Fair Value Measurements - Quantitative Information for Level 3 Inputs (Details) - Market Yield - Significant Unobservable Inputs (Level 3) $ in Millions	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
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	0.185	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	0.092	0.095
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.059	0.031
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.290	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	0.147	0.137
Private Placement | Investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 89.7	$ 246.7
Private Placement | Measurement Input, Discount Rate | Investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.019	0.014
Private Placement | Measurement Input, Discount Rate | Investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.100	0.130
Private Placement | Measurement Input, Discount Rate | Investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.053	0.038
Senior Debt | Non-investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 40.9	$ 111.1
Junior Debt | Non-investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	68.0	64.6
Other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	35.4	26.8
Investments in Fixed Maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 234.0	$ 449.2