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Goodwill and Franchise Value - Franchise Value (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Indefinite-lived Intangible Assets [Roll Forward]      
Balance   $ 187.0 $ 184.3
Additions through acquisitions   103.5 0.5
Adjustments to purchase price allocations $ 2.2   2.2
Reductions through divestitures   (1.8)  
Balance   $ 288.7 $ 187.0