Fair Value Measurements - Quantitative Information for Level 3 Measurements (Details) $ in Millions	Mar. 31, 2023 USD ($) item	Dec. 31, 2022 USD ($) item
Investment and universal life contracts | Lapse rate
Unobservable inputs
Embedded derivative, Input		0.000
Investment and universal life contracts | Lapse rate | Weighted average input
Unobservable inputs
Embedded derivative, Input		0.000
Recurring Fair Value Measurements
Unobservable inputs
Assets measured at fair value | $	$ 234,370.9	$ 225,470.3
Liabilities measured at fair value | $	2,266.9	2,764.4
Recurring Fair Value Measurements | Fair value hierarchy Level 3
Unobservable inputs
Assets measured at fair value | $	3,922.7	3,447.3
Liabilities measured at fair value | $	2,775.3	3,394.9
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Investment and universal life contracts
Unobservable inputs
Liabilities measured at fair value | $	$ (55.5)	$ (46.4)
Embedded derivative, Valuation technique	Discounted cash flow	Discounted cash flow
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long duration interest rate | Minimum
Unobservable inputs
Embedded derivative, Input	0.023	0.024
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long duration interest rate | Maximum
Unobservable inputs
Embedded derivative, Input	0.046	0.047
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long duration interest rate | Weighted average input
Unobservable inputs
Embedded derivative, Input	0.033	0.031
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long-term equity market volatility | Minimum
Unobservable inputs
Embedded derivative, Input	0.151	0.178
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long-term equity market volatility | Maximum
Unobservable inputs
Embedded derivative, Input	0.254	0.369
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long-term equity market volatility | Weighted average input
Unobservable inputs
Embedded derivative, Input	0.192	0.220
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Non-performance risk
Unobservable inputs
Embedded derivative, Input	0.009	0.009
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Non-performance risk | Weighted average input
Unobservable inputs
Embedded derivative, Input	0.009	0.009
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Lapse rate
Unobservable inputs
Embedded derivative, Input	0.000
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Lapse rate | Weighted average input
Unobservable inputs
Embedded derivative, Input	0.000
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Fixed maturities | Trading
Unobservable inputs
Assets measured at fair value | $	$ 128.6	$ 119.7
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Discount rate | Minimum
Unobservable inputs
Fixed maturities, trading, Input	0.100	0.096
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Discount rate | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input		0.152
Fixed maturities, trading, Input	0.166
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Discount rate | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input		0.110
Fixed maturities, trading, Input	0.123
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale
Unobservable inputs
Assets measured at fair value | $	$ 1,569.7	$ 1,479.9
Fixed maturities, available-for-sale, Valuation technique	Discounted cash flow	Discounted cash flow
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Discount rate | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.050	0.027
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Discount rate | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.249	0.331
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Discount rate | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.120	0.110
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Illiquidity premium | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0	0
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Illiquidity premium | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0480	0.0467
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Illiquidity premium | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0080	0.0050
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Comparability adjustment | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input		(0.0016)
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Comparability adjustment | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	(0.0003)	0
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Comparability adjustment | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	(0.0003)	(0.0011)
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale
Unobservable inputs
Assets measured at fair value | $	$ 40.0	$ 39.5
Fixed maturities, available-for-sale, Valuation technique	Discounted cash flow	Discounted cash flow
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Discount rate
Unobservable inputs
Fixed maturities, available-for-sale, Input		0.044
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Discount rate | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.054
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Discount rate | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.054	0.044
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Comparability adjustment
Unobservable inputs
Fixed maturities, available-for-sale, Input		0.0055
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Comparability adjustment | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0052
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Comparability adjustment | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0052	0.0055
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale
Unobservable inputs
Assets measured at fair value | $	$ 625.7	$ 467.8
Fixed maturities, available-for-sale, Valuation technique	Discounted cash flow	Discounted cash flow
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discounted cash flow | Discount rate | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.057	0.056
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discounted cash flow | Discount rate | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.091	0.082
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discounted cash flow | Discount rate | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.082	0.076
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discounted cash flow | Illiquidity premium | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0	0
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discounted cash flow | Illiquidity premium | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0415	0.0260
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discounted cash flow | Illiquidity premium | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0239	0.0220
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discounted cash flow | Comparability adjustment | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0032	0.0001
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discounted cash flow | Comparability adjustment | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0152	0.0139
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discounted cash flow | Comparability adjustment | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0086	0.0077
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets
Unobservable inputs
Assets measured at fair value | $	$ 894.2	$ 1,034.1
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Minimum
Unobservable inputs
Separate account assets, Input	0.060	0.055
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Maximum
Unobservable inputs
Separate account assets, Input	0.100	0.100
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Weighted average input
Unobservable inputs
Separate account assets, Input	0.073	0.070
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Terminal capitalization rate | Minimum
Unobservable inputs
Separate account assets, Input	0.050	0.045
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Terminal capitalization rate | Maximum
Unobservable inputs
Separate account assets, Input	0.095	0.095
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Terminal capitalization rate | Weighted average input
Unobservable inputs
Separate account assets, Input	0.060	0.058
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Average market rent growth rate | Minimum
Unobservable inputs
Separate account assets, Input	0.020	0.020
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Average market rent growth rate | Maximum
Unobservable inputs
Separate account assets, Input	0.038	0.038
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Average market rent growth rate | Weighted average input
Unobservable inputs
Separate account assets, Input	0.030	0.030
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Loan to value | Minimum
Unobservable inputs
Separate account assets, Input	0.434	0.436
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Loan to value | Maximum
Unobservable inputs
Separate account assets, Input	0.649	0.622
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Loan to value | Weighted average input
Unobservable inputs
Separate account assets, Input	0.522	0.506
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Market interest rate | Minimum
Unobservable inputs
Separate account assets, Input	0.055	0.053
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Market interest rate | Maximum
Unobservable inputs
Separate account assets, Input	0.086	0.086
Recurring Fair Value Measurements | Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Market interest rate | Weighted average input
Unobservable inputs
Separate account assets, Input	0.070	0.066