Fair Value Measurements - Quantitative Information for Level 3 Measurements (Details) - Recurring Fair Value Measurements $ in Millions	Mar. 31, 2024 USD ($) item	Dec. 31, 2023 USD ($) item
Unobservable inputs
Assets measured at fair value	$ 243,922.6	$ 240,637.6
Liabilities measured at fair value	1,957.5	1,823.4
Fair value hierarchy Level 3
Unobservable inputs
Assets measured at fair value	5,106.4	5,130.2
Liabilities measured at fair value	2,487.3	2,317.6
Fair value hierarchy Level 3 | Investment and universal life contracts
Unobservable inputs
Liabilities measured at fair value	$ (174.7)	$ (115.5)
Embedded derivative, Valuation technique	Discounted cash flow	Discounted cash flow
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long duration interest rate | Minimum
Unobservable inputs
Embedded derivative, Input	0.027	0.025
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long duration interest rate | Maximum
Unobservable inputs
Embedded derivative, Input	0.050	0.048
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	0.042	0.040
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long-term equity market volatility | Minimum
Unobservable inputs
Embedded derivative, Input	0.153	0.155
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long-term equity market volatility | Maximum
Unobservable inputs
Embedded derivative, Input	0.471	0.401
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	0.214	0.192
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Non-performance risk | Minimum
Unobservable inputs
Embedded derivative, Input	0.006	0.008
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Non-performance risk | Maximum
Unobservable inputs
Embedded derivative, Input	0.013	0.016
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Non-performance risk | Weighted average input
Unobservable inputs
Embedded derivative, Input	0.010	0.011
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Lapse rate | Minimum
Unobservable inputs
Embedded derivative, Input	0.000	0.000
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Lapse rate | Maximum
Unobservable inputs
Embedded derivative, Input	0.550	0.550
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Lapse rate | Weighted average input
Unobservable inputs
Embedded derivative, Input	0.091	0.070
Fair value hierarchy Level 3 | Fixed maturities | Trading
Unobservable inputs
Assets measured at fair value	$ 265.4	$ 203.9
Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Discount rate | Minimum
Unobservable inputs
Fixed maturities, trading, Input	0.108	0.114
Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Discount rate | Maximum
Unobservable inputs
Fixed maturities, trading, Input	0.206	0.223
Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Discount rate | Weighted average input
Unobservable inputs
Fixed maturities, trading, Input	0.127	0.133
Fair value hierarchy Level 3 | Corporate debt securities | Discounted cash flow | Earnings before interest, taxes, depreciation and amortization multiple | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input | item		3.25
Fair value hierarchy Level 3 | Corporate debt securities | Discounted cash flow | Earnings before interest, taxes, depreciation and amortization multiple | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input		3.25
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale
Unobservable inputs
Assets measured at fair value	$ 1,930.9	$ 1,997.4
Fixed maturities, available-for-sale, Valuation technique	Discounted cash flow	Discounted cash flow
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	0.052	0.049
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	0.206	0.242
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	0.110	0.120
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Earnings before interest, taxes, depreciation and amortization multiple
Unobservable inputs
Fixed maturities, available-for-sale, Input	3.66
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Earnings before interest, taxes, depreciation and amortization multiple | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	3.66
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	0.0030
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	0.0791	0.0483
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	0.0147	0.0121
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	0.0067
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	0.0188	0.0217
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	0.0119	0.0141
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale
Unobservable inputs
Assets measured at fair value	$ 98.6	$ 74.6
Fixed maturities, available-for-sale, Valuation technique	Discounted cash flow	Discounted cash flow
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	0.041
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	0.044	0.041
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Comparability adjustment
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0012	0.0020
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	0.0012	0.0020
Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale
Unobservable inputs
Assets measured at fair value	$ 898.8	$ 879.5
Fixed maturities, available-for-sale, Valuation technique	Discounted cash flow	Discounted cash flow
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	0.052	0.050
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	0.117	0.106
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	0.077	0.074
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	0.0069	0.0069
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	0.0429	0.0650
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	0.0330	0.0337
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	(0.0021)	(0.0020)
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	0.0121	0.0213
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	0.0060	0.0092
Fair value hierarchy Level 3 | Other investments
Unobservable inputs
Assets measured at fair value	$ 163.9	$ 163.2
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Discount rate | Minimum
Unobservable inputs
Other investments, Input	0.120
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Discount rate | Maximum
Unobservable inputs
Other investments, Input	0.135
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Discount rate | Weighted average input
Unobservable inputs
Other investments, Input	0.124
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Probability of default | Minimum
Unobservable inputs
Other investments, Input	0.060
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Probability of default | Maximum
Unobservable inputs
Other investments, Input	0.100
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Probability of default | Weighted average input
Unobservable inputs
Other investments, Input	0.083
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Potential loss severity | Minimum
Unobservable inputs
Other investments, Input	0.870
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Potential loss severity | Maximum
Unobservable inputs
Other investments, Input	1.000
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Potential loss severity | Weighted average input
Unobservable inputs
Other investments, Input	0.914
Fair value hierarchy Level 3 | Other investments | Available-for-sale | Discounted cash flow | Discount rate | Minimum
Unobservable inputs
Other investments, Input		0.120
Fair value hierarchy Level 3 | Other investments | Available-for-sale | Discounted cash flow | Discount rate | Maximum
Unobservable inputs
Other investments, Input		0.135
Fair value hierarchy Level 3 | Other investments | Available-for-sale | Discounted cash flow | Discount rate | Weighted average input
Unobservable inputs
Other investments, Input		0.126
Fair value hierarchy Level 3 | Other investments | Available-for-sale | Discounted cash flow | Probability of default | Minimum
Unobservable inputs
Other investments, Input		0.060
Fair value hierarchy Level 3 | Other investments | Available-for-sale | Discounted cash flow | Probability of default | Maximum
Unobservable inputs
Other investments, Input		0.100
Fair value hierarchy Level 3 | Other investments | Available-for-sale | Discounted cash flow | Probability of default | Weighted average input
Unobservable inputs
Other investments, Input		0.085
Fair value hierarchy Level 3 | Other investments | Available-for-sale | Discounted cash flow | Potential loss severity | Minimum
Unobservable inputs
Other investments, Input		0.870
Fair value hierarchy Level 3 | Other investments | Available-for-sale | Discounted cash flow | Potential loss severity | Maximum
Unobservable inputs
Other investments, Input		1.000
Fair value hierarchy Level 3 | Other investments | Available-for-sale | Discounted cash flow | Potential loss severity | Weighted average input
Unobservable inputs
Other investments, Input		0.907
Fair value hierarchy Level 3 | Separate account assets
Unobservable inputs
Assets measured at fair value	$ 725.7	$ 752.8
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Minimum
Unobservable inputs
Separate account assets, Input	0.065	0.065
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
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Weighted average input
Unobservable inputs
Separate account assets, Input	0.071	0.075
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Terminal capitalization rate | Minimum
Unobservable inputs
Separate account assets, Input	0.052	0.053
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
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	0.058	0.061
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
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	0.037	0.037
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	0.027	0.029
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Loan to value | Minimum
Unobservable inputs
Separate account assets, Input	0.456	0.460
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Loan to value | Maximum
Unobservable inputs
Separate account assets, Input	0.720	0.720
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	0.589	0.553
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Market interest rate | Minimum
Unobservable inputs
Separate account assets, Input	0.053	0.053
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Market interest rate | Maximum
Unobservable inputs
Separate account assets, Input	0.081	0.081
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	0.063	0.064