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Fair Value Measurements - Reconciliation of Liabilities Using Level 3 Inputs (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Accrued expenses (current liabilities) $ 6.3 $ 5.8 $ 6.3 $ 5.8  
Fair Value, Measurements, Recurring          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Debt Instrument, Face Amount, Issued By Third Parties 20.3   20.3   $ 44.2
Level 3 | Fair Value, Measurements, Recurring          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Debt Instrument, Face Amount, Issued By Third Parties 20.3   20.3   $ 44.2
Level 3 | Fair Value, Measurements, Recurring | Silverton 2016          
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning balance   115.1   223.4  
Distributed to third party   (2.9)   (114.1)  
Total change in fair value included in the statement of operations   3.3   6.2  
Ending balance   115.5   115.5  
Level 3 | Fair Value, Measurements, Recurring | Silverton 2017          
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning balance 39.8   86.6    
Distributed to third party (17.9)   (63.7)    
Total change in fair value included in the statement of operations 3.4   2.4    
Ending balance 25.3   25.3    
Third party          
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Accrued expenses (current liabilities) $ 5.0 $ 4.7 $ 5.0 $ 4.7