Fair Value Measurements - Quantitative Information for Level 3 Measurements (Details) - Recurring Fair Value Measurements $ in Millions	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Unobservable inputs
Assets measured at fair value	$ 210,374.7	$ 223,689.0
Liabilities measured at fair value	(293.8)	(688.4)
Fair value hierarchy Level 3
Unobservable inputs
Assets measured at fair value	8,790.1	7,958.1
Liabilities measured at fair value	(60.7)	(166.4)
Fair value hierarchy Level 3 | Investment contracts
Unobservable inputs
Liabilities measured at fair value	$ (45.2)	$ (160.3)
Embedded derivative, Valuation technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Fair value hierarchy Level 3 | Investment contracts | Long duration interest rate | Minimum
Unobservable inputs
Embedded derivative, Input	2.8	2.5
Fair value hierarchy Level 3 | Investment contracts | Long duration interest rate | Maximum
Unobservable inputs
Embedded derivative, Input	2.9	2.7
Fair value hierarchy Level 3 | Investment contracts | Long-term equity market volatility | Minimum
Unobservable inputs
Embedded derivative, Input	16.7	18.7
Fair value hierarchy Level 3 | Investment contracts | Long-term equity market volatility | Maximum
Unobservable inputs
Embedded derivative, Input	27.8	41.1
Fair value hierarchy Level 3 | Investment contracts | Non-performance risk | Minimum
Unobservable inputs
Embedded derivative, Input	0.6	0.2
Fair value hierarchy Level 3 | Investment contracts | Non-performance risk | Maximum
Unobservable inputs
Embedded derivative, Input	1.6	1.2
Fair value hierarchy Level 3 | Investment contracts | Lapse rate | Minimum
Unobservable inputs
Embedded derivative, Input	1.3	1.3
Fair value hierarchy Level 3 | Investment contracts | Lapse rate | Maximum
Unobservable inputs
Embedded derivative, Input	16.0	16.0
Fair value hierarchy Level 3 | Non-U.S. governments | Available-for-sale
Unobservable inputs
Assets measured at fair value	$ 4.6	$ 6.1
Fixed maturities, available-for-sale, Valuation technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Fair value hierarchy Level 3 | Non-U.S. governments | Available-for-sale | Discount rate
Unobservable inputs
Fixed maturities, available-for-sale, Input	3.2	2.7
Fair value hierarchy Level 3 | Non-U.S. governments | Available-for-sale | Discount rate | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	3.2	2.7
Fair value hierarchy Level 3 | Non-U.S. governments | Available-for-sale | Illiquidity premium
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.50	0.50
Fair value hierarchy Level 3 | Non-U.S. governments | Available-for-sale | Illiquidity premium | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.50	0.50
Fair value hierarchy Level 3 | Non-U.S. governments | Available-for-sale | Comparability adjustment
Unobservable inputs
Fixed maturities, available-for-sale, Input	(0.25)	(0.25)
Fair value hierarchy Level 3 | Non-U.S. governments | Available-for-sale | Comparability adjustment | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	(0.25)	(0.25)
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale
Unobservable inputs
Assets measured at fair value	$ 25.4	$ 51.0
Fixed maturities, available-for-sale, Valuation technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discount rate | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input	3.3	1.9
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discount rate | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	4.5	7.5
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Discount rate | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	3.9	4.6
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Illiquidity premium | Minimum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.00	0.00
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Illiquidity premium | Maximum
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.60	0.60
Fair value hierarchy Level 3 | Corporate debt securities | Available-for-sale | Illiquidity premium | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	0.36	0.21
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale
Unobservable inputs
Assets measured at fair value		$ 0.5
Fixed maturities, available-for-sale, Valuation technique		us-gaap:ValuationTechniqueDiscountedCashFlowMember
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale | Discount rate
Unobservable inputs
Fixed maturities, available-for-sale, Input		6.0
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale | Discount rate | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input		6.0
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale | Probability of default
Unobservable inputs
Fixed maturities, available-for-sale, Input		85.0
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale | Probability of default | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input		85.0
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale | Potential loss severity
Unobservable inputs
Fixed maturities, available-for-sale, Input		32.0
Fair value hierarchy Level 3 | Commercial mortgage-backed securities | Available-for-sale | Potential loss severity | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input		32.0
Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale
Unobservable inputs
Assets measured at fair value	$ 1.7	$ 2.3
Fixed maturities, available-for-sale, Valuation technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discount rate
Unobservable inputs
Fixed maturities, available-for-sale, Input	5.0	5.0
Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Discount rate | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	5.0	5.0
Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Illiquidity premium
Unobservable inputs
Fixed maturities, available-for-sale, Input	5.00	5.00
Fair value hierarchy Level 3 | Other debt obligations | Available-for-sale | Illiquidity premium | Weighted average input
Unobservable inputs
Fixed maturities, available-for-sale, Input	5.00	5.00
Fair value hierarchy Level 3 | Separate account assets
Unobservable inputs
Assets measured at fair value	$ 8,440.8	$ 7,484.6
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Discount rate | Minimum
Unobservable inputs
Separate account assets, Input	3.3	2.3
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Discount rate | Maximum
Unobservable inputs
Separate account assets, Input	4.7	8.0
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Discount rate | Weighted average input
Unobservable inputs
Separate account assets, Input	4.2	4.8
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Illiquidity premium | Minimum
Unobservable inputs
Separate account assets, Input	0.00	0.00
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Illiquidity premium | Maximum
Unobservable inputs
Separate account assets, Input	0.60	0.60
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Illiquidity premium | Weighted average input
Unobservable inputs
Separate account assets, Input	0.56	0.17
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Credit spread rate | Minimum
Unobservable inputs
Separate account assets, Input	0.85	0.62
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Credit spread rate | Maximum
Unobservable inputs
Separate account assets, Input	1.72	6.90
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	1.68	2.93
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Minimum
Unobservable inputs
Separate account assets, Input	5.6	5.8
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Maximum
Unobservable inputs
Separate account assets, Input	11.5	17.2
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Discount rate | Weighted average input
Unobservable inputs
Separate account assets, Input	6.7	6.9
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Terminal capitalization rate | Minimum
Unobservable inputs
Separate account assets, Input	4.3	4.3
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Terminal capitalization rate | Maximum
Unobservable inputs
Separate account assets, Input	9.3	9.3
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	5.8	6.1
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	2.0	0.5
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	4.7	4.7
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	2.9	2.9
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Loan to value | Minimum
Unobservable inputs
Separate account assets, Input	11.0	12.1
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Loan to value | Maximum
Unobservable inputs
Separate account assets, Input	69.3	71.4
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	45.9	45.8
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Market interest rate | Minimum
Unobservable inputs
Separate account assets, Input	3.9	3.1
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Market interest rate | Maximum
Unobservable inputs
Separate account assets, Input	6.0	4.5
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	4.3	3.8