareholders' Equity (Details) - USD ($)
shares in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 31, 2018
Jan. 26, 2018
Jan. 31, 2016
Dec. 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         $ 250,000,000 $ 170,000,000 $ 300,000,000 $ 250,000,000
Common stock reserved for conversion and incentive plans                
Common stock reserved for conversion and incentive plans   9.0            
Stock Repurchased During Period, Shares (2.4) (1.7) (2.7) (2.4)        
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   6.8            
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