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Fair Value Measurements - Loan Notes Issued by Variable Interest Entities (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, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Gross written premiums | $ 873.2 | $ 852.5 | $ 2,843.8 | $ 2,672.6 |
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
Loan notes held by third parties | 325.0 |  | 325.0 |  | 325.0
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
Loan notes held by third parties | 325.0 |  | 325.0 |  | 325.0
Fair Value, Measurements, Recurring |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Total | 6,837.2 |  | 6,837.2 |  | 7,485.4
Loan notes held by third parties |  |  |  |  | 44.2
Fair Value, Measurements, Recurring | Level 3 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Total | (12.3) |  | (12.3) |  | (86.6)
Loan notes held by third parties |  |  |  |  | 44.2
Fair Value, Measurements, Recurring | Level 3 | Silverton 2016 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Total |  |  |  |  | 86.6
Amount of Level 3 liabilities settled |  | 1.4 |  | 115.9 |
Total change in fair value included in the statement of operations |  | $ (9.8) |  | $ (3.6) |
Fair Value, Measurements, Recurring | Level 3 | Silverton 2017 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Total | 12.3 |  | 12.3 |  |
Amount of Level 3 liabilities settled | 14.7 |  | 78.4 |  |
Total change in fair value included in the statement of operations | 1.7 |  | 4.1 |  |
Accrued expenses and other payables | Fair Value, Measurements, Recurring |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes held by third parties | 12.3 |  | 12.3 |  | 42.4
Accrued expenses and other payables | Fair Value, Measurements, Recurring | Level 3 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes held by third parties | $ 12.3 |  | $ 12.3 |  | $ 42.4
Measurement Input, Expected Term | Maximum | Level 3 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Contract period |  |  | 365 days |  | 365 days

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