Fair Value Measurements (Details 2) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Available-for-sale | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	$ 273.6	$ 321.9	$ 313.0	$ 449.7
Total realized/unrealized gains (losses) included in net income, assets	(0.4)	(5.0)	(3.2)	(12.8)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	3.0	13.7	2.6	19.0
Net purchases, sales, issuances and settlements, assets	34.1	(33.2)	(64.3)	(82.5)
Transfers into Level 3, assets	19.7	39.9	154.2	86.4
Transfers out of Level 3, assets	(32.1)	(7.0)	(104.4)	(129.5)
Ending balance, assets	297.9	330.3	297.9	330.3
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.4)	(1.0)	(2.4)	(5.4)
Gross purchases, sales, issuances and settlements
Purchases, assets	36.9	9.6	70.8	39.5
Sales, assets	(0.7)	(34.4)	(83.7)	(84.9)
Settlements, assets	(2.1)	(8.4)	(51.4)	(37.1)
Net purchases, sales, issuances and settlements, assets	34.1	(33.2)	(64.3)	(82.5)
Available-for-sale | Non-U.S. governments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	44.7	39.6	44.3	22.9
Total realized/unrealized gains (losses) included in net income, assets			(0.1)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(0.3)		(0.8)
Net purchases, sales, issuances and settlements, assets	3.4	6.3	4.4	8.5
Transfers into Level 3, assets				14.5
Ending balance, assets	47.8	45.9	47.8	45.9
Changes in unrealized gains (losses) included in net income relating to positions still held, assets			(0.1)
Gross purchases, sales, issuances and settlements
Purchases, assets	3.7	6.5	9.2	13.2
Sales, assets			(3.9)	(3.9)
Settlements, assets	(0.3)	(0.2)	(0.9)	(0.8)
Net purchases, sales, issuances and settlements, assets	3.4	6.3	4.4	8.5
Available-for-sale | States and political subdivisions
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	1.7		1.9
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.1	0.2		0.2
Net purchases, sales, issuances and settlements, assets			(0.1)
Transfers into Level 3, assets		1.8		1.8
Ending balance, assets	1.8	2.0	1.8	2.0
Gross purchases, sales, issuances and settlements
Settlements, assets			(0.1)
Net purchases, sales, issuances and settlements, assets			(0.1)
Available-for-sale | Corporate debt securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	168.1	199.6	174.5	297.0
Total realized/unrealized gains (losses) included in net income, assets	(0.1)	(1.5)	(4.9)	(6.9)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	2.7	4.4	(7.0)	13.4
Net purchases, sales, issuances and settlements, assets	17.1	(38.8)	(25.3)	(70.3)
Transfers into Level 3, assets	4.7	28.8	105.3	54.8
Transfers out of Level 3, assets	(29.5)	(2.8)	(79.6)	(98.3)
Ending balance, assets	163.0	189.7	163.0	189.7
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.1)	(1.0)	(2.0)	(3.0)
Gross purchases, sales, issuances and settlements
Purchases, assets	18.2	3.1	29.6	21.2
Sales, assets	(0.7)	(34.1)	(32.4)	(80.7)
Settlements, assets	(0.4)	(7.8)	(22.5)	(10.8)
Net purchases, sales, issuances and settlements, assets	17.1	(38.8)	(25.3)	(70.3)
Available-for-sale | Collateralized debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	40.7	77.6	77.6	102.5
Total realized/unrealized gains (losses) included in net income, assets		(3.5)	2.1	(3.7)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(0.1)	7.1	7.4	4.1
Net purchases, sales, issuances and settlements, assets		(0.2)	(56.0)	5.1
Transfers into Level 3, assets			31.7
Transfers out of Level 3, assets	(2.6)	(4.2)	(24.8)	(31.2)
Ending balance, assets	38.0	76.8	38.0	76.8
Changes in unrealized gains (losses) included in net income relating to positions still held, assets				(0.2)
Gross purchases, sales, issuances and settlements
Purchases, assets			17.0	5.1
Sales, assets		(0.3)	(47.4)	(0.3)
Settlements, assets		0.1	(25.6)	0.3
Net purchases, sales, issuances and settlements, assets		(0.2)	(56.0)	5.1
Available-for-sale | Other debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	18.4	5.1	14.7	27.3
Total realized/unrealized gains (losses) included in net income, assets	(0.3)		(0.3)	(2.2)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.6	2.0	3.0	1.3
Net purchases, sales, issuances and settlements, assets	13.6	(0.5)	12.7	(25.8)
Transfers into Level 3, assets	15.0	9.3	17.2	15.3
Ending balance, assets	47.3	15.9	47.3	15.9
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.3)		(0.3)	(2.2)
Gross purchases, sales, issuances and settlements
Purchases, assets	15.0		15.0
Settlements, assets	(1.4)	(0.5)	(2.3)	(25.8)
Net purchases, sales, issuances and settlements, assets	13.6	(0.5)	12.7	(25.8)
