Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details)	Sep. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 5,272,100,000	$ 6,239,300,000
Equity securities at fair value	113,800,000	147,200,000
Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Equity securities at fair value	113,800,000	147,200,000
Derivatives embedded in fixed indexed annuity products	89,900,000	106,600,000
Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	1,307,900,000	1,703,400,000
Municipal bonds | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	1,307,900,000	1,703,400,000
Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	1,927,800,000	2,388,700,000
Corporate bonds | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	1,927,800,000	2,388,700,000
Level 3 | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Equity securities at fair value	2,000,000.0	1,400,000
Derivatives embedded in fixed indexed annuity products	89,900,000	106,600,000
Level 3 | Municipal bonds | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	53,300,000	60,800,000
Level 3 | Corporate bonds | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 239,800,000	$ 210,300,000
Level 3 | Corporate bonds | exit cap rate | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.062
Level 3 | Corporate bonds | EV / Fwd EBITDA (x) | market comparable
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	5.1
Level 3 | Mortgage-backed and other asset-backed securities | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 107,200,000
Minimum | Level 3 | volatility | black-scholes
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	0.320
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	0.0330
Minimum | Level 3 | Corporate bonds | option adjusted spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0242
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	0.0016
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	0.0463
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	0.0226
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	0.070
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	0.160
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	0.304
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	0.172
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	0.470
Maximum | Level 3 | option budget | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.033
Maximum | Level 3 | Municipal bonds | option adjusted spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0446
Maximum | Level 3 | Corporate bonds | option adjusted spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0393
Maximum | Level 3 | Corporate bonds | T Spread | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0403
Maximum | Level 3 | Corporate bonds | yield | discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.112
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	0.074
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	0.210
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	0.331
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	0.050