Fair Value Measurements (Details 4) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Assets (liabilities) measured at fair value
Unobservable inputs
Assets measured at fair value	$ 185,848.6	$ 182,651.2
Liabilities measured at fair value	(1,258.7)	(1,353.6)
Assets (liabilities) measured at fair value | Available-for-sale | Non-U.S. governments
Unobservable inputs
Assets measured at fair value	11.3	11.7
Assets (liabilities) measured at fair value | Available-for-sale | Corporate debt securities
Unobservable inputs
Assets measured at fair value	76.3	70.1
Assets (liabilities) measured at fair value | Available-for-sale | Commercial mortgage-backed securities
Unobservable inputs
Assets measured at fair value	6.1	1.6
Assets (liabilities) measured at fair value | Available-for-sale | Collateralized debt obligations
Unobservable inputs
Assets measured at fair value	13.8	13.6
Assets (liabilities) measured at fair value | Available-for-sale | Other debt obligations
Unobservable inputs
Assets measured at fair value	33.0	33.6
Assets (liabilities) measured at fair value | Trading | Fixed maturities
Unobservable inputs
Assets measured at fair value	40.8	36.2
Assets (liabilities) measured at fair value | Trading | Collateralized private investment vehicle | Fixed maturities
Unobservable inputs
Assets measured at fair value	110.4	110.4
Assets (liabilities) measured at fair value | Other investments | Discounted cash flow, commercial mortgage loans of consolidated VIEs
Unobservable inputs
Assets measured at fair value	57.9	68.1
Assets (liabilities) measured at fair value | Other investments | Discounted cash flow, equity method real estate investment
Unobservable inputs
Assets measured at fair value	75.6	74.8
Assets (liabilities) measured at fair value | Separate account assets
Unobservable inputs
Assets measured at fair value	5,164.9	5,090.4
Assets (liabilities) measured at fair value | Investment-type insurance contracts
Unobservable inputs
Liabilities measured at fair value	(25.8)	(6.9)
Assets (liabilities) measured at fair value | Derivative liabilities
Unobservable inputs
Liabilities measured at fair value	(17.8)	(19.9)
Assets (liabilities) measured at fair value | Other liabilities
Unobservable inputs
Liabilities measured at fair value	(75.9)	(73.9)
Fair value hierarchy Level 3
Unobservable inputs
Assets measured at fair value	6,035.6	6,005.9
Liabilities measured at fair value	$ (136.1)	$ (120.4)
Fair value hierarchy Level 3 | Available-for-sale | Non-U.S. governments | Discounted cash flow | Input
Unobservable inputs
Discount rate (as a percent)	2.10%	2.00%
Illiquidity premium (as a percent)	0.50%	0.50%
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)	2.10%	2.00%
Illiquidity premium (as a percent)	0.50%	0.50%
Fair value hierarchy Level 3 | Available-for-sale | Corporate debt securities | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	1.70%	1.90%
Illiquidity premium (as a percent)	0.00%	0.00%
Earnings before interest, taxes, depreciation and amortization multiple	0.00	0.00
Comparability adjustment (as a percent)	0.00%	0.00%
Probability of default (as a percent)	0.00%	0.00%
Potential loss severity (as a percent)	0.00%	0.00%
Fair value hierarchy Level 3 | Available-for-sale | Corporate debt securities | Discounted cash flow | Maximum
Unobservable inputs
Discount rate (as a percent)	9.80%	7.70%
Illiquidity premium (as a percent)	0.25%	0.25%
Earnings before interest, taxes, depreciation and amortization multiple	5.00	4.50
Comparability adjustment (as a percent)	1.25%	1.25%
Probability of default (as a percent)	25.00%	100.00%
Potential loss severity (as a percent)	16.00%	16.00%
Fair value hierarchy Level 3 | Available-for-sale | Corporate debt securities | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	5.30%	4.40%
Illiquidity premium (as a percent)	0.14%	0.15%
Earnings before interest, taxes, depreciation and amortization multiple	0.40	0.20
Comparability adjustment (as a percent)	0.38%	0.43%
Probability of default (as a percent)	7.60%	5.40%
Potential loss severity (as a percent)	4.80%	0.90%
Fair value hierarchy Level 3 | Available-for-sale | Commercial mortgage-backed securities | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	1.50%	1.50%
Fair value hierarchy Level 3 | Available-for-sale | Commercial mortgage-backed securities | Discounted cash flow | Maximum
Unobservable inputs
Discount rate (as a percent)	13.00%	4.50%
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)	11.00%	0.00%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Input
Unobservable inputs
Discount rate (as a percent)	1.30%	1.50%
Illiquidity premium (as a percent)	4.00%	4.00%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	1.30%	1.50%
Illiquidity premium (as a percent)	4.00%	4.00%
Fair value hierarchy Level 3 | Available-for-sale | Other debt obligations | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	2.00%	2.00%
Illiquidity premium (as a percent)	0.00%	0.00%
Fair value hierarchy Level 3 | Available-for-sale | Other debt obligations | Discounted cash flow | Maximum
Unobservable inputs
Discount rate (as a percent)	15.00%	15.00%
Illiquidity premium (as a percent)	0.50%	0.50%
