Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 5,350.0	$ 5,185.0
Equity securities at fair value	98.8	99.6
Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	1,290.8	1,269.7
Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 1,943.5	$ 1,901.7
Level 3 | Municipal bonds | discounted cash flow | option adjusted spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0308
Level 3 | Corporate bonds | discounted cash flow | option adjusted spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0241
Level 3 | Corporate bonds | discounted cash flow | exit cap
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	6.2
Level 3 | Corporate bonds | vendor priced | vendor priced
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	79.64
Level 3 | Corporate bonds | market comparable | EV / Fwd EBITDA (x)
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	5.92
Level 3 | Mortgage-backed and other asset-backed securities | Fair Value
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 104.6
Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | discount margin
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.395
Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | PDI spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.055
Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | SBL spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.045
Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | CPR
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.200
Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | default rate annual
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.040
Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | recovery
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.650
Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | I spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	175
Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | N spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0463
Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | T Spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0226
Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | LIBOR
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.023
Level 3 | Mortgage-backed and other asset-backed securities | market comparable | median price
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	81.34
Minimum | Level 3 | discounted cash flow | option budget
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.009
Minimum | Level 3 | black-scholes | volatility
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	0.280
Minimum | Level 3 | black-scholes | time to exit
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	2.67
Minimum | Level 3 | market comparable | price/book ExAOCI
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	1.06
Minimum | Level 3 | Corporate bonds | discounted cash flow | yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.061
Minimum | Level 3 | Corporate bonds | discounted cash flow | discount rate
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	6.2
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.064
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | discount rate
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.160
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | median comparable yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.207
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | weighting
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.170
Minimum | Level 3 | Mortgage-backed and other asset-backed securities | vendor priced | haircut
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0001
Maximum | Level 3 | discounted cash flow | option budget
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.035
Maximum | Level 3 | black-scholes | volatility
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	0.440
Maximum | Level 3 | Corporate bonds | discounted cash flow | yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	11.0
Maximum | Level 3 | Corporate bonds | discounted cash flow | discount rate
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	10.7
Maximum | Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.065
Maximum | Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | discount rate
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.210
Maximum | Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | median comparable yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.432
Maximum | Level 3 | Mortgage-backed and other asset-backed securities | discounted cash flow | weighting
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.830
Maximum | Level 3 | Mortgage-backed and other asset-backed securities | vendor priced | haircut
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.003
Weighted Average | Level 3 | discounted cash flow | lapse rate
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.054
Weighted Average | Level 3 | discounted cash flow | mortality multiplier
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.678
Weighted Average | Level 3 | discounted cash flow | non-performance adjustment
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.050