Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 6,239,300,000	$ 6,345,300,000
Equity securities at fair value	$ 147,200,000	121,600,000
Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | volatility
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	100.00
Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | option budget
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.009
Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | volatility
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	121.95
Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | option budget
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.025
Weighted Average | discounted cash flow | Fair Value, Inputs, Level 3 | lapse rate
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.053
Weighted Average | discounted cash flow | Fair Value, Inputs, Level 3 | Mortality multiplier
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.668
Weighted Average | discounted cash flow | Fair Value, Inputs, Level 3 | Non-performance adjustment
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.050
Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 1,703,400,000	1,827,500,000
Municipal bonds | Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | I spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0374
Municipal bonds | Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | I spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0425
Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 2,388,700,000	2,122,900,000
Corporate bonds | Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | I spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	333
Corporate bonds | Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | N spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0211
Corporate bonds | Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | T spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	130
Corporate bonds | Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.034
Corporate bonds | Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | discount rate
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.113
Corporate bonds | Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | I spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	365
Corporate bonds | Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | N spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0531
Corporate bonds | Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | T spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	540
Corporate bonds | Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.088
Corporate bonds | Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | discount rate
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.120
Corporate bonds | Weighted Average | market comparable | Fair Value, Inputs, Level 3 | option adjusted spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.125
Other asset-backed securities
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 1,075,200,000	$ 1,231,800,000
Other asset-backed securities | discounted cash flow | Fair Value, Inputs, Level 3 | N spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0341
Other asset-backed securities | discounted cash flow | Fair Value, Inputs, Level 3 | T spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0196
Other asset-backed securities | Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | discount rate
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.085
Other asset-backed securities | Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | median comparable yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.073
Other asset-backed securities | Minimum | market comparable | Fair Value, Inputs, Level 3 | median price
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	54.79
Other asset-backed securities | Minimum | vendor price | Fair Value, Inputs, Level 3 | haircut
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.030
Other asset-backed securities | Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | discount rate
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.200
Other asset-backed securities | Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | median comparable yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.161
Other asset-backed securities | Maximum | market comparable | Fair Value, Inputs, Level 3 | median price
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	97.95
Other asset-backed securities | Maximum | vendor price | Fair Value, Inputs, Level 3 | haircut
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.050
Other asset-backed securities | Weighted Average | discounted cash flow | Fair Value, Inputs, Level 3 | discount margin
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.146