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Fair Value Measurements - Quantitative Information for Level 3 Measurements (Details) - Recurring Fair Value Measurements $ in Millions	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Unobservable inputs
Assets measured at fair value	$ 249,740.8	$ 240,637.6
Liabilities measured at fair value	1,840.8	1,823.4
Fair value hierarchy Level 3
Unobservable inputs
Assets measured at fair value	5,680.2	5,130.2
Liabilities measured at fair value	2,366.1	2,317.6
Fair value hierarchy Level 3 | Investment and universal life contracts
Unobservable inputs
Liabilities measured at fair value	$ (578.4)	$ (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	0.03	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	0.049	0.048
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long duration interest rate | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input		0.04
Embedded derivative, Input	0.048
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long-term equity market volatility | Minimum
Unobservable inputs
Embedded derivative, Input	0.145	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	0.493	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	0.004	0.008
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Non-performance risk | Maximum
Unobservable inputs
Embedded derivative, Input	0.011	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	0.008	0.011
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Lapse rate | Minimum
Unobservable inputs
Embedded derivative, Input	0	0
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Lapse rate | Maximum
Unobservable inputs
Embedded derivative, Input	0.55	0.55
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Lapse rate | Weighted average input
Unobservable inputs
Embedded derivative, Input	0.085	0.07
Fair value hierarchy Level 3 | Fixed maturities | Trading
Unobservable inputs
Assets measured at fair value	$ 289.6	$ 203.9
Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Discount rate | Minimum
Unobservable inputs
Fixed maturities, trading, Input	0.095	0.114
Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Discount rate | Maximum
Unobservable inputs
Fixed maturities, trading, Input	0.13	0.223
Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Discount rate | Weighted average input
Unobservable inputs
Fixed maturities, trading, Input	0.099	0.133
Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Earnings before interest, taxes, depreciation and amortization multiple
Unobservable inputs
Fixed maturities, trading, Input	1.1
Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Earnings before interest, taxes, depreciation and amortization multiple | Weighted average input
Unobservable inputs
Fixed maturities, trading, Input	1.1
Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Comparability adjustment | Minimum
Unobservable inputs
Fixed maturities, trading, Input	(0.0032)
Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Comparability adjustment | Maximum
Unobservable inputs
Fixed maturities, trading, Input	0.2947
Fair value hierarchy Level 3 | Fixed maturities | Trading | Discounted cash flow | Comparability adjustment | Weighted average input
Unobservable inputs
Fixed maturities, trading, Input	0.0152
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale
Unobservable inputs
Assets measured at fair value	$ 1,817.8	$ 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	0.021	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	0.127	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	0.08	0.12
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		3.25
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Earnings before interest, taxes, depreciation and amortization multiple | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input	1.1
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Earnings before interest, taxes, depreciation and amortization multiple | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	1.7
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	1.3	3.25
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	0.0030	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	0.0142	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	(0.0042)	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	0.2947	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	0.0149	0.0141
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale
Unobservable inputs
Assets measured at fair value		$ 74.6
Fixed maturities, available-for-sale, Valuation technique		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		0.041
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		0.041
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Comparability adjustment
Unobservable inputs
Fixed maturities, available-for-sale, Input		0.0020
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		0.0020
Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale
Unobservable inputs
Assets measured at fair value	$ 1,343.1	$ 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	0.049	0.05
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	0.077	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	0.054	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	(0.0083)	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	0.0260	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	0.0120	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	(0.0019)	(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	0.0415	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	0.0128	0.0092
Fair value hierarchy Level 3 | Other investments
Unobservable inputs
Assets measured at fair value	$ 129.0	$ 163.2
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Discount rate | Minimum
Unobservable inputs
Other investments, Input	0.115	0.12
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Discount rate | Maximum
Unobservable inputs
Other investments, Input	0.135	0.135
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Discount rate | Weighted average input
Unobservable inputs
Other investments, Input	0.124
Separate account assets, Input		0.126
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Probability of default | Minimum
Unobservable inputs
Other investments, Input	0.06	0.06
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Probability of default | Maximum
Unobservable inputs
Other investments, Input	0.10	0.10
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Probability of default | Weighted average input
Unobservable inputs
Other investments, Input	0.081	0.085
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Potential loss severity | Minimum
Unobservable inputs
Other investments, Input	0.87	0.87
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Potential loss severity | Maximum
Unobservable inputs
Other investments, Input	1	1
Fair value hierarchy Level 3 | Other investments | Discounted cash flow | Potential loss severity | Weighted average input
Unobservable inputs
Other investments, Input	0.92	0.907
Fair value hierarchy Level 3 | Separate account assets
Unobservable inputs
Assets measured at fair value	$ 726.7	$ 752.8
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Minimum
Unobservable inputs
Separate account assets, Input	0.07	0.065
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Maximum
Unobservable inputs
Separate account assets, Input	0.11	0.10
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Weighted average input
Unobservable inputs
Separate account assets, Input	0.072	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	0.055	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	0.06	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	0.02	0.02
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	0.045	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	0.464	0.46
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Loan to value | Maximum
Unobservable inputs
Separate account assets, Input	0.695	0.72
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	0.592	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	0.049	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	0.072	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	0.061	0.064

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