Fair Value Measurements (Details 2) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Available-for-sale | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	$ 324.6	$ 788.3	$ 449.7	$ 790.9
Total realized/unrealized gains (losses) included in net income, assets	(4.4)	(1.8)	(7.8)	(20.0)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	1.8	(3.7)	5.3	18.8
Purchases, sales, issuances and settlements, assets	(7.7)	(54.0)	(49.3)	(67.5)
Transfers into Level 3, assets	22.6	26.6	46.5	54.5
Transfers out of Level 3, assets	(15.0)	(38.2)	(122.5)	(59.5)
Ending balance, assets	321.9	717.2	321.9	717.2
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(1.8)	(1.8)	(5.1)	(3.0)
Gross purchases, sales, issuances and settlements
Purchases, assets	13.7	3.3	29.9	11.2
Sales, assets	(20.0)	(3.9)	(50.5)	(20.8)
Settlements, assets	(1.4)	(53.4)	(28.7)	(57.9)
Purchases, sales, issuances and settlements, assets	(7.7)	(54.0)	(49.3)	(67.5)
Available-for-sale | Non-U.S. governments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	36.8	24.5	22.9	24.5
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.3	0.2		0.2
Purchases, sales, issuances and settlements, assets	2.5	(1.4)	2.2	(1.4)
Transfers into Level 3, assets			14.5
Ending balance, assets	39.6	23.3	39.6	23.3
Gross purchases, sales, issuances and settlements
Purchases, assets	2.8	2.5	6.7	2.5
Sales, assets		(3.9)	(3.9)	(3.9)
Settlements, assets	(0.3)		(0.6)
Purchases, sales, issuances and settlements, assets	2.5	(1.4)	2.2	(1.4)
Available-for-sale | Corporate debt securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	202.7	545.2	297.0	552.1
Total realized/unrealized gains (losses) included in net income, assets	(2.8)	(1.0)	(5.4)	(8.9)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	7.0	(4.6)	9.0	0.1
Purchases, sales, issuances and settlements, assets	(14.9)	(23.4)	(31.5)	(34.6)
Transfers into Level 3, assets	22.6	18.4	26.0	45.9
Transfers out of Level 3, assets	(15.0)		(95.5)	(20.0)
Ending balance, assets	199.6	534.6	199.6	534.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.1)	(1.1)	(2.7)	(2.3)
Gross purchases, sales, issuances and settlements
Purchases, assets	5.8	0.8	18.1	8.4
Sales, assets	(20.0)		(46.6)	(16.5)
Settlements, assets	(0.7)	(24.2)	(3.0)	(26.5)
Purchases, sales, issuances and settlements, assets	(14.9)	(23.4)	(31.5)	(34.6)
Available-for-sale | Commercial mortgage-backed securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets		19.0		16.2
Total realized/unrealized gains (losses) included in other comprehensive income, assets		(0.3)		2.3
Purchases, sales, issuances and settlements, assets		(0.3)		(0.1)
Transfers out of Level 3, assets		(7.1)		(7.1)
Ending balance, assets		11.3		11.3
Gross purchases, sales, issuances and settlements
Settlements, assets		(0.3)		(0.1)
Purchases, sales, issuances and settlements, assets		(0.3)		(0.1)
Available-for-sale | Collateralized debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	79.0	111.1	102.5	109.3
Total realized/unrealized gains (losses) included in net income, assets	(0.1)	(0.8)	(0.2)	(11.1)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(6.1)	0.6	(3.0)	15.3
Purchases, sales, issuances and settlements, assets	4.8		5.3	(1.3)
Transfers out of Level 3, assets			(27.0)	(1.3)
Ending balance, assets	77.6	110.9	77.6	110.9
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.1)	(0.7)	(0.2)	(0.7)
Gross purchases, sales, issuances and settlements
Purchases, assets	5.1		5.1	0.3
Sales, assets				(0.4)
Settlements, assets	(0.3)		0.2	(1.2)
Purchases, sales, issuances and settlements, assets	4.8		5.3	(1.3)
Available-for-sale | Other debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	6.1	88.5	27.3	88.8
Total realized/unrealized gains (losses) included in net income, assets	(1.5)		(2.2)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.6	0.4	(0.7)	0.9
Purchases, sales, issuances and settlements, assets	(0.1)	(28.9)	(25.3)	(30.1)
Transfers into Level 3, assets		8.2	6.0	8.6
Transfers out of Level 3, assets		(31.1)		(31.1)
Ending balance, assets	5.1	37.1	5.1	37.1
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(1.6)		(2.2)
Gross purchases, sales, issuances and settlements
Settlements, assets	(0.1)	(28.9)	(25.3)	(30.1)
Purchases, sales, issuances and settlements, assets	(0.1)	(28.9)	(25.3)	(30.1)
Available-for-sale | Equity securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	17.5	48.2	18.0	43.2
Total realized/unrealized gains (losses) included in net income, assets		(4.5)		(4.5)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(0.4)	4.7	(0.9)	9.7
Ending balance, assets	17.1	48.4	17.1	48.4
Changes in unrealized gains (losses) included in net income relating to positions still held, assets		(4.5)		(4.5)
Trading | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	206.2	269.6	220.8	269.1
