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Fair Value Measurements - Reconciliation of Level 3 Inputs (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |
Liability for unpaid claims and claims adjustment expense amount ceded |  | $ 125.5 | $ (85.8)
Amount classified within accrued expenses and other payables | $ (5.7) | (52.5) |
Recurring |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Debt Instrument, Accrued Interest and Payables, Fair Value Disclosure | 4.6 | 42.4 |
Silverton 2017 | Recurring | Level 3 |  |  |
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |
Balance at the beginning of the period | 86.6 |  |
Distributed to third parties | (86.4) |  |
Total change in fair value included in the statement of operations | 4.4 |  |
Balance at the end of the period | 4.6 | 86.6 |
Silverton 2016 | Recurring | Level 3 |  |  |
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |
Balance at the beginning of the period | 86.6 | 223.4 |
Distributed to third parties |  | (115.6) |
Total change in fair value included in the statement of operations |  | (21.2) |
Balance at the end of the period |  | 86.6 | $ 223.4
Third party |  |  |
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |
Amount classified within accrued expenses and other payables | $ (4.6) | $ (42.4) |

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