Fair Value Measurements - Quantitative Information about Significant Unobservable Inputs (Details) - Valuation, Market Approach - Significant Unobservable Inputs (Level 3) $ in Millions	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collateralized Loan Obligations
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 613.5	$ 504.9
Other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	21.5	15.9
Total Level 3 Fixed Maturity Investments in Corporate Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	1,044.2	897.4
Investment-grade | Investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	204.2	146.7
Non-investment-grade | Senior Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	123.7	142.3
Non-investment-grade | Junior Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 81.3	$ 87.6
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.024	0.037
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.085	0.109
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.041	0.052
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.024	0.048
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.215	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	0.091	0.115
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.096	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	0.032	0.041
Collateralized Loan Obligations | Measurement Input, Discount Rate | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.125	0.134
Collateralized Loan Obligations | Measurement Input, Discount Rate | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.051	0.061