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Fair Value Measurements - Loan Notes Issued by Variable Interest Entities (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Gross written premiums | $ 852.5 | $ 763.5 | $ 2,672.6 | $ 2,540.9 |
Maximum | Level 3 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Gross written premiums |  |  | 57.6 | 43.4 |
Reserve for losses |  |  | $ 37.5 | $ 11.8 |
Contract period |  |  | 365 days | 365 days |
Loan notes held by third parties | (325.0) |  | $ (325.0) |  | $ (220.0)
Minimum | Level 3 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Gross written premiums |  |  | 50.1 | $ 38.9 |
Reserve for losses |  |  | 4.2 | 2.7 |
Loan notes held by third parties | (325.0) |  | (325.0) |  | (220.0)
Fair Value, Measurements, Recurring |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Total | 7,555.6 |  | 7,555.6 |  | 7,653.6
Loan notes held by third parties | (101.5) |  | (101.5) |  | (115.0)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) | (2.4) |  | (2.4) |  | (108.4)
Fair Value, Measurements, Recurring | Level 3 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Total | 103.9 |  | 103.9 |  | 223.4
Loan notes held by third parties | (101.5) |  | (101.5) |  | (115.0)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) | (2.4) | $ (2.3) | (2.4) | $ (2.3) | (108.4)
Fair Value, Measurements, Recurring | Level 3 | Silverton 2016 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Total | $ 103.9 |  | $ 103.9 |  |
Fair Value, Measurements, Recurring | Level 3 | Silverton 2015 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Total |  |  |  |  | $ 223.4

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