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Fair Value Measurements - Inputs Used (Details) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Gross written premiums | $ 1,116.8 | $ 982.7 | $ 3,598.6 | $ 3,107.9 | $ 3,967.6 | $ 4,338.7 | $ 3,938.4
Privately-held Investments, available for sale | 25.3 |  | 25.3 |  | 14.9 | $ 0.0 |
Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Privately-held Investments, available for sale | 25.3 |  | 25.3 |  |  |  |
Recurring |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Privately-held Investments trading, at fair value |  |  |  |  | 475.0 |  |
Recurring | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Privately-held Investments trading, at fair value |  |  |  |  | 475.0 |  |
Total, Privately-held Investments | $ 366.5 |  | $ 366.5 |  | $ 489.9 |  |
Discounted cash flow | Commercial mortgage loans | Minimum | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Measurement inputs | 0.038 |  | 0.038 |  | 0.038 |  |
Discounted cash flow | Commercial mortgage loans | Maximum | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Measurement inputs | 0.260 |  | 0.260 |  | 0.200 |  |
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 | 0.090 |  | 0.090 |  | 0.077 |  |
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 trading, at fair value | $ 133.7 |  | $ 133.7 |  | $ 274.9 |  |
Discounted cash flow | Middle market loans | Minimum | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Measurement inputs | 0.069 |  | 0.069 |  | 0.077 |  |
Discounted cash flow | Middle market loans | Maximum | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Measurement inputs | 0.168 |  | 0.168 |  | 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 | 0.094 |  | 0.094 |  | 0.110 |  |
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 trading, at fair value | $ 64.0 |  | $ 64.0 |  | $ 84.8 |  |
Discounted cash flow | Asset-backed | Minimum | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Measurement inputs | 0.049 |  | 0.049 |  | 0.059 |  |
Discounted cash flow | Asset-backed | Maximum | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Measurement inputs | 0.08 |  | 0.08 |  | 0.097 |  |
Discounted cash flow | Asset-backed | Weighted Average | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Measurement inputs | 0.068 |  | 0.068 |  | 0.070 |  |
Discounted cash flow | Asset-backed | Recurring | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Privately-held Investments trading, at fair value | $ 129.2 |  | $ 129.2 |  | $ 82.9 |  |
Privately-held Investments, available for sale | $ 15.3 |  | $ 15.3 |  |  |  |
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 |  | 0.064 |  | 0.066 |  |
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 |  | 0.064 |  | 0.066 |  |
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 |  | 0.064 |  | 0.066 |  |
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 trading, at fair value | $ 14.3 |  | $ 14.3 |  | $ 14.4 |  |
Discounted cash flow | Short-term investments | Minimum | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Measurement inputs |  |  |  |  | 0.093 |  |
Discounted cash flow | Short-term investments | Maximum | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Measurement inputs |  |  |  |  | 0.185 |  |
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 |  |  |  |  | 0.093 |  |
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 trading, at fair value |  |  |  |  | $ 18.0 |  |
Valuation, Market Approach | Asset-backed | Recurring | Level 3 |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Privately-held Investments trading, at fair value |  |  |  |  | $ 14.9 |  |
Privately-held Investments, available for sale | $ 10.0 |  | $ 10.0 |  |  |  |

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