Fair Value Measurements - Quantitative Information about Significant Unobservable Inputs (Details) - Valuation, Market Approach - Significant Unobservable Inputs (Level 3) $ in Millions	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 32.0	$ 26.8
Total Level 3 Fixed Maturity Investments
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	249.9	449.2
Investment-grade | Investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	87.9	246.7
Non-investment-grade | Senior Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	76.1	111.1
Non-investment-grade | Junior Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 53.9	$ 64.6
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.023	0.014
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.103	0.130
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.054	0.038
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.051	0.024
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.202	0.234
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.085	0.095
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.060	0.031
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.275	0.279
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.150	0.137