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Intangible Assets Schedule of Intangible Assets and Excess Reorganization Value (Details) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]    
Amortizable intangible assets, gross $ 123.5 $ 123.5
Accumulated intangible asset amortization (43.6) (40.8)
Amortizable intangible assets, net $ 79.9 82.7
Average useful life of finite-lived intangible assets 15 years  
Contract-Based Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Amortizable intangible assets, gross $ 54.6 54.6
Accumulated intangible asset amortization (26.4) (25.9)
Amortizable intangible assets, net 28.2 28.7
Customer-Related Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Amortizable intangible assets, gross 68.9 68.9
Accumulated intangible asset amortization (17.2) (14.9)
Amortizable intangible assets, net $ 51.7 $ 54.0