Fair Value Measurements - Quantitative Information for Level 3 Inputs (Details) - Market Yield - Significant Unobservable Inputs (Level 3) $ in Millions	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.024	0.024
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.203	0.215
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.109	0.091
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.089	0.096
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.205	0.180
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.146	0.131
Collateralized Loan Obligations | Measurement Input, Discount Rate | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.029	0.032
Collateralized Loan Obligations | Measurement Input, Discount Rate | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.201	0.125
Collateralized Loan Obligations | Measurement Input, Discount Rate | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.073	0.051
Private Placement | Investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 186.0	$ 204.2
Private Placement | Measurement Input, Discount Rate | Investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.024	0.024
Private Placement | Measurement Input, Discount Rate | Investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.100	0.085
Private Placement | Measurement Input, Discount Rate | Investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.043	0.041
Senior Debt | Non-investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 118.0	$ 123.7
Junior Debt | Non-investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	77.5	81.3
Collateralized Loan Obligations
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	77.9	613.5
Other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	25.9	21.5
Investments in Fixed Maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 485.3	$ 1,044.2