Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	$ 5,987,400,000	$ 6,239,300,000
Equity securities at fair value	126,800,000	147,200,000
Carrying Amount
Fair Value Measurement Inputs and Valuation Techniques
Equity securities at fair value	126,800,000	147,200,000
Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	1,579,500,000	1,703,400,000
Municipal bonds | Carrying Amount
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	1,579,500,000	1,703,400,000
Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	2,296,000,000	2,388,700,000
Corporate bonds | Carrying Amount
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	$ 2,296,000,000	2,388,700,000
Other asset-backed securities | Carrying Amount
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale		$ 1,075,200,000
Level 3 | discounted cash flow | volatility | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	100.00
Level 3 | discounted cash flow | volatility | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	121.95
Level 3 | discounted cash flow | lapse rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.053
Level 3 | discounted cash flow | mortality multiplier | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.668
Level 3 | discounted cash flow | option budget | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.009
Level 3 | discounted cash flow | option budget | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.025
Level 3 | discounted cash flow | non-performance adjustment | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.050
Level 3 | Municipal bonds | discounted cash flow | I Spread | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0360
Level 3 | Municipal bonds | discounted cash flow | I Spread | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0409
Level 3 | Corporate bonds | discounted cash flow | N Spread | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0213
Level 3 | Corporate bonds | discounted cash flow | N Spread | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0379
Level 3 | Corporate bonds | discounted cash flow | T spread | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	100
Level 3 | Corporate bonds | discounted cash flow | T spread | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	560
Level 3 | Corporate bonds | discounted cash flow | yield | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.027
Level 3 | Corporate bonds | discounted cash flow | yield | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.107
Level 3 | Corporate bonds | discounted cash flow | discount rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.113
Level 3 | Corporate bonds | discounted cash flow | discount rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.120
Level 3 | Corporate bonds | market comparable | option adjusted spread | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.125
Level 3 | Other asset-backed securities | discounted cash flow | N Spread | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0213
Level 3 | Other asset-backed securities | discounted cash flow | N Spread | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0379
Level 3 | Other asset-backed securities | discounted cash flow | T spread | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0100
Level 3 | Other asset-backed securities | discounted cash flow | T spread | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0560
Level 3 | Other asset-backed securities | discounted cash flow | discount rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.085
Level 3 | Other asset-backed securities | discounted cash flow | discount rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.200
Level 3 | Other asset-backed securities | discounted cash flow | discount margin | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.146
Level 3 | Other asset-backed securities | discounted cash flow | median comparable yield | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.100
Level 3 | Other asset-backed securities | discounted cash flow | median comparable yield | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.220
Level 3 | Other asset-backed securities | market comparable | median price | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	28.54
Level 3 | Other asset-backed securities | market comparable | median price | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	92.06
Level 3 | Other asset-backed securities | vendor price | haircut | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.030
Level 3 | Other asset-backed securities | vendor price | haircut | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.050