Fair Value Measurements (Details 4) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified																3 Months Ended
	Mar. 31, 2012 Assets (liabilities) measured at fair value	Dec. 31, 2011 Assets (liabilities) measured at fair value	Mar. 31, 2012 Assets (liabilities) measured at fair value Available-for-sale Non-U.S. governments	Mar. 31, 2012 Assets (liabilities) measured at fair value Available-for-sale Corporate debt securities	Mar. 31, 2012 Assets (liabilities) measured at fair value Available-for-sale Collateralized debt obligations	Mar. 31, 2012 Assets (liabilities) measured at fair value Available-for-sale Other debt obligations	Mar. 31, 2012 Assets (liabilities) measured at fair value Trading Fixed maturities Discounted cash flow	Mar. 31, 2012 Assets (liabilities) measured at fair value Trading Collateralized private investment vehicles Fixed maturities	Mar. 31, 2012 Assets (liabilities) measured at fair value Other investments	Mar. 31, 2012 Assets (liabilities) measured at fair value Separate account assets	Mar. 31, 2012 Assets (liabilities) measured at fair value Investment-type insurance contracts	Mar. 31, 2012 Assets (liabilities) measured at fair value Derivative liabilities	Mar. 31, 2012 Assets (liabilities) measured at fair value Other liabilities	Mar. 31, 2012 Fair value hierarchy Level 3	Dec. 31, 2011 Fair value hierarchy Level 3	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Non-U.S. governments Discounted cash flow Input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Non-U.S. governments Discounted cash flow Weighted average input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Discounted cash flow Input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Discounted cash flow Minimum input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Discounted cash flow Maximum input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Discounted cash flow Weighted average input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Recovery value Input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Recovery value Weighted average input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Collateralized debt obligations Discounted cash flow Minimum input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Collateralized debt obligations Discounted cash flow Maximum input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Collateralized debt obligations Discounted cash flow Weighted average input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Other debt obligations Discounted cash flow Input	Mar. 31, 2012 Fair value hierarchy Level 3 Available-for-sale Other debt obligations Discounted cash flow Weighted average input	Mar. 31, 2012 Fair value hierarchy Level 3 Trading Fixed maturities Discounted cash flow Minimum input	Mar. 31, 2012 Fair value hierarchy Level 3 Trading Fixed maturities Discounted cash flow Maximum input	Mar. 31, 2012 Fair value hierarchy Level 3 Trading Fixed maturities Discounted cash flow Weighted average input	Mar. 31, 2012 Fair value hierarchy Level 3 Other investments Discounted cash flow Input	Mar. 31, 2012 Fair value hierarchy Level 3 Other investments Discounted cash flow Weighted average input	Mar. 31, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, mortgage loans Minimum input	Mar. 31, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, mortgage loans Maximum input	Mar. 31, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, mortgage loans Weighted average input	Mar. 31, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, real estate Minimum input	Mar. 31, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, real estate Maximum input	Mar. 31, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, real estate Weighted average input	Mar. 31, 2012 Fair value hierarchy Level 3 Investment-type insurance contracts Discounted cash flow Minimum input Y	Mar. 31, 2012 Fair value hierarchy Level 3 Investment-type insurance contracts Discounted cash flow Maximum input Y
Unobservable inputs
Assets measured at fair value	$ 130,474.0	$ 124,868.9	$ 13.9	$ 69.5	$ 41.2	$ 6.1	$ 31.2	$ 132.4	$ 89.8	$ 4,123.8				$ 4,972.6	$ 5,044.4
Liabilities measured at fair value	$ (1,766.1)	$ (1,948.4)									$ (129.0)	$ (86.5)	$ (40.7)	$ (312.0)	$ (397.1)
Illiquidity premium (as a percent)																0.25%	0.25%		0.00%	2.00%	0.83%	6.00%	6.00%	4.00%	10.00%	8.15%			0.00%	14.00%	4.40%	3.45%	3.45%	0.00%	0.50%
Discount rate (as a percent)																2.30%	2.30%		0.30%	41.00%	13.00%	6.20%	6.20%	2.10%	11.50%	8.00%	20.00%	20.00%	2.30%	61.00%	8.00%	4.30%	4.30%	0.90%	9.30%	4.10%	6.50%	10.50%	8.20%
Comparability adjustment (as a percent)																			0.00%	1.30%	0.12%
Earnings before interest, taxes, depreciation and amortization multiple																		4.50			4.50
Probability of default (as a percent)																						50.00%	50.00%
Potential loss severity (as a percent)																						47.00%	47.00%
Credit spread rate (as a percent)																																		0.60%	8.85%	3.32%
Terminal capitalization rate (as a percent)																																					5.50%	9.50%	7.30%
Average market rent growth rate (as a percent)																																					2.20%	6.10%	3.40%
Long duration interest rate (as a percent)																																								2.90%	3.00%
Long-term equity market volatility (as a percent)																																								16.10%	43.40%
Non-performance risk (as a percent)																																								0.70%	2.30%
Lapse rate (as a percent)																																								0.50%	16.00%
Period of swap rate used for calculating Long duration rate (in years)																																								20	30