Fair Value Measurements (Details 4) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Assets (liabilities) measured at fair value
Unobservable inputs
Assets measured at fair value	196,885.8	193,493.8
Liabilities measured at fair value	(1,440.6)	(1,278.3)
Assets (liabilities) measured at fair value | Available-for-sale | Non-U.S. governments
Unobservable inputs
Assets measured at fair value	10.1	10.3
Assets (liabilities) measured at fair value | Available-for-sale | Corporate debt securities
Unobservable inputs
Assets measured at fair value	60.9	83.0
Assets (liabilities) measured at fair value | Available-for-sale | Collateralized debt obligations
Unobservable inputs
Assets measured at fair value	3.0	12.7
Assets (liabilities) measured at fair value | Available-for-sale | Other debt obligations
Unobservable inputs
Assets measured at fair value	39.6	49.3
Assets (liabilities) measured at fair value | Trading | Fixed maturities
Unobservable inputs
Assets measured at fair value	15.4	15.2
Assets (liabilities) measured at fair value | Trading | Collateralized private investment vehicle | Fixed maturities
Unobservable inputs
Assets measured at fair value		100.4
Assets (liabilities) measured at fair value | Other investments | Discounted cash flow, commercial mortgage loans of consolidated VIEs
Unobservable inputs
Assets measured at fair value		35.0
Assets (liabilities) measured at fair value | Other investments | Discounted cash flow, equity method real estate investment
Unobservable inputs
Assets measured at fair value	28.6	92.2
Assets (liabilities) measured at fair value | Separate account assets
Unobservable inputs
Assets measured at fair value	5,989.0	5,857.4
Assets (liabilities) measured at fair value | Investment-type insurance contracts
Unobservable inputs
Liabilities measured at fair value	(228.7)	(176.4)
Assets (liabilities) measured at fair value | Derivative liabilities
Unobservable inputs
Liabilities measured at fair value		(19.3)
Assets (liabilities) measured at fair value | Other liabilities
Unobservable inputs
Liabilities measured at fair value		(66.3)
Fair value hierarchy Level 3
Unobservable inputs
Assets measured at fair value	6,660.2	6,628.3
Liabilities measured at fair value	(344.8)	(278.2)
Fair value hierarchy Level 3 | Available-for-sale | Non-U.S. governments | Discounted cash flow | Input
Unobservable inputs
Discount rate (as a percent)	1.80%	2.20%
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)	1.80%	2.20%
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)	1.00%	1.80%
Illiquidity premium (as a percent)	0.00%	0.00%
Comparability adjustment (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)	5.50%	6.70%
Illiquidity premium (as a percent)	0.25%	0.25%
Comparability adjustment (as a percent)	0.39%	0.01%
Fair value hierarchy Level 3 | Available-for-sale | Corporate debt securities | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	3.40%	4.00%
Illiquidity premium (as a percent)	0.02%	0.13%
Comparability adjustment (as a percent)	0.12%	0.00%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Input
Unobservable inputs
Discount rate (as a percent)	18.80%
Probability of default (as a percent)	100.00%
Potential loss severity (as a percent)	68.00%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)		2.70%
Probability of default (as a percent)		0.00%
Potential loss severity (as a percent)		0.00%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Maximum
Unobservable inputs
Discount rate (as a percent)		17.10%
Probability of default (as a percent)		100.00%
Potential loss severity (as a percent)		70.00%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	18.80%	6.10%
Probability of default (as a percent)	100.00%	23.50%
Potential loss severity (as a percent)	68.00%	16.40%
Fair value hierarchy Level 3 | Available-for-sale | Other debt obligations | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	1.30%	1.40%
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)	5.00%	5.00%
Illiquidity premium (as a percent)	10.00%	10.00%
Fair value hierarchy Level 3 | Available-for-sale | Other debt obligations | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	2.20%	2.30%
Illiquidity premium (as a percent)	2.15%	1.75%
Fair value hierarchy Level 3 | Trading | Fixed maturities | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	1.70%	1.80%
Illiquidity premium (as a percent)	2.00%	2.00%