Available-for-sale | Equity securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	16.9	17.1	15.3	18.0
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.3	(0.4)	1.9	(1.3)
Ending balance, assets	17.2	16.7	17.2	16.7
Trading | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	170.7	185.7	166.8	220.8
Total realized/unrealized gains (losses) included in net income, assets	2.1	5.1	6.0	3.6
Net purchases, sales, issuances and settlements, assets	0.1		0.1	(43.1)
Transfers into Level 3, assets				9.5
Ending balance, assets	172.9	190.8	172.9	190.8
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	2.1	5.0	6.1	4.3
Gross purchases, sales, issuances and settlements
Sales, assets				(0.9)
Settlements, assets	0.1		0.1	(42.2)
Net purchases, sales, issuances and settlements, assets	0.1		0.1	(43.1)
Derivative assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	56.9	58.8	75.1	60.2
Total realized/unrealized gains (losses) included in net income, assets	1.0	10.7	(21.3)	6.9
Net purchases, sales, issuances and settlements, assets	(0.4)	0.5	3.7	2.9
Ending balance, assets	57.5	70.0	57.5	70.0
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	1.2	12.6	(21.6)	8.0
Gross purchases, sales, issuances and settlements
Purchases, assets		0.5	7.2	3.7
Sales, assets	(0.4)		(3.5)	(0.8)
Net purchases, sales, issuances and settlements, assets	(0.4)	0.5	3.7	2.9
Other investments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	116.5	83.6	113.9	97.5
Total realized/unrealized gains (losses) included in net income, assets	1.8	(0.1)	7.1	(0.8)
Net purchases, sales, issuances and settlements, assets	22.2	30.7	19.5	17.5
Ending balance, assets	140.5	114.2	140.5	114.2
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	1.9	(0.1)	7.1	(0.8)
Gross purchases, sales, issuances and settlements
Purchases, assets	25.7	33.3	28.3	33.3
Settlements, assets	(3.5)	(2.6)	(8.8)	(15.8)
Net purchases, sales, issuances and settlements, assets	22.2	30.7	19.5	17.5
Separate account assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	4,841.7	4,440.5	4,616.0	4,198.2
Total realized/unrealized gains (losses) included in net income, assets	134.6	106.0	427.2	318.4
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.1			0.3
Net purchases, sales, issuances and settlements, assets	(33.7)	(26.6)	(108.1)	3.2
Transfers into Level 3, assets	4.9		12.7	1.6
Transfers out of Level 3, assets	(6.3)		(6.5)	(1.8)
Ending balance, assets	4,941.3	4,519.9	4,941.3	4,519.9
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	122.0	105.9	406.3	307.1
Gross purchases, sales, issuances and settlements
Purchases, assets	33.6	42.6	143.2	211.1
Sales, assets	(54.7)	(72.9)	(223.4)	(191.9)
Issuances, assets	(1.4)	(57.8)	(7.6)	(204.1)
Settlements, assets	(11.2)	61.5	(20.3)	188.1
Net purchases, sales, issuances and settlements, assets	(33.7)	(26.6)	(108.1)	3.2
Investment-type insurance contracts
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(74.2)	(179.4)	(170.5)	(195.8)
Total realized/unrealized gains (losses) included in net income, liabilities	17.6	(37.0)	111.2	(14.6)
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities		(0.1)		(0.1)
Net purchases, sales, issuances and settlements, liabilities	7.3	(5.2)	10.0	(11.2)
Ending balance, liabilities	(49.3)	(221.7)	(49.3)	(221.7)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	19.8	(37.6)	109.2	(16.8)
Gross purchases, sales, issuances and settlements
Issuances, liabilities	5.4	(6.7)	5.6	(14.8)
Settlements, liabilities	1.9	1.5	4.4	3.6
Net purchases, sales, issuances and settlements, liabilities	7.3	(5.2)	10.0	(11.2)
Derivative liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(70.0)	(147.1)	(102.6)	(177.1)
Total realized/unrealized gains (losses) included in net income, liabilities	14.1	19.6	44.4	32.4
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities	0.3	0.5	(0.2)	0.7
Net purchases, sales, issuances and settlements, liabilities	5.6	7.3	8.4	24.3
Ending balance, liabilities	(50.0)	(119.7)	(50.0)	(119.7)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	14.2	19.3	43.8	34.9
Gross purchases, sales, issuances and settlements
Purchases, liabilities		(1.9)	(3.1)	(3.6)
Sales, liabilities	5.6	9.2	11.5	27.9
Net purchases, sales, issuances and settlements, liabilities	5.6	7.3	8.4	24.3
Other liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(59.6)	(33.6)	(39.6)	(24.2)
Total realized/unrealized gains (losses) included in net income, liabilities	(7.5)	(14.7)	(27.5)	(24.1)
Ending balance, liabilities	(67.1)	(48.3)	(67.1)	(48.3)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	$ (7.6)	$ (14.7)	$ (27.5)	$ (24.1)