Fair Value Measurements - Quantitative Information about Significant Unobservable Inputs (Details) - Valuation, Market Approach - Significant Unobservable Inputs (Level 3) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Collateralized Loan Obligations
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value		$ 613.5
Other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 26.8	21.5
Total Level 3 Fixed Maturity Investments in Corporate Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	449.2	1,044.2
Investment-grade | Investment-grade
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	246.7	204.2
Non-investment-grade | Senior Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	111.1	123.7
Non-investment-grade | Junior Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 64.6	$ 81.3
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.014	0.024
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.130	0.085
Measurement Input, Discount Rate | Investment-grade | Investment-grade | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.038	0.041
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	0.234	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	0.095	0.091
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.031	0.096
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.279	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	0.137	0.131
Collateralized Loan Obligations | Measurement Input, Discount Rate | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input		0.032
Collateralized Loan Obligations | Measurement Input, Discount Rate | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input		0.125
Collateralized Loan Obligations | Measurement Input, Discount Rate | Weighted-average Yield
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input		0.051