Fair Value Measurements (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Transfers into Level 3, assets	$ 159.2	$ 226.1	$ 531.9
Transfers out of Level 3, assets	(454.8)	(497.1)	(414.1)
Transfers out of Level 3, liabilities	136.3
Recurring Fair Value Measurements | Available-for-sale | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	790.9	1,172.6	1,173.0
Total realized/unrealized gains (losses) included in net income, assets	(33.8)	(14.4)	(103.3)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(3.0)	73.5	341.9
Purchases, sales, issuances and settlements, assets	(85.2)	(261.9)	(366.4)
Transfers into Level 3, assets	112.2	197.5
Transfers out of Level 3, assets	(331.4)	(376.4)
Transfers in (out) of Level 3, assets			127.4
Ending balance, assets	449.7	790.9	1,172.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(15.3)	(4.3)	(95.8)
Gross purchases, sales, issuances and settlements
Purchases, assets	26.1
Sales, assets	(41.9)
Settlements, assets	(69.4)
Purchases, sales, issuances and settlements, assets	(85.2)	(261.9)	(366.4)
Recurring Fair Value Measurements | Available-for-sale | Non-U.S. governments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	24.5	16.1	45.3
Total realized/unrealized gains (losses) included in net income, assets	0.2	(0.1)	(10.3)
Total realized/unrealized gains (losses) included in other comprehensive income, assets		0.1	2.4
Purchases, sales, issuances and settlements, assets	(1.8)	8.4	(21.3)
Ending balance, assets	22.9	24.5	16.1
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	0.1	(0.1)	(0.1)
Gross purchases, sales, issuances and settlements
Purchases, assets	3.6
Sales, assets	(5.4)
Purchases, sales, issuances and settlements, assets	(1.8)	8.4	(21.3)
Recurring Fair Value Measurements | Available-for-sale | States and political subdivisions
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets		11.5
Total realized/unrealized gains (losses) included in other comprehensive income, assets		1.0	1.3
Transfers into Level 3, assets		11.5
Transfers out of Level 3, assets		(24.0)
Transfers in (out) of Level 3, assets			10.2
Ending balance, assets			11.5
Recurring Fair Value Measurements | Available-for-sale | Corporate debt securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	552.1	737.3	750.9
Total realized/unrealized gains (losses) included in net income, assets	(10.8)	0.7	(26.7)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(20.8)	26.9	160.6
Purchases, sales, issuances and settlements, assets	(42.7)	(193.2)	(348.3)
Transfers into Level 3, assets	103.2	152.2
Transfers out of Level 3, assets	(284.0)	(171.8)
Transfers in (out) of Level 3, assets			200.8
Ending balance, assets	297.0	552.1	737.3
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(6.1)	(2.2)	(32.2)
Gross purchases, sales, issuances and settlements
Purchases, assets	21.2
Sales, assets	(25.6)
Settlements, assets	(38.3)
Purchases, sales, issuances and settlements, assets	(42.7)	(193.2)	(348.3)
Recurring Fair Value Measurements | Available-for-sale | Commercial mortgage-backed securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	16.2	34.3	58.0
Total realized/unrealized gains (losses) included in net income, assets	(3.7)	(0.1)	(0.3)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	5.1	1.0	9.8
Purchases, sales, issuances and settlements, assets	(10.5)	11.2	(12.1)
Transfers out of Level 3, assets	(7.1)	(30.2)
Transfers in (out) of Level 3, assets			(21.1)
Ending balance, assets		16.2	34.3
Changes in unrealized gains (losses) included in net income relating to positions still held, assets		(0.1)
Gross purchases, sales, issuances and settlements
Sales, assets	(10.5)
Purchases, sales, issuances and settlements, assets	(10.5)	11.2	(12.1)
Recurring Fair Value Measurements | Available-for-sale | Collateralized debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	109.3	296.8	236.8
Total realized/unrealized gains (losses) included in net income, assets	(19.6)	(14.9)	(63.9)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	13.8	40.0	150.4
Purchases, sales, issuances and settlements, assets	0.3	(125.2)	(10.6)
Transfers into Level 3, assets		0.9
Transfers out of Level 3, assets	(1.3)	(88.3)
Transfers in (out) of Level 3, assets			(15.9)
Ending balance, assets	102.5	109.3	296.8
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(9.3)	(1.9)	(63.5)
Gross purchases, sales, issuances and settlements
Purchases, assets	1.3
Sales, assets	(0.4)
Settlements, assets	(0.6)
Purchases, sales, issuances and settlements, assets	0.3	(125.2)	(10.6)
Recurring Fair Value Measurements | Available-for-sale | Other debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	88.8	76.6	82.0
Total realized/unrealized gains (losses) included in net income, assets	0.1		(2.1)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(1.1)	4.5	17.4
Purchases, sales, issuances and settlements, assets	(30.5)	36.9	25.9
Transfers into Level 3, assets	9.0	32.9
Transfers out of Level 3, assets	(39.0)	(62.1)
Transfers in (out) of Level 3, assets			(46.6)
Ending balance, assets	27.3	88.8	76.6
Gross purchases, sales, issuances and settlements
Settlements, assets	(30.5)
Purchases, sales, issuances and settlements, assets	(30.5)	36.9	25.9
Recurring Fair Value Measurements | Available-for-sale | Equity securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	43.2	71.7	56.2
