Fair Value Measurements (Details 4) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified																	9 Months Ended
	Sep. 30, 2012 Assets (liabilities) measured at fair value	Dec. 31, 2011 Assets (liabilities) measured at fair value	Sep. 30, 2012 Assets (liabilities) measured at fair value Available-for-sale Non-U.S. governments	Sep. 30, 2012 Assets (liabilities) measured at fair value Available-for-sale Corporate debt securities	Sep. 30, 2012 Assets (liabilities) measured at fair value Available-for-sale Collateralized debt obligations	Sep. 30, 2012 Assets (liabilities) measured at fair value Available-for-sale Other debt obligations	Sep. 30, 2012 Assets (liabilities) measured at fair value Trading Fixed maturities	Sep. 30, 2012 Assets (liabilities) measured at fair value Trading Collateralized private investment vehicles Fixed maturities	Sep. 30, 2012 Assets (liabilities) measured at fair value Other investments Discounted cash flow, commercial mortgage loans of consolidated VIEs	Sep. 30, 2012 Assets (liabilities) measured at fair value Other investments Discounted cash flow, equity method real estate investment	Sep. 30, 2012 Assets (liabilities) measured at fair value Separate account assets	Sep. 30, 2012 Assets (liabilities) measured at fair value Investment-type insurance contracts	Sep. 30, 2012 Assets (liabilities) measured at fair value Derivative liabilities	Sep. 30, 2012 Assets (liabilities) measured at fair value Other liabilities	Sep. 30, 2012 Fair value hierarchy Level 3	Dec. 31, 2011 Fair value hierarchy Level 3	Sep. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Non-U.S. governments Discounted cash flow Input	Sep. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Non-U.S. governments Discounted cash flow Weighted average input	Sep. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Discounted cash flow Minimum	Sep. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Discounted cash flow Maximum	Sep. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Discounted cash flow Weighted average input	Sep. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Collateralized debt obligations Discounted cash flow Minimum	Sep. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Collateralized debt obligations Discounted cash flow Maximum	Sep. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Collateralized debt obligations Discounted cash flow Weighted average input	Sep. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Other debt obligations Discounted cash flow Minimum	Sep. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Other debt obligations Discounted cash flow Maximum	Sep. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Other debt obligations Discounted cash flow Weighted average input	Sep. 30, 2012 Fair value hierarchy Level 3 Trading Fixed maturities Discounted cash flow Minimum	Sep. 30, 2012 Fair value hierarchy Level 3 Trading Fixed maturities Discounted cash flow Maximum	Sep. 30, 2012 Fair value hierarchy Level 3 Trading Fixed maturities Discounted cash flow Weighted average input	Sep. 30, 2012 Fair value hierarchy Level 3 Other investments Discounted cash flow, commercial mortgage loans of consolidated VIEs Input	Sep. 30, 2012 Fair value hierarchy Level 3 Other investments Discounted cash flow, commercial mortgage loans of consolidated VIEs Weighted average input	Sep. 30, 2012 Fair value hierarchy Level 3 Other investments Discounted cash flow, equity method real estate investment Input	Sep. 30, 2012 Fair value hierarchy Level 3 Other investments Discounted cash flow, equity method real estate investment Weighted average input	Sep. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, mortgage loans Minimum	Sep. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, mortgage loans Maximum	Sep. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, mortgage loans Weighted average input	Sep. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, real estate Minimum	Sep. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, real estate Maximum	Sep. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, real estate Weighted average input	Sep. 30, 2012 Fair value hierarchy Level 3 Investment-type insurance contracts Discounted cash flow Minimum	Sep. 30, 2012 Fair value hierarchy Level 3 Investment-type insurance contracts Discounted cash flow Maximum
Unobservable inputs
Assets measured at fair value	$ 135,671.1	$ 124,868.9	$ 13.5	$ 61.5	$ 38.4	$ 15.9	$ 39.4	$ 132.4	$ 81.3	$ 32.8	$ 4,359.4				$ 5,241.9	$ 5,044.4
Liabilities measured at fair value	$ (1,810.8)	$ (1,948.4)										$ (221.7)	$ (78.4)	$ (48.3)	$ (389.7)	$ (397.1)
Discount rate (as a percent)																	1.70%	1.70%	1.70%	34.00%	9.40%	1.10%	20.70%	13.80%	6.80%	20.00%	12.50%	1.30%	72.20%	4.90%	3.30%	3.30%	12.50%	12.50%	0.80%	12.50%	4.00%	6.50%	10.50%	8.20%
Illiquidity premium (as a percent)																	0.50%	0.50%	0.00%	1.00%	0.40%	4.00%	10.00%	7.92%	0.00%	0.50%	0.28%	0.00%	14.00%	3.60%	4.17%	4.17%			0.00%	0.50%
Earnings before interest, taxes, depreciation and amortization multiple																			0.0	4.4	1							0.0	4.4	0.3
Probability of default (as a percent)																			0.00%	100.00%	13.30%
Potential loss severity (as a percent)																			0.00%	60.00%	6.30%							0.00%	66.00%	14.90%
Credit spread rate (as a percent)																																			0.62%	12.30%	3.40%
Terminal capitalization rate (as a percent)																																	5.50%	5.50%				4.80%	9.50%	7.30%
Average market rent growth rate (as a percent)																																	5.40%	5.40%				2.30%	5.70%	3.40%
Long duration interest rate (as a percent)																																									2.50%	2.60%
Long-term equity market volatility (as a percent)																																									16.10%	42.30%
Non-performance risk (as a percent)																																									0.50%	1.50%
Lapse rate (as a percent)																																									0.50%	16.00%
Period of swap rate used for calculating Long duration rate																																									20 years	30 years