Fair Value Measurements (Details 4) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Assets (liabilities) measured at fair value
Unobservable inputs
Assets measured at fair value	$ 175,674.4	$ 136,670.5
Liabilities measured at fair value	(1,364.5)	(1,613.0)
Assets (liabilities) measured at fair value | Available-for-sale | Non-U.S. governments
Unobservable inputs
Assets measured at fair value	12.1	12.9
Assets (liabilities) measured at fair value | Available-for-sale | Corporate debt securities
Unobservable inputs
Assets measured at fair value	50.2	66.6
Assets (liabilities) measured at fair value | Available-for-sale | Collateralized debt obligations
Unobservable inputs
Assets measured at fair value	13.6	38.2
Assets (liabilities) measured at fair value | Available-for-sale | Other debt obligations
Unobservable inputs
Assets measured at fair value	16.1	14.7
Assets (liabilities) measured at fair value | Trading | Fixed maturities
Unobservable inputs
Assets measured at fair value	40.0	35.9
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	72.6	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	67.9	33.6
Assets (liabilities) measured at fair value | Separate account assets
Unobservable inputs
Assets measured at fair value	4,780.1	4,449.0
Assets (liabilities) measured at fair value | Investment-type insurance contracts
Unobservable inputs
Liabilities measured at fair value	(49.3)	(170.5)
Assets (liabilities) measured at fair value | Derivative liabilities
Unobservable inputs
Liabilities measured at fair value	(27.7)	(65.1)
Assets (liabilities) measured at fair value | Other liabilities
Unobservable inputs
Liabilities measured at fair value	(67.1)	(39.6)
Fair value hierarchy Level 3
Unobservable inputs
Assets measured at fair value	5,627.3	5,300.1
Liabilities measured at fair value	$ (166.4)	$ (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)	2.30%	1.60%
Illiquidity premium (as a percent)		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)	2.30%	1.60%
Illiquidity premium (as a percent)		0.50%
Fair value hierarchy Level 3 | Available-for-sale | Corporate debt securities | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	2.00%	1.70%
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%
Illiquidity premium (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)	8.20%	29.00%
Earnings before interest, taxes, depreciation and amortization multiple	4.50	3.50
Probability of default (as a percent)	100.00%	100.00%
Potential loss severity (as a percent)	17.00%	30.00%
Illiquidity premium (as a percent)	0.50%	1.00%
Fair value hierarchy Level 3 | Available-for-sale | Corporate debt securities | Discounted cash flow | Weighted average input
Unobservable inputs
Discount rate (as a percent)	3.80%	8.40%
Earnings before interest, taxes, depreciation and amortization multiple	0.40	0.20
Probability of default (as a percent)	7.90%	6.40%
Potential loss severity (as a percent)	1.30%	1.90%
Illiquidity premium (as a percent)	0.31%	0.39%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Input
Unobservable inputs
Discount rate (as a percent)	1.60%
Illiquidity premium (as a percent)	4.00%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)		1.00%
Illiquidity premium (as a percent)		4.00%
Fair value hierarchy Level 3 | Available-for-sale | Collateralized debt obligations | Discounted cash flow | Maximum
Unobservable inputs
Discount rate (as a percent)		19.80%
Illiquidity premium (as a percent)		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)	1.60%	13.30%
Illiquidity premium (as a percent)	4.00%	7.91%
Fair value hierarchy Level 3 | Available-for-sale | Other debt obligations | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	7.30%	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)	11.40%	11.80%
Illiquidity premium (as a percent)	0.23%	0.30%
Fair value hierarchy Level 3 | Trading | Fixed maturities | Discounted cash flow | Minimum
Unobservable inputs
Discount rate (as a percent)	1.70%	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)	82.70%	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)	4.50%	4.10%
Illiquidity premium (as a percent)	3.50%	3.90%
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%	3.50%
Illiquidity premium (as a percent)	1.55%	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)	4.30%	3.50%
Illiquidity premium (as a percent)	1.55%	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)		9.30%
Terminal capitalization rate (as a percent)		5.50%
Average market rent growth rate (as a percent)		3.60%
Fair value hierarchy Level 3 | Other investments | Discounted cash flow, equity method real estate investment | Minimum
Unobservable inputs
Discount rate (as a percent)	8.00%
Terminal capitalization rate (as a percent)	5.50%
Average market rent growth rate (as a percent)	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)	8.40%
Terminal capitalization rate (as a percent)	6.80%
Average market rent growth rate (as a percent)	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)	8.20%	9.30%
Terminal capitalization rate (as a percent)	6.10%	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 debt | Input
Unobservable inputs
Loan to value (as a percent)		49.40%
Credit spread rate (as a percent)		3.30%
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)	42.80%
Credit spread rate (as a percent)	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)	60.60%
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)	51.70%	49.40%
Credit spread rate (as a percent)	1.80%	3.30%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Minimum
Unobservable inputs
Discount rate (as a percent)	0.70%	0.80%
Illiquidity premium (as a percent)	0.00%	0.00%
Credit spread rate (as a percent)	0.51%	0.44%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, mortgage loans | Maximum
Unobservable inputs
Discount rate (as a percent)	7.90%	10.40%
Illiquidity premium (as a percent)	0.50%	0.50%
Credit spread rate (as a percent)	7.00%	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)	3.60%	3.30%
Illiquidity premium (as a percent)	0.13%	2.00%
Credit spread rate (as a percent)	2.54%	2.86%
Fair value hierarchy Level 3 | Separate account assets | Discounted cash flow, real estate | Minimum
Unobservable inputs
Discount rate (as a percent)	6.00%	6.50%
Terminal capitalization rate (as a percent)	4.50%	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)	5.00%	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)	7.80%	8.30%
Terminal capitalization rate (as a percent)	6.70%	7.20%
Average market rent growth rate (as a percent)	3.30%	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)	13.60%	17.00%
Credit spread rate (as a percent)	1.60%	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)	69.60%	86.00%
Credit spread rate (as a percent)	4.90%	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)	51.90%	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)	3.50%	2.60%
Long-term equity market volatility (as a percent)	15.00%	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 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)	3.70%	2.80%
Long-term equity market volatility (as a percent)	39.80%	38.30%
Non-performance risk (as a percent)	1.30%	1.60%
Lapse rate (as a percent)	14.60%	14.60%
Period of swap rate used for calculating long duration interest rate	30 years	30 years