Fair Value Measurements - Quantitative Information for Level 3 Inputs (Details) - Market Yield - Significant Unobservable Inputs (Level 3) $ in Millions	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.027	0.048
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.199	0.300
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.098	0.115
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.099	0.110
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.180	0.285
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.131	0.142
Collateralized Loan Obligations | Measurement Input, Discount Rate | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.037	0.041
Collateralized Loan Obligations | Measurement Input, Discount Rate | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.126	0.134
Collateralized Loan Obligations | Measurement Input, Discount Rate | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.054	0.061
Private Placement | Investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 249.8	$ 146.7
Private Placement | Measurement Input, Discount Rate | Investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.025	0.037
Private Placement | Measurement Input, Discount Rate | Investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.102	0.109
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	0.052
Senior Debt | Non-investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 117.5	$ 142.3
Junior Debt | Non-investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	82.4	87.6
Collateralized Loan Obligations
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	536.5	504.9
Other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	8.6	15.9
Investments in Fixed Maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 994.8	$ 897.4