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Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Joint_Venture
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Sep. 30, 2015
Investments in and Advances to Affiliates [Line Items]        
Advances to Affiliate $ 6.5 $ 9.7    
Asset Impairment Charges 8.6 38.5 $ 33.9  
Equity Method Investment, Net Sales Proceeds 2.3 1.0 24.7  
Portfolio proceeds and other 165.6 223.7 482.2  
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]        
Investments in affiliated companies 441.0 387.0 348.5  
Fair Value of Assets Acquired     17.8  
Cardinal Marine [Domain]        
Investments in and Advances to Affiliates [Line Items]        
Asset Impairment Charges     19.0  
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]        
GATX's Percentage Ownership       50.00%
Rolls Royce Partners Finance [Member]        
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]        
Investments in affiliated companies [1] $ 434.2      
GATX's Percentage Ownership [1] 50.00%      
Adler Funding Llc [Member]        
Investments in and Advances to Affiliates [Line Items]        
Asset Impairment Charges $ 3.0      
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]        
Investments in affiliated companies $ 6.5      
GATX's Percentage Ownership 12.50%      
RRPF Joint Ventures [Member]        
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]        
Number of joint venture investments | Joint_Venture 16      
Southern Capital LLC [Member]        
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]        
Investments in affiliated companies $ 0.3      
GATX's Percentage Ownership 50.00%      
Portfolio Management [Member]        
Investments in and Advances to Affiliates [Line Items]        
Asset Impairment Charges $ 3.7 6.7 31.0  
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]        
Investments in affiliated companies 434.2 375.3 335.1  
Portfolio Management [Member] | Cardinal Marine [Domain]        
Investments in and Advances to Affiliates [Line Items]        
Asset Impairment Charges     19.0  
Portfolio Management [Member] | Cardinal Marine [Domain]        
Investments in and Advances to Affiliates [Line Items]        
Portfolio proceeds and other $ 1.1 $ 1.0 $ 24.7  
[1] (1) Combined investment balances of sixteen separate joint ventures (collectively, the "RRPF affiliates")