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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 |

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