Fair value hierarchy Level 3 | Available-for-sale | Other debt obligations | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	6.30%	6.70%
Illiquidity premium (as a percent)	0.10%	0.11%
Fair value hierarchy Level 3 | Trading | Fixed maturities | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	1.40%	1.60%
Illiquidity premium (as a percent)	0.00%	0.00%
Fair value hierarchy Level 3 | Trading | Fixed maturities | Discounted cash flow | Maximum
Unobservable inputs
Discount rate (as a percent)	79.90%	83.00%
Illiquidity premium (as a percent)	14.00%	14.00%
Fair value hierarchy Level 3 | Trading | Fixed maturities | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	3.10%	3.40%
Illiquidity premium (as a percent)	3.40%	3.70%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, commercial mortgage loans of consolidated VIEs | Input
Unobservable inputs
Discount rate (as a percent)	4.30%	4.80%
Illiquidity premium (as a percent)	1.91%	0.94%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, commercial mortgage loans of consolidated VIEs | Weighted average input
Unobservable inputs
Discount rate (as a percent)	4.30%	4.80%
Illiquidity premium (as a percent)	1.91%	0.94%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment | Minimum
Unobservable inputs
Discount rate (as a percent)	7.80%	7.80%
Terminal capitalization rate (as a percent)	5.50%	5.50%
Average market rent growth rate (as a percent)	3.50%	3.50%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment | Maximum
Unobservable inputs
Discount rate (as a percent)	8.10%	8.10%
Terminal capitalization rate (as a percent)	6.80%	6.80%
Average market rent growth rate (as a percent)	3.60%	3.60%
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)	7.90%	7.90%
Terminal capitalization rate (as a percent)	6.10%	6.10%
Average market rent growth rate (as a percent)	3.60%	3.60%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment debt | Minimum
Unobservable inputs
Loan to value (as a percent)	39.90%	40.50%
Credit spread rate (as a percent)	1.80%	1.50%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment debt | Maximum
Unobservable inputs
Loan to value (as a percent)	61.50%	61.00%
Credit spread rate (as a percent)	2.00%	2.00%
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)	50.70%	50.70%
Credit spread rate (as a percent)	1.90%	1.80%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Minimum
Unobservable inputs
Discount rate (as a percent)	0.60%	0.60%
Illiquidity premium (as a percent)	0.00%	0.00%
Credit spread rate (as a percent)	0.53%	0.32%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Maximum
Unobservable inputs
Discount rate (as a percent)	5.40%	5.60%
Illiquidity premium (as a percent)	0.60%	0.60%
Credit spread rate (as a percent)	4.80%	4.40%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Weighted average input
Unobservable inputs
Discount rate (as a percent)	3.20%	3.30%
Illiquidity premium (as a percent)	0.11%	0.12%
Credit spread rate (as a percent)	2.17%	2.14%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Minimum
Unobservable inputs
Discount rate (as a percent)	6.00%	6.00%
Terminal capitalization rate (as a percent)	4.50%	4.50%
Average market rent growth rate (as a percent)	1.40%	2.40%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Maximum
Unobservable inputs
Discount rate (as a percent)	22.80%	16.00%
Terminal capitalization rate (as a percent)	9.00%	9.00%
Average market rent growth rate (as a percent)	5.00%	4.70%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Weighted average input
Unobservable inputs
Discount rate (as a percent)	7.60%	7.60%
Terminal capitalization rate (as a percent)	6.60%	6.60%
Average market rent growth rate (as a percent)	3.20%	3.00%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Minimum
Unobservable inputs
Loan to value (as a percent)	10.70%	11.00%
Credit spread rate (as a percent)	1.60%	1.50%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Maximum
Unobservable inputs
Loan to value (as a percent)	66.80%	55.90%
Credit spread rate (as a percent)	4.90%	5.20%
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)	50.30%	50.30%
Credit spread rate (as a percent)	3.20%	3.30%
Fair value hierarchy Level 3 | Investment-type insurance contracts | Discounted cash flow | Input
Unobservable inputs
Long duration interest rate (as a percent)	3.50%
Fair value hierarchy Level 3 | Investment-type insurance contracts | Discounted cash flow | Minimum
Unobservable inputs
Long duration interest rate (as a percent)		3.80%
Long-term equity market volatility (as a percent)	15.00%	15.00%
Non-performance risk (as a percent)	0.20%	0.20%
Lapse rate (as a percent)	0.50%	0.50%
Period of swap rate used for calculating long duration interest rate	20 years	20 years
Fair value hierarchy Level 3 | Investment-type insurance contracts | Discounted cash flow | Maximum
Unobservable inputs
Long duration interest rate (as a percent)		3.90%
Long-term equity market volatility (as a percent)	39.00%	40.10%
Non-performance risk (as a percent)	1.10%	1.20%
Lapse rate (as a percent)	14.60%	14.60%
Period of swap rate used for calculating long duration interest rate	30 years	30 years