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Fair Value Measurements - Quantitative Information for Level 3 Measurements (Details) - Recurring Fair Value Measurements $ in Millions	Mar. 31, 2021 USD ($) item	Dec. 31, 2020 USD ($) item
Unobservable inputs
Assets measured at fair value | $	$ 258,433.9	$ 259,883.4
Liabilities measured at fair value | $	(351.6)	(654.3)
Fair value hierarchy Level 3
Unobservable inputs
Assets measured at fair value | $	1,952.6	9,286.1
Liabilities measured at fair value | $	(188.5)	(473.5)
Fair value hierarchy Level 3 | Investment and universal life contracts
Unobservable inputs
Liabilities measured at fair value | $	$ (177.1)	$ (467.8)
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	0.018	0.012
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long duration interest rate | Maximum
Unobservable inputs
Embedded derivative, Input	0.022	0.014
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	0.021	0.013
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long-term equity market volatility | Minimum
Unobservable inputs
Embedded derivative, Input	0.176	0.176
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Long-term equity market volatility | Maximum
Unobservable inputs
Embedded derivative, Input	0.303	0.269
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	0.219	0.196
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Non-performance risk | Minimum
Unobservable inputs
Embedded derivative, Input	0.001	0.001
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Non-performance risk | Maximum
Unobservable inputs
Embedded derivative, Input	0.013	0.014
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
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	0.160	0.160
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Lapse rate | Weighted average input
Unobservable inputs
Embedded derivative, Input	0.062	0.058
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale
Unobservable inputs
Assets measured at fair value | $	$ 339.9	$ 286.1
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	0.010	0.009
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	0.123	0.117
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	0.085	0.073
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
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	0.0060	0.0060
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	0.0011	0.0019
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
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	0.0854	0.0769
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	0.0324	0.0359
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Probability of default | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.000	0.000
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Probability of default | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	1.000	1.000
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Probability of default | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.002	0.003
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Potential loss severity | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.000	0.000
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Potential loss severity | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.546	0.546
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discounted cash flow | Potential loss severity | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.001	0.002
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale
Unobservable inputs
Assets measured at fair value | $	$ 0.2	$ 1.1
Fixed maturities, available-for-sale, Valuation technique	Discounted cash flow	Discounted cash flow
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale | Discounted cash flow | Probability of default
Unobservable inputs
Fixed maturities, available-for-sale, Input	1.000	1.000
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale | Discounted cash flow | Probability of default | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	1.000	1.000
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale | Discounted cash flow | Potential loss severity
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.963	0.784
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale | Discounted cash flow | Potential loss severity | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.963	0.784
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale
Unobservable inputs
Assets measured at fair value | $	$ 97.2	$ 0.7
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 | Probability of default
Unobservable inputs
Fixed maturities, available-for-sale, Input	1.000	1.000
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Probability of default | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	1.000	1.000
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Potential loss severity
Unobservable inputs
Fixed maturities, available-for-sale, Input		0.405
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Potential loss severity | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.300
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Potential loss severity | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.387
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Potential loss severity | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.301	0.405
Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale
Unobservable inputs
Assets measured at fair value | $	$ 22.4	$ 0.8
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
Unobservable inputs
Fixed maturities, available-for-sale, Input		0.100
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	0.025
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	0.100
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	0.028	0.100
Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discounted cash flow | Illiquidity premium
Unobservable inputs
Fixed maturities, available-for-sale, Input		0.0500
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	0.0225
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	0.0500
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	0.0236	0.0500
Fair value hierarchy Level 3 | Other investments
Unobservable inputs
Assets measured at fair value | $	$ 39.7	$ 30.4
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, other investments | Discount rate | Minimum
Unobservable inputs
Other investments, Input	0.250	0.250
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, other investments | Discount rate | Maximum
Unobservable inputs
Other investments, Input	0.300	0.300
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, other investments | Discount rate | Weighted average input
Unobservable inputs
Other investments, Input	0.275	0.275
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, other investments | Terminal earnings before interest, taxes, depreciation and amortization multiple | Minimum
Unobservable inputs
Other investments, Input	3.8	3.8
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, other investments | Terminal earnings before interest, taxes, depreciation and amortization multiple | Maximum
Unobservable inputs
Other investments, Input	4.7	4.7
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, other investments | Terminal earnings before interest, taxes, depreciation and amortization multiple | Weighted average input
Unobservable inputs
Other investments, Input	4.2	4.2
Fair value hierarchy Level 3 | Other investments | Market comparables | Revenue multiples | Minimum
Unobservable inputs
Other investments, Input	6.0	6.0
Fair value hierarchy Level 3 | Other investments | Market comparables | Revenue multiples | Maximum
Unobservable inputs
Other investments, Input	9.0	8.0
Fair value hierarchy Level 3 | Other investments | Market comparables | Revenue multiples | Weighted average input
Unobservable inputs
Other investments, Input	7.5	7.0
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate | Discount rate
Unobservable inputs
Other investments, Input	0.055	0.065
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate | Discount rate | Weighted average input
Unobservable inputs
Other investments, Input	0.055	0.065
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate | Terminal capitalization rate
Unobservable inputs
Other investments, Input	0.045	0.053
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate | Terminal capitalization rate | Weighted average input
Unobservable inputs
Other investments, Input	0.045	0.053
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate | Average market rent growth rate
Unobservable inputs
Other investments, Input	0.030	0.026
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate | Average market rent growth rate | Weighted average input
Unobservable inputs
Other investments, Input	0.030	0.026
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate debt | Credit spread rate
Unobservable inputs
Other investments, Input	0.028	0.033
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate debt | Credit spread rate | Weighted average input
Unobservable inputs
Other investments, Input	0.028	0.033
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate debt | Loan to value
Unobservable inputs
Other investments, Input	0.433	0.526
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate debt | Loan to value | Weighted average input
Unobservable inputs
Other investments, Input	0.433	0.526
Fair value hierarchy Level 3 | Separate account assets
Unobservable inputs
Assets measured at fair value | $	$ 1,197.1	$ 8,893.2
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Discount rate
Unobservable inputs
Separate account assets, Input	0.013	0.012
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Discount rate | Weighted average input
Unobservable inputs
Separate account assets, Input	0.013	0.012
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Illiquidity premium
Unobservable inputs
Separate account assets, Input	0.0060	0.0060
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Illiquidity premium | Weighted average input
Unobservable inputs
Separate account assets, Input	0.0060	0.0060
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Credit spread rate
Unobservable inputs
Separate account assets, Input	0.0119	0.0110
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Credit spread rate | Weighted average input
Unobservable inputs
Separate account assets, Input	0.0119	0.0110
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Minimum
Unobservable inputs
Separate account assets, Input	0.058	0.056
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Maximum
Unobservable inputs
Separate account assets, Input	0.119	0.119
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Weighted average input
Unobservable inputs
Separate account assets, Input	0.075	0.069
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
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Terminal capitalization rate | Maximum
Unobservable inputs
Separate account assets, Input	0.093	0.093
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	0.057
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	0.015	0.015
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	0.033	0.048
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	0.028	0.030
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Loan to value | Minimum
Unobservable inputs
Separate account assets, Input	0.260	0.063
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Loan to value | Maximum
Unobservable inputs
Separate account assets, Input	0.595	0.742
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	0.429	0.475
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Market interest rate | Minimum
Unobservable inputs
Separate account assets, Input	0.025	0.020
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Market interest rate | Maximum
Unobservable inputs
Separate account assets, Input	0.045	0.050
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	0.029	0.034

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Fair value of financial and nonfinancial obligations.

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