Fair Value Measurements (Details 2) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Available-for-sale | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	$ 321.9	$ 717.2	$ 449.7	$ 790.9
Total realized/unrealized gains (losses) included in net income, assets	(5.0)	(10.7)	(12.8)	(30.7)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	13.7	(17.1)	19.0	1.7
Net purchases, sales, issuances and settlements, assets	(33.2)	(19.0)	(82.5)	(86.5)
Transfers into Level 3, assets	39.9	10.4	86.4	64.9
Transfers out of Level 3, assets	(7.0)	(271.9)	(129.5)	(331.4)
Ending balance, assets	330.3	408.9	330.3	408.9
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(1.0)	(7.3)	(5.4)	(11.6)
Gross purchases, sales, issuances and settlements
Purchases, assets	9.6	2.1	39.5	13.3
Sales, assets	(34.4)	(13.3)	(84.9)	(34.1)
Settlements, assets	(8.4)	(7.8)	(37.1)	(65.7)
Net purchases, sales, issuances and settlements, assets	(33.2)	(19.0)	(82.5)	(86.5)
Available-for-sale | Non-U.S. governments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	39.6	23.3	22.9	24.5
Total realized/unrealized gains (losses) included in other comprehensive income, assets		0.3		0.5
Net purchases, sales, issuances and settlements, assets	6.3	(0.9)	8.5	(2.3)
Transfers into Level 3, assets			14.5
Ending balance, assets	45.9	22.7	45.9	22.7
Gross purchases, sales, issuances and settlements
Purchases, assets	6.5	0.6	13.2	3.1
Sales, assets		(1.5)	(3.9)	(5.4)
Settlements, assets	(0.2)		(0.8)
Net purchases, sales, issuances and settlements, assets	6.3	(0.9)	8.5	(2.3)
Available-for-sale | States and political subdivisions
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.2		0.2
Transfers into Level 3, assets	1.8		1.8
Ending balance, assets	2.0		2.0
Available-for-sale | Corporate debt securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	199.6	534.6	297.0	552.1
Total realized/unrealized gains (losses) included in net income, assets	(1.5)	0.1	(6.9)	(8.8)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	4.4	(13.9)	13.4	(13.8)
Net purchases, sales, issuances and settlements, assets	(38.8)	(7.9)	(70.3)	(42.5)
Transfers into Level 3, assets	28.8	10.0	54.8	55.9
Transfers out of Level 3, assets	(2.8)	(264.0)	(98.3)	(284.0)
Ending balance, assets	189.7	258.9	189.7	258.9
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(1.0)	(0.1)	(3.0)	(3.8)
Gross purchases, sales, issuances and settlements
Purchases, assets	3.1	1.4	21.2	9.8
Sales, assets	(34.1)	(1.3)	(80.7)	(17.8)
Settlements, assets	(7.8)	(8.0)	(10.8)	(34.5)
Net purchases, sales, issuances and settlements, assets	(38.8)	(7.9)	(70.3)	(42.5)
Available-for-sale | Commercial mortgage-backed securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets		11.3		16.2
Total realized/unrealized gains (losses) included in net income, assets		(3.7)		(3.7)
Total realized/unrealized gains (losses) included in other comprehensive income, assets		2.8		5.1
Net purchases, sales, issuances and settlements, assets		(10.4)		(10.5)
Transfers out of Level 3, assets				(7.1)
Gross purchases, sales, issuances and settlements
Sales, assets		(10.5)		(10.5)
Settlements, assets		0.1
Net purchases, sales, issuances and settlements, assets		(10.4)		(10.5)
Available-for-sale | Collateralized debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	77.6	110.9	102.5	109.3
Total realized/unrealized gains (losses) included in net income, assets	(3.5)	(7.2)	(3.7)	(18.3)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	7.1	(5.2)	4.1	10.1
Net purchases, sales, issuances and settlements, assets	(0.2)	0.4	5.1	(0.9)
Transfers out of Level 3, assets	(4.2)		(31.2)	(1.3)
Ending balance, assets	76.8	98.9	76.8	98.9
Changes in unrealized gains (losses) included in net income relating to positions still held, assets		(7.2)	(0.2)	(7.9)
Gross purchases, sales, issuances and settlements
Purchases, assets		0.1	5.1	0.4
Sales, assets	(0.3)		(0.3)	(0.4)
Settlements, assets	0.1	0.3	0.3	(0.9)
Net purchases, sales, issuances and settlements, assets	(0.2)	0.4	5.1	(0.9)
Available-for-sale | Other debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	5.1	37.1	27.3	88.8
Total realized/unrealized gains (losses) included in net income, assets		0.1	(2.2)	0.1
Total realized/unrealized gains (losses) included in other comprehensive income, assets	2.0	(1.1)	1.3	(0.2)
Net purchases, sales, issuances and settlements, assets	(0.5)	(0.2)	(25.8)	(30.3)
Transfers into Level 3, assets	9.3	0.4	15.3	9.0
Transfers out of Level 3, assets		(7.9)		(39.0)
Ending balance, assets	15.9	28.4	15.9	28.4
Changes in unrealized gains (losses) included in net income relating to positions still held, assets			(2.2)	0.1
Gross purchases, sales, issuances and settlements
Settlements, assets	(0.5)	(0.2)	(25.8)	(30.3)
Net purchases, sales, issuances and settlements, assets	(0.5)	(0.2)	(25.8)	(30.3)
Available-for-sale | Equity securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	17.1	48.4	18.0	43.2
Total realized/unrealized gains (losses) included in net income, assets				(4.5)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(0.4)	1.4	(1.3)	11.1
Net purchases, sales, issuances and settlements, assets		(27.6)		(27.6)
Transfers out of Level 3, assets		(16.1)		(16.1)
