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Fair Value Measurements - Inputs Used (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross written premiums | $ 3,938.4 | $ 3,698.5 | $ 3,442.4
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 | 307.1 | 299.3 |
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 | $ 307.1 | $ 299.3 |
Discounted cash flow | Commercial mortgage loans | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.035 |  |
Discounted cash flow | Commercial mortgage loans | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.095 |  |
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 | $ 173.9 |  |
Discounted cash flow | Middle market loans | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.062 |  |
Discounted cash flow | Middle market loans | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.155 |  |
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 | 0.091 |  |
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 | $ 65.3 |  |
Discounted cash flow | Asset-backed | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.030 |  |
Discounted cash flow | Asset-backed | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.073 |  |
Discounted cash flow | Asset-backed | Weighted Average | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.054 |  |
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 | $ 26.7 |  |
Discounted cash flow | Equity securities — Trading | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.092 |  |
Discounted cash flow | Equity securities — Trading | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.092 |  |
Discounted cash flow | Equity securities — Trading | Weighted Average | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.092 |  |
Discounted cash flow | Equity securities — Trading | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | $ 2.8 |  |
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 | 37.6 |  |
Transaction value | Equity securities — Trading | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | 0.8 |  |
Liquidation method | 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 | 0.0 |  |
Recovery approach | 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 | $ 0.0 |  |

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- Definition Value of input used to measure investment in debt security measured at fair value with change in fair value recognized in net income (trading). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258 + Details
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