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Fair Value Measurements - Inputs Used (Details) $ in Millions	12 Months Ended
	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	$ 3,967.6	$ 4,338.7	$ 3,938.4
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	$ 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
Discounted cash flow | Commercial mortgage loans | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.200
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	0.077
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	$ 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	0.077
Discounted cash flow | Middle market loans | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.185
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	0.110
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	$ 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	0.059
Discounted cash flow | Asset-backed | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.097
Discounted cash flow | Asset-backed | Weighted Average | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.070
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	$ 82.9
Discounted cash flow | Global corporate securities | Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.066
Discounted cash flow | Global corporate securities | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.066
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	0.066
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	$ 14.4
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

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of direct and assumed premiums written.

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