Fair Value Measurements - Quantitative Information about Significant Unobservable Inputs (Details) - Valuation, Market Approach - Significant Unobservable Inputs (Level 3) $ in Millions	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 64.4	$ 56.4
Total Level 3 Fixed Maturity Investments
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	189.5	227.9
Investment-grade | Investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	60.0	56.5
Non-investment-grade | Senior Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	32.6	72.9
Non-investment-grade | Junior Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 32.5	$ 42.1
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.042	0.046
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.158	0.145
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.087	0.092
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.092	0.046
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.367	0.367
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.135	0.109
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.118	0.088
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.225	0.225
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.138	0.151