Fair Value Measurements - Quantitative Information for Level 3 Measurements (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Unobservable inputs
Assets measured at fair value	$ 223,689.0	$ 199,900.1
Liabilities measured at fair value	(688.4)	(1,021.7)
Fair value hierarchy Level 3
Unobservable inputs
Assets measured at fair value	7,958.1	8,038.6
Liabilities measured at fair value	(166.4)	(259.0)
Fair value hierarchy Level 3 | Available-for-sale | Non-U.S. governments
Unobservable inputs
Assets measured at fair value	$ 6.1	$ 7.6
Fair value hierarchy Level 3 | Available-for-sale | Non-U.S. governments | Discounted cash flow
Unobservable inputs
Discount rate (as a percent)	2.70%	2.30%
Illiquidity premium (as a percent)	0.50%	0.50%
Comparability adjustment (as a percent)	(0.25%)	(0.25%)
Fair value hierarchy Level 3 | Available-for-sale | Non-U.S. governments | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	2.70%	2.30%
Illiquidity premium (as a percent)	0.50%	0.50%
Comparability adjustment (as a percent)	(0.25%)	(0.25%)
Fair value hierarchy Level 3 | Available-for-sale | Corporate debt securities
Unobservable inputs
Assets measured at fair value	$ 51.0	$ 49.8
Fair value hierarchy Level 3 | Available-for-sale | Corporate debt securities | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	1.90%	1.50%
Illiquidity premium (as a percent)	0.00%	0.00%
Comparability adjustment (as a percent)		0.00%
Fair value hierarchy Level 3 | Available-for-sale | Corporate debt securities | Discounted cash flow | Maximum
Unobservable inputs
Discount rate (as a percent)	7.50%	7.60%
Illiquidity premium (as a percent)	0.60%	0.60%
Comparability adjustment (as a percent)		0.20%
Fair value hierarchy Level 3 | Available-for-sale | Corporate debt securities | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	4.60%	4.00%
Illiquidity premium (as a percent)	0.21%	0.27%
Comparability adjustment (as a percent)		0.06%
Fair value hierarchy Level 3 | Available-for-sale | Commercial mortgage-backed securities
Unobservable inputs
Assets measured at fair value	$ 0.5	$ 49.3
Fair value hierarchy Level 3 | Available-for-sale | Commercial mortgage-backed securities | Discounted cash flow
Unobservable inputs
Discount rate (as a percent)	6.00%
Probability of default (as a percent)	85.00%
Potential loss severity (as a percent)	32.00%
Fair value hierarchy Level 3 | Available-for-sale | Commercial mortgage-backed securities | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)		3.10%
Probability of default (as a percent)		0.00%
Potential loss severity (as a percent)		0.00%
Fair value hierarchy Level 3 | Available-for-sale | Commercial mortgage-backed securities | Discounted cash flow | Maximum
Unobservable inputs
Discount rate (as a percent)		12.80%
Probability of default (as a percent)		10.00%
Potential loss severity (as a percent)		99.50%
Fair value hierarchy Level 3 | Available-for-sale | Commercial mortgage-backed securities | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	6.00%	10.20%
Probability of default (as a percent)	85.00%	7.80%
Potential loss severity (as a percent)	32.00%	39.50%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations
Unobservable inputs
Assets measured at fair value		$ 0.2
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow
Unobservable inputs
Discount rate (as a percent)		95.10%
Probability of default (as a percent)		100.00%
Potential loss severity (as a percent)		91.20%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)		95.10%
Probability of default (as a percent)		100.00%
Potential loss severity (as a percent)		91.20%
Fair value hierarchy Level 3 | Available-for-sale | Other debt obligations
Unobservable inputs
Assets measured at fair value	$ 2.3	$ 6.8
Fair value hierarchy Level 3 | Available-for-sale | Other debt obligations | Discounted cash flow
Unobservable inputs
Discount rate (as a percent)	5.00%	5.00%
Illiquidity premium (as a percent)	5.00%	5.00%
Fair value hierarchy Level 3 | Available-for-sale | Other debt obligations | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	5.00%	5.00%
Illiquidity premium (as a percent)	5.00%	5.00%
Fair value hierarchy Level 3 | Trading | Fixed maturities
Unobservable inputs
Assets measured at fair value		$ 10.5
