Fair Value Measurements (Details 2) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended						3 Months Ended
	Mar. 31, 2013 Available-for-sale Fixed maturities	Mar. 31, 2012 Available-for-sale Fixed maturities	Mar. 31, 2013 Available-for-sale Non-U.S. governments	Mar. 31, 2012 Available-for-sale Non-U.S. governments	Mar. 31, 2013 Available-for-sale States and political subdivisions	Dec. 31, 2012 Available-for-sale States and political subdivisions	Mar. 31, 2013 Available-for-sale Corporate debt securities	Mar. 31, 2012 Available-for-sale Corporate debt securities	Mar. 31, 2013 Available-for-sale Collateralized debt obligations	Mar. 31, 2012 Available-for-sale Collateralized debt obligations	Mar. 31, 2013 Available-for-sale Other debt obligations	Mar. 31, 2012 Available-for-sale Other debt obligations	Mar. 31, 2013 Available-for-sale Equity securities	Mar. 31, 2012 Available-for-sale Equity securities	Mar. 31, 2013 Trading Fixed maturities	Mar. 31, 2012 Trading Fixed maturities	Mar. 31, 2013 Derivative assets	Mar. 31, 2012 Derivative assets	Mar. 31, 2013 Other investments	Mar. 31, 2012 Other investments	Mar. 31, 2013 Separate account assets	Mar. 31, 2012 Separate account assets	Mar. 31, 2013 Investment-type insurance contracts	Mar. 31, 2012 Investment-type insurance contracts	Mar. 31, 2013 Derivative liabilities	Mar. 31, 2012 Derivative liabilities	Mar. 31, 2013 Other liabilities	Mar. 31, 2012 Other liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	$ 313.0	$ 449.7	$ 44.3	$ 22.9	$ 1.9	$ 1.9	$ 174.5	$ 297.0	$ 77.6	$ 102.5	$ 14.7	$ 27.3	$ 15.3	$ 18.0	$ 166.8	$ 220.8	$ 75.1	$ 60.2	$ 113.9	$ 97.5	$ 4,616.0	$ 4,198.2
Total realized/unrealized gains (losses) included in net income, assets	(1.0)	(3.4)					(3.1)	(2.6)	2.1	(0.1)		(0.7)			2.7	3.6	(11.5)	(14.6)	(0.6)	(0.9)	119.0	86.1
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(1.0)	3.5	0.3	(0.3)			(10.1)	2.0	7.1	3.1	1.7	(1.3)	0.8	(0.5)							(0.2)	0.1
Net purchases, sales, issuances and settlements, assets	(55.3)	(41.6)	(4.2)	(0.3)			(17.6)	(16.6)	(33.0)	0.5	(0.5)	(25.2)				(18.2)	3.6	1.7	(2.4)	(6.8)	(77.0)	(2.7)
Transfers into Level 3, assets	68.9	23.9		14.5			47.5	3.4	21.4			6.0									1.4	0.3
Transfers out of Level 3, assets	(23.6)	(107.5)					(23.6)	(80.5)		(27.0)												(1.7)
Ending balance, assets	301.0	324.6	40.4	36.8	1.9	1.9	167.6	202.7	75.2	79.0	15.9	6.1	16.1	17.5	169.5	206.2	67.2	47.3	110.9	89.8	4,659.2	4,280.3
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(3.1)	(2.8)					(3.1)	(2.7)		(0.1)					2.8	(2.4)	(11.0)	(13.6)	(0.6)	(0.8)	117.2	76.9
Beginning balance, liabilities																							(170.5)	(195.8)	(102.6)	(177.1)	(39.6)	(24.2)
Total realized/unrealized gains (losses) included in net income, liabilities																							51.7	68.8	25.8	25.4	(14.6)	(16.5)
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities																									0.3	1.3
Net purchases, sales, issuances and settlements, liabilities																							1.0	(2.0)	0.9	8.1
Ending balance, liabilities																							(117.8)	(129.0)	(75.6)	(142.3)	(54.2)	(40.7)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities																							50.9	68.1	25.7	26.4	(14.6)	(16.5)
Gross purchases, sales, issuances and settlements
Purchases, assets	4.2	16.2		3.9			4.2	12.3									6.7	2.5	0.2		66.5	127.5
Sales, assets	(45.7)	(30.5)	(3.9)	(3.9)			(9.4)	(26.6)	(32.4)							(0.9)	(3.1)	(0.8)			(136.8)	(90.3)
Issuances, assets																					(4.5)	(134.9)
Settlements, assets	(13.8)	(27.3)	(0.3)	(0.3)			(12.4)	(2.3)	(0.6)	0.5	(0.5)	(25.2)				(17.3)			(2.6)	(6.8)	(2.2)	95.0
Net purchases, sales, issuances and settlements, assets	(55.3)	(41.6)	(4.2)	(0.3)			(17.6)	(16.6)	(33.0)	0.5	(0.5)	(25.2)				(18.2)	3.6	1.7	(2.4)	(6.8)	(77.0)	(2.7)
Purchases, liabilities																									(1.6)	(0.7)
Sales, liabilities																									2.5	8.8
Issuances, liabilities																							(0.3)	(3.3)
Settlements, liabilities																							1.3	1.3
Net purchases, sales, issuances and settlements, liabilities																							$ 1.0	$ (2.0)	$ 0.9	$ 8.1