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Fair Value Measurements - Reconciliation of Liabilities Using Level 3 Inputs (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Accrued expenses (current liabilities) $ 15.2 $ 3.0 $ 15.2 $ 3.0  
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         $ 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         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   223.4 223.4
Distributed to third party   (1.4)   (115.9)  
Total change in fair value included in the statement of operations   (9.8)   (3.6)  
Ending balance   103.9   103.9  
Level 3 | Fair Value, Measurements, Recurring | Silverton 2017          
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning balance 25.3   86.6    
Distributed to third party (14.7)   (78.4)    
Total change in fair value included in the statement of operations 1.7   4.1    
Ending balance 12.3   12.3   $ 86.6
Third party          
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Accrued expenses (current liabilities) $ 12.3 $ 2.4 $ 12.3 $ 2.4