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Fair Value Measurements - Inputs Used (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross written premiums | $ 4,338.7 | $ 3,938.4 | $ 3,698.5
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 | 533.0 | 307.1 |
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 | $ 533.0 | $ 307.1 |
Discounted cash flow | Commercial mortgage loans | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.039 |  |
Discounted cash flow | Commercial mortgage loans | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.094 |  |
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 | 0.053 |  |
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 | $ 200.7 |  |
Discounted cash flow | Middle market loans | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.074 |  |
Discounted cash flow | Middle market loans | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.185 |  |
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 | 0.099 |  |
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 | $ 106.9 |  |
Discounted cash flow | Asset-backed | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.058 |  |
Discounted cash flow | Asset-backed | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.093 |  |
Discounted cash flow | Asset-backed | Weighted Average | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.067 |  |
Discounted cash flow | Global corporate securities | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.064 |  |
Discounted cash flow | Global corporate securities | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.064 |  |
Discounted cash flow | Global corporate securities | Weighted Average | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.064 |  |
Discounted cash flow | Global corporate securities | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | $ 15.0 |  |
Discounted cash flow | Equity securities — Trading | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.090 |  |
Discounted cash flow | Equity securities — Trading | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.125 |  |
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 | 0.091 |  |
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 | $ 5.3 |  |
Discounted cash flow | Short-term investments | Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.064 |  |
Discounted cash flow | Short-term investments | Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.064 |  |
Discounted cash flow | Short-term investments | Weighted Average | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Measurement inputs | 0.064 |  |
Discounted cash flow | Short-term investments | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | $ 25.6 |  |
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 | 85.8 |  |
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 | 1.3 |  |
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 | 25.6 |  |
Recovery approach | Asset-backed | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | $ 66.8 |  |

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