Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details)	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	$ 5,688,000,000	$ 6,239,300,000
Equity securities at fair value	117,300,000	147,200,000
Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Equity securities at fair value	117,300,000	147,200,000
Derivatives embedded in fixed indexed annuity products	93,200,000	106,600,000
Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	1,426,300,000	1,703,400,000
Municipal bonds | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	1,426,300,000	1,703,400,000
Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	2,150,500,000	2,388,700,000
Corporate bonds | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	2,150,500,000	2,388,700,000
Level 3 | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Equity securities at fair value	1,400,000	1,400,000
Derivatives embedded in fixed indexed annuity products	93,200,000	106,600,000
Level 3 | Municipal bonds | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	51,000,000.0	60,800,000
Level 3 | Corporate bonds | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	$ 270,500,000	$ 210,300,000
Level 3 | Corporate bonds | exit cap rate | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	6.2
Level 3 | Corporate bonds | EV / Fwd EBITDA (x) | market comparable
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	5.9
Level 3 | Mortgage-backed and other asset-backed securities | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	$ 97,100,000
Minimum | Level 3 | volatility | black scholes
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	0.300
Minimum | Level 3 | variable square per unit | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	21,544
Minimum | Level 3 | variable square meter | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	100.00
Minimum | Level 3 | option budget | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.009
Minimum | Level 3 | Municipal bonds | option adjusted spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0309
Minimum | Level 3 | Corporate bonds | option adjusted spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	242
Minimum | Level 3 | Corporate bonds | N spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0363
Minimum | Level 3 | Corporate bonds | T Spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	16
Minimum | Level 3 | Corporate bonds | yield | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.038
Minimum | Level 3 | Corporate bonds | occupancy rate | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.310
Minimum | Level 3 | Corporate bonds | discount rate | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.113
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | London Interbank Offered Rate (LIBOR)
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.010
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | N spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	416
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | T Spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	272
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | yield | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.056
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | discount rate | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.155
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | discount margin | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.229
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | median comparable yield | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.129
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | PDI spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.068
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | SBL spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.045
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | weighting | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.170
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | CPR | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.200
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | default rate annual | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.040
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | recovery | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.650
Maximum | Level 3 | volatility | black scholes
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	0.460
Maximum | Level 3 | variable square per unit | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	38,345
Maximum | Level 3 | variable square meter | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	624.19
Maximum | Level 3 | option budget | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.025
Maximum | Level 3 | Municipal bonds | option adjusted spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0425
Maximum | Level 3 | Corporate bonds | option adjusted spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	393
Maximum | Level 3 | Corporate bonds | T Spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	403
Maximum | Level 3 | Corporate bonds | yield | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.106
Maximum | Level 3 | Corporate bonds | occupancy rate | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	1.000
Maximum | Level 3 | Corporate bonds | discount rate | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.120
Maximum | Level 3 | Mortgage-backed and other asset-backed securities | yield | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.073
Maximum | Level 3 | Mortgage-backed and other asset-backed securities | discount rate | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.205
Maximum | Level 3 | Mortgage-backed and other asset-backed securities | median comparable yield | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.250
Maximum | Level 3 | Mortgage-backed and other asset-backed securities | weighting | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.830
Weighted Average | Level 3 | lapse rate | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.053
Weighted Average | Level 3 | mortality multiplier | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.668
Weighted Average | Level 3 | non-performance adjustment | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.050
Weighted Average | Level 3 | Corporate bonds | weighted average cost of capital | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.05