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Goodwill - Changes in Carrying Amount of Definite-Lived Intangible Assets Excluding Goodwill (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]    
Definite-lived intangible assets, gross carrying value $ 93.8 $ 93.8
Accumulated amortization 13.4 5.5
Definite-lived intangible assets, net carrying value 80.4 88.3
Intangible assets (excluding goodwill), gross carrying value 156.5 157.3
Intangible assets, net (excluding goodwill) $ 143.1 151.8
Definite-lived intangible asset, useful life 12 years  
Accumulated expense $ 8.0 4.8
Estimated amortization of intangibles, year one 8.0  
Estimated amortization of intangibles, year two 8.0  
Estimated amortization of intangibles, year three 7.9  
Estimated amortization of intangibles, year four 7.8  
Estimated amortization of intangibles, year five 7.6  
Estimated amortization of intangibles, thereafter 41.0  
Customer Relationships    
Finite-Lived Intangible Assets [Line Items]    
Definite-lived intangible assets, gross carrying value [1] 90.9 90.9
Accumulated amortization [1] 12.1 4.6
Definite-lived intangible assets, net carrying value [1] $ 78.8 86.3
Definite-lived intangible asset, useful life 12 years  
Other Intangible Assets    
Finite-Lived Intangible Assets [Line Items]    
Definite-lived intangible assets, gross carrying value [1] $ 2.9 2.9
Accumulated amortization [1] 1.3 0.9
Definite-lived intangible assets, net carrying value [1] $ 1.6 $ 2.0
Definite-lived intangible asset, useful life 7 years  
[1] Average useful life of customer relationships and other definite-lived intangible assets are estimated to be approximately 12 years and 7 years, respectively. The average useful life across all definite-lived intangible assets is estimated to be approximately 12 years.