Fair Value Measurements - Quantitative Information for Level 3 Inputs (Details) - Market Yield - Significant Unobservable Inputs (Level 3) $ in Millions	Jun. 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	0.016	0.024
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.190	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	0.067	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	0.265	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	$ 263.7	$ 246.7
Private Placement | Measurement Input, Discount Rate | Investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.009	0.014
Private Placement | Measurement Input, Discount Rate | Investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.094	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	0.032	0.038
Senior Debt | Non-investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 60.3	$ 111.1
Junior Debt | Non-investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	68.6	64.6
Other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	68.8	26.8
Investments in Fixed Maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 461.4	$ 449.2