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Goodwill and Franchise Value (Schedule of Goodwill) (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2008
Goodwill [Roll Forward]      
Balance $ 434.9 $ 256.3  
Adjustments to purchase price allocations 5.1    
Additions through acquisitions 16.8 180.7  
Reductions through divestitures   (2.1)  
Balance 456.8 434.9  
Accumulated impairment loss     $ 299.3
Goodwill acquired including purchase accounting adjustments 21.9    
Domestic      
Goodwill [Roll Forward]      
Balance 164.5 114.0  
Adjustments to purchase price allocations 1.6    
Additions through acquisitions 5.9 51.4  
Reductions through divestitures   (0.9)  
Balance 172.0 164.5  
Import      
Goodwill [Roll Forward]      
Balance 188.9 104.3  
Adjustments to purchase price allocations 1.6    
Additions through acquisitions 8.8 85.8  
Reductions through divestitures   (1.2)  
Balance 199.3 188.9  
Luxury      
Goodwill [Roll Forward]      
Balance 81.5 38.0  
Adjustments to purchase price allocations 1.9    
Additions through acquisitions 2.1 43.5  
Reductions through divestitures   0.0  
Balance $ 85.5 $ 81.5