Fair Value Measurements (Details 4) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Assets (liabilities) measured at fair value
Unobservable inputs
Assets measured at fair value	175,385.5	136,670.5
Liabilities measured at fair value	(1,587.7)	(1,613.0)
Assets (liabilities) measured at fair value | Available-for-sale | Non-U.S. governments
Unobservable inputs
Assets measured at fair value	13.0	12.9
Assets (liabilities) measured at fair value | Available-for-sale | Corporate debt securities
Unobservable inputs
Assets measured at fair value	31.4	66.6
Assets (liabilities) measured at fair value | Available-for-sale | Collateralized debt obligations
Unobservable inputs
Assets measured at fair value	35.6	38.2
Assets (liabilities) measured at fair value | Available-for-sale | Other debt obligations
Unobservable inputs
Assets measured at fair value	15.9	14.7
Assets (liabilities) measured at fair value | Trading | Fixed maturities
Unobservable inputs
Assets measured at fair value	37.9	35.9
Assets (liabilities) measured at fair value | Trading | Collateralized private investment vehicles | 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	77.2	80.3
Assets (liabilities) measured at fair value | Other investments | Discounted cash flow, equity method real estate investment
Unobservable inputs
Assets measured at fair value	33.7	33.6
Assets (liabilities) measured at fair value | Separate account assets
Unobservable inputs
Assets measured at fair value	4,510.9	4,449.0
Assets (liabilities) measured at fair value | Investment-type insurance contracts
Unobservable inputs
Liabilities measured at fair value	(117.8)	(170.5)
Assets (liabilities) measured at fair value | Derivative liabilities
Unobservable inputs
Liabilities measured at fair value	(46.2)	(65.1)
Assets (liabilities) measured at fair value | Other liabilities
Unobservable inputs
Liabilities measured at fair value	(54.2)	(39.6)
Fair value hierarchy Level 3
Unobservable inputs
Assets measured at fair value	5,323.9	5,300.1
Liabilities measured at fair value	(247.6)	(312.7)
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%	1.60%
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%	1.60%
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%	1.70%
Illiquidity premium (as a percent)		0.00%
Earnings before interest, taxes, depreciation and amortization multiple	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)	22.00%	29.00%
Illiquidity premium (as a percent)		1.00%
Earnings before interest, taxes, depreciation and amortization multiple	4.80	3.50
Probability of default (as a percent)	100.00%	100.00%
Potential loss severity (as a percent)	30.00%	30.00%
Fair value hierarchy Level 3 | Available-for-sale | Corporate debt securities | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	11.40%	8.40%
Illiquidity premium (as a percent)		0.39%
Earnings before interest, taxes, depreciation and amortization multiple	3.10	0.20
Probability of default (as a percent)	65.10%	6.40%
Potential loss severity (as a percent)	14.40%	1.90%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	1.20%	1.00%
Illiquidity premium (as a percent)	0.00%	4.00%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Maximum
Unobservable inputs
Discount rate (as a percent)	2.70%	19.80%
Illiquidity premium (as a percent)	4.00%	10.00%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	2.10%	13.30%
Illiquidity premium (as a percent)	1.50%	7.91%
Fair value hierarchy Level 3 | Available-for-sale | Other debt obligations | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	6.50%	6.50%
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%	20.00%
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)	10.40%	11.80%
Illiquidity premium (as a percent)	0.27%	0.30%
Fair value hierarchy Level 3 | Trading | Fixed maturities | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	1.30%	1.20%
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)	62.10%	60.50%
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)	4.00%	4.10%
Illiquidity premium (as a percent)	3.90%	3.90%
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)	48.40%	49.40%
Credit spread rate (as a percent)	3.30%	3.30%
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)	48.40%	49.40%
Credit spread rate (as a percent)	3.30%	3.30%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, commercial mortgage loans of consolidated VIEs | Input
Unobservable inputs
Discount rate (as a percent)	3.70%	3.50%
Illiquidity premium (as a percent)	2.72%	2.87%
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)	3.70%	3.50%
Illiquidity premium (as a percent)	2.72%	2.87%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment | Input
Unobservable inputs
Discount rate (as a percent)	9.00%	9.30%
Terminal capitalization rate (as a percent)	5.50%	5.50%
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)	9.00%	9.30%
Terminal capitalization rate (as a percent)	5.50%	5.50%
Average market rent growth rate (as a percent)	3.60%	3.60%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Minimum
Unobservable inputs
Discount rate (as a percent)	0.80%	0.80%
Illiquidity premium (as a percent)	0.00%	0.00%
Credit spread rate (as a percent)	0.47%	0.44%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Maximum
Unobservable inputs
Discount rate (as a percent)	9.40%	10.40%
Illiquidity premium (as a percent)	0.50%	0.50%
Credit spread rate (as a percent)	8.80%	9.75%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Weighted average input
Unobservable inputs
Discount rate (as a percent)	3.30%	3.30%
Illiquidity premium (as a percent)	0.20%	0.20%
Credit spread rate (as a percent)	2.40%	2.86%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Minimum
Unobservable inputs
Discount rate (as a percent)	6.50%	6.50%
Terminal capitalization rate (as a percent)	4.80%	4.80%
Average market rent growth rate (as a percent)	1.90%	2.30%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Maximum
Unobservable inputs
Discount rate (as a percent)	16.00%	16.00%
Terminal capitalization rate (as a percent)	9.00%	9.00%
Average market rent growth rate (as a percent)	5.60%	5.50%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Weighted average input
Unobservable inputs
Discount rate (as a percent)	7.90%	8.30%
Terminal capitalization rate (as a percent)	6.80%	7.20%
Average market rent growth rate (as a percent)	3.50%	3.30%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Minimum
Unobservable inputs
Loan to value (as a percent)	16.10%	17.00%
Credit spread rate (as a percent)	1.40%	1.60%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate debt | Maximum
Unobservable inputs
Loan to value (as a percent)	85.80%	86.00%
Credit spread rate (as a percent)	5.10%	5.30%
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)	54.20%	54.80%
Credit spread rate (as a percent)	3.30%	3.50%
Fair value hierarchy Level 3 | Investment-type insurance contracts | Discounted cash flow | Minimum
Unobservable inputs
Long duration interest rate (as a percent)	2.80%	2.60%
Long-term equity market volatility (as a percent)	16.10%	16.10%
Non-performance risk (as a percent)	0.20%	0.30%
Lapse rate (as a percent)	0.50%	0.50%
Period of swap rate used for calculating Long duration 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)	3.00%	2.80%
Long-term equity market volatility (as a percent)	37.80%	38.30%
Non-performance risk (as a percent)	1.50%	1.60%
Lapse rate (as a percent)	14.60%	14.60%
Period of swap rate used for calculating Long duration rate	30 years	30 years