Total realized/unrealized gains (losses) included in net income, assets	(6.1)	2.6	(0.2)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	12.0	(8.2)	30.3
Purchases, sales, issuances and settlements, assets	(28.0)	(21.4)	(43.7)
Transfers into Level 3, assets	13.0	0.1
Transfers out of Level 3, assets	(16.1)	(1.6)
Transfers in (out) of Level 3, assets			29.1
Ending balance, assets	18.0	43.2	71.7
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(4.5)	3.3	(2.0)
Gross purchases, sales, issuances and settlements
Purchases, assets	0.3
Sales, assets	(28.3)
Purchases, sales, issuances and settlements, assets	(28.0)	(21.4)	(43.7)
Recurring Fair Value Measurements | Trading | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	269.1	63.5	60.7
Total realized/unrealized gains (losses) included in net income, assets	(16.6)	13.5	13.0
Purchases, sales, issuances and settlements, assets	(27.2)	194.1
Transfers into Level 3, assets	20.5
Transfers out of Level 3, assets	(25.0)	(2.0)
Transfers in (out) of Level 3, assets			(10.2)
Ending balance, assets	220.8	269.1	63.5
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(15.8)	13.2	13.1
Gross purchases, sales, issuances and settlements
Purchases, assets	10.0
Sales, assets	(8.7)
Settlements, assets	(28.5)
Purchases, sales, issuances and settlements, assets	(27.2)	194.1
Recurring Fair Value Measurements | Derivative assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	33.3	54.4	100.7
Total realized/unrealized gains (losses) included in net income, assets	37.8	(18.3)	(43.6)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(0.1)	(0.1)	(0.2)
Purchases, sales, issuances and settlements, assets	(10.8)	(2.7)	(2.5)
Ending balance, assets	60.2	33.3	54.4
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	33.4	(17.1)	(30.5)
Gross purchases, sales, issuances and settlements
Purchases, assets	19.0
Sales, assets	(29.8)
Purchases, sales, issuances and settlements, assets	(10.8)	(2.7)	(2.5)
Recurring Fair Value Measurements | Other investments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	128.3
Total realized/unrealized gains (losses) included in net income, assets	(2.5)	25.9
Purchases, sales, issuances and settlements, assets	(28.3)	102.4
Ending balance, assets	97.5	128.3
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(2.6)	25.9
Gross purchases, sales, issuances and settlements
Settlements, assets	(28.3)
Purchases, sales, issuances and settlements, assets	(28.3)	102.4
Recurring Fair Value Measurements | Separate account assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	3,771.5	4,120.7	6,042.3
Total realized/unrealized gains (losses) included in net income, assets	406.6	304.0	(1,601.5)
Total realized/unrealized gains (losses) included in other comprehensive income, assets		(0.4)
Purchases, sales, issuances and settlements, assets	88.9	(564.2)	(291.6)
Transfers into Level 3, assets	13.5	28.5
Transfers out of Level 3, assets	(82.3)	(117.1)
Transfers in (out) of Level 3, assets			(28.5)
Ending balance, assets	4,198.2	3,771.5	4,120.7
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	400.9	249.0	(1,488.3)
Gross purchases, sales, issuances and settlements
Purchases, assets	342.7
Sales, assets	(191.8)
Settlements, assets	(62.0)
Purchases, sales, issuances and settlements, assets	88.9	(564.2)	(291.6)
Recurring Fair Value Measurements | Investment-type insurance contracts
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(6.6)	(23.6)	(60.2)
Total realized/unrealized gains (losses) included in net income, liabilities	(206.3)	(8.2)	10.8
Purchases, sales, issuances and settlements, liabilities	17.1	25.2	25.8
Ending balance, liabilities	(195.8)	(6.6)	(23.6)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(206.6)	(8.6)	10.8
Gross purchases, sales, issuances and settlements
Issuances, liabilities	9.2
Settlements, liabilities	7.9
Purchases, sales, issuances and settlements, liabilities	17.1	25.2	25.8
Recurring Fair Value Measurements | Derivative liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(181.5)	(93.7)	(266.9)
Total realized/unrealized gains (losses) included in net income, liabilities	(11.4)	9.9	141.4
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities	0.2	(1.4)	7.2
Purchases, sales, issuances and settlements, liabilities	15.6	(96.3)	24.6
Ending balance, liabilities	(177.1)	(181.5)	(93.7)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(8.6)	8.0	88.8
Gross purchases, sales, issuances and settlements
Purchases, liabilities	(12.1)
Sales, liabilities	27.7
Purchases, sales, issuances and settlements, liabilities	15.6	(96.3)	24.6
Recurring Fair Value Measurements | Other liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(156.8)	(89.1)	(103.8)
Total realized/unrealized gains (losses) included in net income, liabilities	(1.2)	9.3
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities	13.4	(28.3)	33.2
Purchases, sales, issuances and settlements, liabilities	(15.9)	(48.7)	(18.5)
Transfers out of Level 3, liabilities	136.3
Ending balance, liabilities	(24.2)	(156.8)	(89.1)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(1.1)	2.3
Gross purchases, sales, issuances and settlements
Purchases, liabilities	(2.1)
Settlements, liabilities	(13.8)
Purchases, sales, issuances and settlements, liabilities	$ (15.9)	$ (48.7)	$ (18.5)