Fair value hierarchy Level 3 | Trading | Fixed maturities | Discounted cash flow | Maximum
Unobservable inputs
Discount rate (as a percent)	4.00%	126.90%
Illiquidity premium (as a percent)	8.00%	14.00%
Fair value hierarchy Level 3 | Trading | Fixed maturities | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	2.50%	3.50%
Illiquidity premium (as a percent)	4.50%	4.60%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, commercial mortgage loans of consolidated VIEs | Input
Unobservable inputs
Discount rate (as a percent)		4.20%
Illiquidity premium (as a percent)		0.76%
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)		4.20%
Illiquidity premium (as a percent)		0.76%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment | Input
Unobservable inputs
Discount rate (as a percent)	7.90%
Terminal capitalization rate (as a percent)	6.80%
Average market rent growth rate (as a percent)	3.30%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment | Minimum
Unobservable inputs
Discount rate (as a percent)		7.30%
Terminal capitalization rate (as a percent)		5.50%
Average market rent growth rate (as a percent)		3.30%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment | Maximum
Unobservable inputs
Discount rate (as a percent)		8.00%
Terminal capitalization rate (as a percent)		6.80%
Average market rent growth rate (as a percent)		3.70%
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%	7.60%
Terminal capitalization rate (as a percent)	6.80%	6.10%
Average market rent growth rate (as a percent)	3.30%	3.50%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment debt | Input
Unobservable inputs
Loan to value (as a percent)	57.30%
Credit spread rate (as a percent)	2.00%
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)		34.20%
Credit spread rate (as a percent)		1.80%
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)		58.90%
Credit spread rate (as a percent)		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)	57.30%	46.50%
Credit spread rate (as a percent)	2.00%	1.90%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Minimum
Unobservable inputs
Discount rate (as a percent)	1.10%	1.10%
Illiquidity premium (as a percent)	0.00%	0.00%
Credit spread rate (as a percent)	0.68%	0.70%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Maximum
Unobservable inputs
Discount rate (as a percent)	6.20%	6.90%
Illiquidity premium (as a percent)	0.60%	0.60%
Credit spread rate (as a percent)	5.72%	6.32%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Weighted average input
Unobservable inputs
Discount rate (as a percent)	3.10%	3.20%
Illiquidity premium (as a percent)	0.07%	0.07%
Credit spread rate (as a percent)	2.21%	2.21%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Minimum
Unobservable inputs
Discount rate (as a percent)	5.60%	6.00%
Terminal capitalization rate (as a percent)	4.50%	4.50%
Average market rent growth rate (as a percent)	2.00%	1.30%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Maximum
Unobservable inputs
Discount rate (as a percent)	17.50%	24.60%
Terminal capitalization rate (as a percent)	9.50%	9.50%
Average market rent growth rate (as a percent)	4.50%	4.40%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Weighted average input
Unobservable inputs
Discount rate (as a percent)	7.30%	7.40%
Terminal capitalization rate (as a percent)	6.30%	6.40%
Average market rent growth rate (as a percent)	3.10%	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)	4.10%	6.80%
Credit spread rate (as a percent)	1.40%	2.10%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Maximum
Unobservable inputs
Loan to value (as a percent)	61.90%	64.10%
Credit spread rate (as a percent)	4.20%	4.80%
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)	46.30%	46.50%
Credit spread rate (as a percent)	2.40%	3.40%
Fair value hierarchy Level 3 | Investment-type insurance contracts | Discounted cash flow | Minimum
Unobservable inputs
Long duration interest rate (as a percent)	2.30%	2.60%
Long-term equity market volatility (as a percent)	14.90%	14.90%
Non-performance risk (as a percent)	0.20%	0.10%
Lapse rate (as a percent)	0.50%	0.50%
Fair value hierarchy Level 3 | Investment-type insurance contracts | Discounted cash flow | Maximum
Unobservable inputs
Long duration interest rate (as a percent)	2.40%	2.70%
Long-term equity market volatility (as a percent)	42.00%	39.50%
Non-performance risk (as a percent)	1.60%	1.40%
Lapse rate (as a percent)	14.10%	14.10%