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Fair Value Measurements - Inputs Used (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross written premiums | $ 3,446.9 | $ 3,360.9 | $ 3,147.0
Amount classified within accrued expenses and other payables | 5.7 | 52.5 |
Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross written premiums | 50.1 | 50.1 |
Reserve for losses | 4.2 | 4.2 |
Initial value of issuance | 325.0 | 325.0 |
Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross written premiums | 61.1 | 61.1 |
Reserve for losses | 61.9 | 61.9 |
Initial value of issuance | 325.0 | 325.0 |
Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Initial value of issuance | 0.0 | 44.2 |
Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Initial value of issuance | 0.0 | 44.2 |
Silverton 2017 | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of Loan Notes | $ 4.6 | 86.6 |
Silverton 2016 | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of Loan Notes |  | $ 86.6 | $ 223.4
Measurement Input, Expected Term [Member] | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Contract period | 365 days | 365 days |
Third party |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Amount classified within accrued expenses and other payables | $ 4.6 | $ 42.4 |

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