Fair Value Measurements - Quantitative Information for Level 3 Inputs (Details) - Market Yield - Significant Unobservable Inputs (Level 3) $ in Millions	Sep. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.046	0.092
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.594	0.367
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.128	0.135
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.096	0.118
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.310	0.225
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.134	0.138
Investment-grade | Investment grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 54.8	$ 60.0
Investment-grade | Measurement Input, Discount Rate | Investment grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.034	0.042
Investment-grade | Measurement Input, Discount Rate | Investment grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.113	0.158
Investment-grade | Measurement Input, Discount Rate | Investment grade | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.080	0.087
Senior Debt | Non-investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 82.0	$ 32.6
Junior Debt | Non-investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	35.0	32.5
Other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	69.9	64.4
Investments in Fixed Maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 241.7	$ 189.5