Ending balance, assets	16.7	6.1	16.7	6.1
Gross purchases, sales, issuances and settlements
Sales, assets		(27.6)		(27.6)
Net purchases, sales, issuances and settlements, assets		(27.6)		(27.6)
Trading | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	185.7	221.5	220.8	269.1
Total realized/unrealized gains (losses) included in net income, assets	5.1	(19.1)	3.6	(23.5)
Net purchases, sales, issuances and settlements, assets		(0.2)	(43.1)	(24.0)
Transfers into Level 3, assets		20.5	9.5	20.5
Transfers out of Level 3, assets		(5.6)		(25.0)
Ending balance, assets	190.8	217.1	190.8	217.1
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	5.0	(19.1)	4.3	(22.6)
Gross purchases, sales, issuances and settlements
Purchases, assets				10.0
Sales, assets			(0.9)	(5.7)
Settlements, assets		(0.2)	(42.2)	(28.3)
Net purchases, sales, issuances and settlements, assets		(0.2)	(43.1)	(24.0)
Derivative assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	58.8	33.7	60.2	33.3
Total realized/unrealized gains (losses) included in net income, assets	10.7	26.6	6.9	38.6
Total realized/unrealized gains (losses) included in other comprehensive income, assets		0.1		(0.1)
Net purchases, sales, issuances and settlements, assets	0.5	5.7	2.9	(5.7)
Ending balance, assets	70.0	66.1	70.0	66.1
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	12.6	24.0	8.0	33.3
Gross purchases, sales, issuances and settlements
Purchases, assets	0.5	16.2	3.7	17.0
Sales, assets		(10.5)	(0.8)	(22.7)
Net purchases, sales, issuances and settlements, assets	0.5	5.7	2.9	(5.7)
Other investments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	83.6	107.4	97.5	128.3
Total realized/unrealized gains (losses) included in net income, assets	(0.1)	0.6	(0.8)	(1.5)
Net purchases, sales, issuances and settlements, assets	30.7	(3.5)	17.5	(22.3)
Ending balance, assets	114.2	104.5	114.2	104.5
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.1)	0.6	(0.8)	(1.5)
Gross purchases, sales, issuances and settlements
Purchases, assets	33.3		33.3
Settlements, assets	(2.6)	(3.5)	(15.8)	(22.3)
Net purchases, sales, issuances and settlements, assets	30.7	(3.5)	17.5	(22.3)
Separate account assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	4,440.5	3,904.4	4,198.2	3,771.5
Total realized/unrealized gains (losses) included in net income, assets	106.0	100.1	318.4	335.7
Total realized/unrealized gains (losses) included in other comprehensive income, assets		(0.1)	0.3	(0.2)
Net purchases, sales, issuances and settlements, assets	(26.6)	79.0	3.2	9.4
Transfers into Level 3, assets		10.4	1.6	13.5
Transfers out of Level 3, assets		(36.8)	(1.8)	(72.9)
Ending balance, assets	4,519.9	4,057.0	4,519.9	4,057.0
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	105.9	105.0	307.1	338.9
Gross purchases, sales, issuances and settlements
Purchases, assets	42.6	122.7	211.1	196.1
Sales, assets	(72.9)	(40.7)	(191.9)	(165.3)
Issuances, assets	(57.8)		(204.1)	2.3
Settlements, assets	61.5	(3.0)	188.1	(23.7)
Net purchases, sales, issuances and settlements, assets	(26.6)	79.0	3.2	9.4
Investment-type insurance contracts
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(179.4)	(49.2)	(195.8)	(6.6)
Total realized/unrealized gains (losses) included in net income, liabilities	(37.0)	(122.0)	(14.6)	(178.4)
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities	(0.1)		(0.1)
Net purchases, sales, issuances and settlements, liabilities	(5.2)	5.1	(11.2)	18.9
Ending balance, liabilities	(221.7)	(166.1)	(221.7)	(166.1)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(37.6)	(123.9)	(16.8)	(178.5)
Gross purchases, sales, issuances and settlements
Issuances, liabilities	(6.7)	(0.3)	(14.8)	11.7
Settlements, liabilities	1.5	5.4	3.6	7.2
Net purchases, sales, issuances and settlements, liabilities	(5.2)	5.1	(11.2)	18.9
Derivative liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(147.1)	(178.8)	(177.1)	(181.5)
Total realized/unrealized gains (losses) included in net income, liabilities	19.6	(20.7)	32.4	(13.8)
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities	0.5	(2.2)	0.7	0.2
Net purchases, sales, issuances and settlements, liabilities	7.3	(2.5)	24.3	(9.1)
Ending balance, liabilities	(119.7)	(204.2)	(119.7)	(204.2)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	19.3	(24.6)	34.9	(14.5)
Gross purchases, sales, issuances and settlements
Purchases, liabilities	(1.9)	(1.2)	(3.6)	(11.1)
Sales, liabilities	9.2	(1.3)	27.9	2.0
Net purchases, sales, issuances and settlements, liabilities	7.3	(2.5)	24.3	(9.1)
Other liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(33.6)	(178.8)	(24.2)	(156.8)
Total realized/unrealized gains (losses) included in net income, liabilities	(14.7)	5.1	(24.1)	3.4
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities		22.4		13.3
Net purchases, sales, issuances and settlements, liabilities		(4.7)		(15.9)
Ending balance, liabilities	(48.3)	(156.0)	(48.3)	(156.0)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(14.7)	(1.0)	(24.1)	3.4
Gross purchases, sales, issuances and settlements
Purchases, liabilities				(2.1)
Settlements, liabilities		(4.7)		(13.8)
Net purchases, sales, issuances and settlements, liabilities		$ (4.7)		$ (15.9)