Fair value hierarchy Level 3 | Trading | Fixed maturities | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)		2.30%
Illiquidity premium (as a percent)		0.00%
Fair value hierarchy Level 3 | Trading | Fixed maturities | Discounted cash flow | Maximum
Unobservable inputs
Discount rate (as a percent)		9.00%
Illiquidity premium (as a percent)		3.00%
Fair value hierarchy Level 3 | Trading | Fixed maturities | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)		2.70%
Illiquidity premium (as a percent)		2.40%
Fair value hierarchy Level 3 | Other investments
Unobservable inputs
Assets measured at fair value		$ 36.9
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment
Unobservable inputs
Discount rate (as a percent)		7.60%
Terminal capitalization rate (as a percent)		6.80%
Average market rent growth rate (as a percent)		2.90%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment | Weighted average input
Unobservable inputs
Discount rate (as a percent)		7.60%
Terminal capitalization rate (as a percent)		6.80%
Average market rent growth rate (as a percent)		2.90%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment debt
Unobservable inputs
Loan to value (as a percent)		52.50%
Credit spread rate (as a percent)		2.10%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment debt | Weighted average input
Unobservable inputs
Loan to value (as a percent)		52.50%
Credit spread rate (as a percent)		2.10%
Fair value hierarchy Level 3 | Separate account assets
Unobservable inputs
Assets measured at fair value	$ 7,484.6	$ 7,225.4
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Minimum
Unobservable inputs
Discount rate (as a percent)	2.30%	1.40%
Illiquidity premium (as a percent)	0.00%	0.00%
Credit spread rate (as a percent)	0.62%	0.83%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Maximum
Unobservable inputs
Discount rate (as a percent)	8.00%	5.30%
Illiquidity premium (as a percent)	0.60%	0.60%
Credit spread rate (as a percent)	6.90%	4.72%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Weighted average input
Unobservable inputs
Discount rate (as a percent)	4.80%	3.70%
Illiquidity premium (as a percent)	0.17%	0.13%
Credit spread rate (as a percent)	2.93%	2.27%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Minimum
Unobservable inputs
Discount rate (as a percent)	5.80%	5.80%
Terminal capitalization rate (as a percent)	4.30%	4.30%
Average market rent growth rate (as a percent)	0.50%	1.80%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Maximum
Unobservable inputs
Discount rate (as a percent)	17.20%	16.20%
Terminal capitalization rate (as a percent)	9.30%	9.30%
Average market rent growth rate (as a percent)	4.70%	4.30%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Weighted average input
Unobservable inputs
Discount rate (as a percent)	6.90%	7.00%
Terminal capitalization rate (as a percent)	6.10%	6.10%
Average market rent growth rate (as a percent)	2.90%	2.90%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Minimum
Unobservable inputs
Loan to value (as a percent)	12.10%	6.30%
Market interest rate (as a percent)	3.10%	3.30%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Maximum
Unobservable inputs
Loan to value (as a percent)	71.40%	69.70%
Market interest rate (as a percent)	4.50%	4.60%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Weighted average input
Unobservable inputs
Loan to value (as a percent)	45.80%	47.00%
Market interest rate (as a percent)	3.80%	3.90%
Fair value hierarchy Level 3 | Investment contracts
Unobservable inputs
Liabilities measured at fair value	$ (160.3)	$ (176.5)
Fair value hierarchy Level 3 | Investment contracts | Discounted cash flow | Minimum
Unobservable inputs
Long duration interest rate (as a percent)	2.50%	2.60%
Long-term equity market volatility (as a percent)	18.70%	16.00%
Non-performance risk (as a percent)	0.20%	0.30%
Lapse rate (as a percent)	1.30%	0.50%
Fair value hierarchy Level 3 | Investment contracts | Discounted cash flow | Maximum
Unobservable inputs
Long duration interest rate (as a percent)	2.70%	2.70%
Long-term equity market volatility (as a percent)	41.10%	45.90%
Non-performance risk (as a percent)	1.20%	1.70%
Lapse rate (as a percent)	16.00%	14.10%