Total realized/unrealized gains (losses) included in net income, assets	(1.1)	(0.3)	(2.8)	(4.4)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(4.0)		1.3
Purchases, sales, issuances and settlements, assets	(24.9)	(28.4)	(43.1)	(23.8)
Transfers into Level 3, assets	9.5		9.5
Transfers out of Level 3, assets		(19.4)		(19.4)
Ending balance, assets	185.7	221.5	185.7	221.5
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(1.1)	(0.4)	(3.6)	(3.6)
Gross purchases, sales, issuances and settlements
Purchases, assets				10.0
Sales, assets		(0.4)	(0.9)	(5.7)
Settlements, assets	(24.9)	(28.0)	(42.2)	(28.1)
Purchases, sales, issuances and settlements, assets	(24.9)	(28.4)	(43.1)	(23.8)
Derivative assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	47.3	39.4	60.2	33.3
Total realized/unrealized gains (losses) included in net income, assets	10.8	5.7	(3.8)	12.0
Total realized/unrealized gains (losses) included in other comprehensive income, assets		(0.1)		(0.2)
Purchases, sales, issuances and settlements, assets	0.7	(11.3)	2.4	(11.4)
Ending balance, assets	58.8	33.7	58.8	33.7
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	11.8	5.7	(2.8)	10.7
Gross purchases, sales, issuances and settlements
Purchases, assets	0.7	0.8	3.2	0.8
Sales, assets		(12.1)	(0.8)	(12.2)
Purchases, sales, issuances and settlements, assets	0.7	(11.3)	2.4	(11.4)
Other investments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	89.8	122.2	97.5	128.3
Total realized/unrealized gains (losses) included in net income, assets	0.2		(0.7)	(2.1)
Purchases, sales, issuances and settlements, assets	(6.4)	(14.8)	(13.2)	(18.8)
Ending balance, assets	83.6	107.4	83.6	107.4
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	0.2		(0.7)	(2.1)
Gross purchases, sales, issuances and settlements
Settlements, assets	(6.4)	(14.8)	(13.2)	(18.8)
Purchases, sales, issuances and settlements, assets	(6.4)	(14.8)	(13.2)	(18.8)
Separate account assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	4,280.3	3,799.5	4,198.2	3,771.5
Total realized/unrealized gains (losses) included in net income, assets	126.3	161.9	212.4	235.6
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.2	0.2	0.3	(0.1)
Purchases, sales, issuances and settlements, assets	32.5	(52.3)	29.8	(69.6)
Transfers into Level 3, assets	1.3		1.6	3.1
Transfers out of Level 3, assets	(0.1)	(4.9)	(1.8)	(36.1)
Ending balance, assets	4,440.5	3,904.4	4,440.5	3,904.4
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	126.9	161.9	203.6	231.1
Gross purchases, sales, issuances and settlements
Purchases, assets	41.0	38.2	168.5	73.4
Sales, assets	(28.7)	(79.9)	(119.0)	(124.6)
Issuances, assets	(11.4)	2.3	(146.3)	2.3
Settlements, assets	31.6	(12.9)	126.6	(20.7)
Purchases, sales, issuances and settlements, assets	32.5	(52.3)	29.8	(69.6)
Investment-type insurance contracts
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(129.0)	(4.2)	(195.8)	(6.6)
Total realized/unrealized gains (losses) included in net income, liabilities	(46.4)	(51.9)	22.4	(56.4)
Purchases, sales, issuances and settlements, liabilities	(4.0)	6.9	(6.0)	13.8
Ending balance, liabilities	(179.4)	(49.2)	(179.4)	(49.2)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(47.6)	(53.5)	21.2	(56.2)
Gross purchases, sales, issuances and settlements
Issuances, liabilities	(4.8)	5.7	(8.1)	12.0
Settlements, liabilities	0.8	1.2	2.1	1.8
Purchases, sales, issuances and settlements, liabilities	(4.0)	6.9	(6.0)	13.8
Derivative liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(142.3)	(185.0)	(177.1)	(181.5)
Total realized/unrealized gains (losses) included in net income, liabilities	(12.6)	5.5	12.8	6.9
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities	(1.1)	0.4	0.2	2.4
Purchases, sales, issuances and settlements, liabilities	8.9	0.3	17.0	(6.6)
Ending balance, liabilities	(147.1)	(178.8)	(147.1)	(178.8)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(13.4)	6.3	13.7	8.8
Gross purchases, sales, issuances and settlements
Purchases, liabilities	(1.0)	(0.5)	(1.7)	(9.9)
Sales, liabilities	9.9	0.8	18.7	3.3
Purchases, sales, issuances and settlements, liabilities	8.9	0.3	17.0	(6.6)
Other liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(40.7)	(158.9)	(24.2)	(156.8)
Total realized/unrealized gains (losses) included in net income, liabilities	7.1	(6.1)	(9.4)	(1.7)
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities		(9.3)		(9.1)
Purchases, sales, issuances and settlements, liabilities		(4.5)		(11.2)
Ending balance, liabilities	(33.6)	(178.8)	(33.6)	(178.8)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	7.2	(6.1)	(9.4)	(1.7)
Gross purchases, sales, issuances and settlements
Purchases, liabilities				(2.1)
Settlements, liabilities		(4.5)		(9.1)
Purchases, sales, issuances and settlements, liabilities		$ (4.5)		$ (11.2)