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Fair Value Measurements - Inputs Used (Details) | 12 Months Ended
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Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross written premiums | $ 3,442,400,000 | $ 3,446,900,000 | $ 3,360,900,000
Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross written premiums |  | 50,100,000 |
Reserve for losses |  | 4,200,000 |
Initial value of issuance |  | 325,000,000 |
Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross written premiums |  | 61,100,000 |
Reserve for losses |  | 61,900,000 |
Initial value of issuance |  | 325,000,000 |
Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) | 0 | (4,600,000) |
Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Loan Notes, fair value |  | 4,600,000 | $ 86,600,000
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) | 0 | (4,600,000) |
Silverton 2016 | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Loan Notes, fair value |  | $ 4,600,000 |
Measurement Input, Expected Term | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Contract period |  | 365 days |
Held for trading financial assets, at fair value | Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | 279,700,000 |  |
Held for trading financial assets, at fair value | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | $ 279,700,000 | $ 0 |
Discounted cash flow | Commercial mortgage loans | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.050 |  |
Discounted cash flow | Commercial mortgage loans | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.063 |  |
Discounted cash flow | Commercial mortgage loans | Weighted Average | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.058 |  |
Discounted cash flow | Commercial mortgage loans | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | $ 125,700,000 |  |
Discounted cash flow | Middle market loans | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.068 |  |
Discounted cash flow | Middle market loans | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.103 |  |
Discounted cash flow | Middle market loans | Weighted Average | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.079 |  |
Discounted cash flow | Middle market loans | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | $ 111,700,000 |  |
Discounted cash flow | Asset-backed | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.064 |  |
Discounted cash flow | Asset-backed | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.064 |  |
Discounted cash flow | Asset-backed | Weighted Average | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.064 |  |
Discounted cash flow | Asset-backed | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | $ 8,700,000 |  |
Transaction value | Commercial mortgage loans | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | 30,900,000 |  |
Transaction value | Equity securities | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | $ 2,700,000 |  |

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