Fair Value Measurements (Details 2) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended					3 Months Ended
	Mar. 31, 2012 Available-for-sale Fixed maturities	Mar. 31, 2011 Available-for-sale Fixed maturities	Mar. 31, 2012 Available-for-sale Non-U.S. governments	Mar. 31, 2011 Available-for-sale Non-U.S. governments	Dec. 31, 2010 Available-for-sale Non-U.S. governments	Mar. 31, 2012 Available-for-sale Corporate debt securities	Mar. 31, 2011 Available-for-sale Corporate debt securities	Mar. 31, 2011 Available-for-sale Commercial mortgage-backed securities	Mar. 31, 2012 Available-for-sale Collateralized debt obligations	Mar. 31, 2011 Available-for-sale Collateralized debt obligations	Mar. 31, 2012 Available-for-sale Other debt obligations	Mar. 31, 2011 Available-for-sale Other debt obligations	Mar. 31, 2012 Available-for-sale Equity securities	Mar. 31, 2011 Available-for-sale Equity securities	Mar. 31, 2012 Trading Fixed maturities	Mar. 31, 2011 Trading Fixed maturities	Mar. 31, 2012 Derivative assets	Mar. 31, 2011 Derivative assets	Mar. 31, 2012 Other investments	Mar. 31, 2011 Other investments	Mar. 31, 2012 Separate account assets	Mar. 31, 2011 Separate account assets	Mar. 31, 2012 Investment-type insurance contracts	Mar. 31, 2011 Investment-type insurance contracts	Mar. 31, 2012 Derivative liabilities	Mar. 31, 2011 Derivative liabilities	Mar. 31, 2012 Other liabilities	Mar. 31, 2011 Other liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	$ 449.7	$ 790.9	$ 22.9	$ 24.5	$ 24.5	$ 297.0	$ 552.1	$ 16.2	$ 102.5	$ 109.3	$ 27.3	$ 88.8	$ 18.0	$ 43.2	$ 220.8	$ 269.1	$ 60.2	$ 33.3	$ 97.5	$ 128.3	$ 4,198.2	$ 3,771.5
Total realized/unrealized gains (losses) included in net income, assets	(5.6)	(18.2)	(2.2)			(2.6)	(7.9)		(0.1)	(10.3)	(0.7)				(1.7)	(4.1)	(14.6)	6.3	(0.9)	(2.1)	86.9	73.7
Total realized/unrealized gains (losses) included in other comprehensive income, assets	3.9	22.5	0.1			2.0	4.7	2.6	3.1	14.7	(1.3)	0.5	(0.5)	5.0	5.3			(0.1)			0.1	(0.3)
Purchases, sales, issuances and settlements, assets	(39.8)	(13.5)	1.5			(16.6)	(11.2)	0.2	0.5	(1.3)	(25.2)	(1.2)			(18.2)	4.6	1.7	(0.1)	(6.8)	(4.0)	3.4	(17.3)
Transfers into Level 3, assets	23.9	27.9	14.5			3.4	27.5				6.0	0.4									0.3	3.1
Transfers out of Level 3, assets	(107.5)	(21.3)				(80.5)	(20.0)		(27.0)	(1.3)											(1.7)	(31.2)
Ending balance, assets	324.6	788.3	36.8	24.5	24.5	202.7	545.2	19.0	79.0	111.1	6.1	88.5	17.5	48.2	206.2	269.6	47.3	39.4	89.8	122.2	4,287.2	3,799.5
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(5.7)	(18.2)	(2.2)			(2.7)	(7.9)		(0.1)	(10.3)	(0.7)				(2.4)	(3.1)	(13.6)	6.2	(0.8)	(2.1)	77.7	71.8
Beginning balance, liabilities																							(195.8)	(6.6)	(177.1)	(181.5)	(24.2)	(156.8)
Total realized/unrealized gains (losses) included in net income, liabilities																							68.8	(4.5)	25.4	1.4	(16.5)	4.4
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities																									1.3	2.0		0.2
Purchases, sales, issuances and settlements, liabilities																							(2.0)	6.9	8.1	(6.9)		(6.7)
Ending balance, liabilities																							(129.0)	(4.2)	(142.3)	(185.0)	(40.7)	(158.9)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities																							68.1	(4.3)	26.4	2.5	(16.5)	4.4
Gross purchases, sales, issuances and settlements
Purchases, assets	83.4	7.9	71.1			12.3	7.6			0.3						10.0	2.5				174.0	35.2
Sales, assets	(95.9)	(16.9)	(69.3)			(26.6)	(16.5)			(0.4)					(0.9)	(5.3)	(0.8)	(0.1)			(130.7)	(44.7)
Issuances, assets																					(134.9)
Settlements, assets	(27.3)	(4.5)	(0.3)			(2.3)	(2.3)	0.2	0.5	(1.2)	(25.2)	(1.2)			(17.3)	(0.1)			(6.8)	(4.0)	95.0	(7.8)
Purchases, sales, issuances and settlements, assets	(39.8)	(13.5)	1.5			(16.6)	(11.2)	0.2	0.5	(1.3)	(25.2)	(1.2)			(18.2)	4.6	1.7	(0.1)	(6.8)	(4.0)	3.4	(17.3)
Purchases, liabilities																									(0.7)	(9.4)		(2.1)
Sales, liabilities																									8.8	2.5
Issuances, liabilities																							(3.3)	6.3
Settlements, liabilities																							1.3	0.6				(4.6)
Purchases, sales, issuances and settlements, liabilities																							$ (2.0)	$ 6.9	$ 8.1	$ (6.9)		$ (6.7)