Fair Value Measurements - Quantitative Information for Level 3 Measurements (Details) - Recurring Fair Value Measurements $ in Millions	Sep. 30, 2021 USD ($) item	Dec. 31, 2020 USD ($) item
Unobservable inputs
Assets measured at fair value | $	$ 260,148.7	$ 259,883.4
Liabilities measured at fair value | $	(600.0)	(654.3)
Fair value hierarchy Level 3
Unobservable inputs
Assets measured at fair value | $	2,163.3	9,286.1
Liabilities measured at fair value | $	(356.0)	(473.5)
Fair value hierarchy Level 3 | Investment and universal life contracts
Unobservable inputs
Liabilities measured at fair value | $	$ (355.3)	$ (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	0.019	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	0.021	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	0.020	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	0.179	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	0.320	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	0.221	0.196
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Non-performance risk | Minimum
Unobservable inputs
Embedded derivative, Input	0.002	0.001
Fair value hierarchy Level 3 | Investment and universal life contracts | Discounted cash flow | Non-performance risk | Maximum
Unobservable inputs
Embedded derivative, Input	0.010	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	0.008	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	0.170	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	0.051	0.058
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale
Unobservable inputs
Assets measured at fair value | $	$ 746.9	$ 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	0.007	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	0.186	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	0.070	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	0.0070	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	0.0007	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
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		0.0769
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		0.0359
Fair value hierarchy Level 3 | Corporate debt 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 | Corporate debt 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 | Corporate debt securities | Available-for-sale | Discounted cash flow | Potential loss severity
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.164	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	0.164	0.546
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale
Unobservable inputs
Assets measured at fair value | $	$ 2.7	$ 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 | Discount rate
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.039
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale | Discounted cash flow | Discount rate | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.039
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
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
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		0.784
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		0.784
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale
Unobservable inputs
Assets measured at fair value | $	$ 1.0	$ 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 | Discount rate
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.030
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	0.030
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Illiquidity premium
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0291
Fair value hierarchy Level 3 | Collateralized debt obligations | Available-for-sale | Discounted cash flow | Illiquidity premium | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.0291
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
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
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 | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input		0.405
Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale
Unobservable inputs
Assets measured at fair value | $	$ 22.2	$ 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	0.0235	0.0500
Fair value hierarchy Level 3 | Other investments
Unobservable inputs
Assets measured at fair value | $	$ 1.3	$ 30.4
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, other investments | Discount rate | Minimum
Unobservable inputs
Other investments, Input		0.250
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, other investments | Discount rate | Maximum
Unobservable inputs
Other investments, Input		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
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
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
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
Fair value hierarchy Level 3 | Other investments | Market comparables | Revenue multiples | Minimum
Unobservable inputs
Other investments, Input	6.8	6.0
Fair value hierarchy Level 3 | Other investments | Market comparables | Revenue multiples | Maximum
Unobservable inputs
Other investments, Input	9.1	8.0
Fair value hierarchy Level 3 | Other investments | Market comparables | Revenue multiples | Weighted average input
Unobservable inputs
Other investments, Input	8.0	7.0
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate | Discount rate
Unobservable inputs
Other investments, Input		0.065
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate | Discount rate | Weighted average input
Unobservable inputs
Other investments, Input		0.065
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate | Terminal capitalization rate
Unobservable inputs
Other investments, Input		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		0.053
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate | Average market rent growth rate
Unobservable inputs
Other investments, Input		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		0.026
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate debt | Credit spread rate
Unobservable inputs
Other investments, Input		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		0.033
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, real estate debt | Loan to value
Unobservable inputs
Other investments, Input		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		0.526
Fair value hierarchy Level 3 | Separate account assets
Unobservable inputs
Assets measured at fair value | $	$ 1,268.0	$ 8,893.2
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Discount rate
Unobservable inputs
Separate account assets, Input	0.012	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	0.012	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	0.0109	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	0.0109	0.0110
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Minimum
Unobservable inputs
Separate account assets, Input	0.055	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	0.045	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	0.059	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	0.036	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	0.290	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	0.602	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	0.436	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	0.023	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	0.028	0.034