Fair Value Measurements (Details 2) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Available-for-sale | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	$ 333.6	$ 273.6	$ 336.8	$ 313.0
Total realized/unrealized gains (losses) included in net income, assets	(2.4)	(0.4)	(3.3)	(3.2)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(2.7)	3.0	1.9	2.6
Net purchases, sales, issuances and settlements, assets	50.9	34.1	51.4	(64.3)
Transfers into Level 3, assets	16.4	19.7	53.4	154.2
Transfers out of Level 3, assets	(17.9)	(32.1)	(62.3)	(104.4)
Ending balance, assets	377.9	297.9	377.9	297.9
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(2.3)	(0.4)	(3.1)	(2.4)
Gross purchases, sales, issuances and settlements
Purchases, assets	58.4	36.9	131.6	70.8
Sales, assets	(5.5)	(0.7)	(50.0)	(83.7)
Settlements, assets	(2.0)	(2.1)	(30.2)	(51.4)
Net purchases, sales, issuances and settlements, assets	50.9	34.1	51.4	(64.3)
Available-for-sale | Non-U.S. governments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	39.6	44.7	47.5	44.3
Total realized/unrealized gains (losses) included in net income, assets			(0.1)	(0.1)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(0.2)	(0.3)	(0.2)	(0.8)
Net purchases, sales, issuances and settlements, assets	1.9	3.4	(5.9)	4.4
Ending balance, assets	41.3	47.8	41.3	47.8
Changes in unrealized gains (losses) included in net income relating to positions still held, assets			(0.1)	(0.1)
Gross purchases, sales, issuances and settlements
Purchases, assets	2.2	3.7	5.8	9.2
Sales, assets			(10.8)	(3.9)
Settlements, assets	(0.3)	(0.3)	(0.9)	(0.9)
Net purchases, sales, issuances and settlements, assets	1.9	3.4	(5.9)	4.4
Available-for-sale | States and political subdivisions
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	1.7	1.7	1.8	1.9
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0	0.1
Net purchases, sales, issuances and settlements, assets			(0.1)	(0.1)
Ending balance, assets	1.7	1.8	1.7	1.8
Gross purchases, sales, issuances and settlements
Settlements, assets			(0.1)	(0.1)
Net purchases, sales, issuances and settlements, assets			(0.1)	(0.1)
Available-for-sale | Corporate debt securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	170.0	168.1	164.0	174.5
Total realized/unrealized gains (losses) included in net income, assets	(1.4)	(0.1)	(1.6)	(4.9)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(2.8)	2.7	(1.7)	(7.0)
Net purchases, sales, issuances and settlements, assets	40.7	17.1	21.0	(25.3)
Transfers into Level 3, assets	15.7	4.7	46.6	105.3
Transfers out of Level 3, assets		(29.5)	(6.1)	(79.6)
Ending balance, assets	222.2	163.0	222.2	163.0
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(1.4)	(0.1)	(1.5)	(2.0)
Gross purchases, sales, issuances and settlements
Purchases, assets	46.5	18.2	64.5	29.6
Sales, assets	(5.5)	(0.7)	(39.2)	(32.4)
Settlements, assets	(0.3)	(0.4)	(4.3)	(22.5)
Net purchases, sales, issuances and settlements, assets	40.7	17.1	21.0	(25.3)
Available-for-sale | Commercial mortgage-backed securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	6.7		1.6
Total realized/unrealized gains (losses) included in net income, assets	(1.0)		(1.6)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.2		1.3
Net purchases, sales, issuances and settlements, assets	0.2		(0.2)
Transfers into Level 3, assets	0.7		6.8
Transfers out of Level 3, assets	(1.4)		(2.5)
Ending balance, assets	5.4		5.4
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.9)		(1.5)
Gross purchases, sales, issuances and settlements
Settlements, assets	0.2		(0.2)
Net purchases, sales, issuances and settlements, assets	0.2		(0.2)
Available-for-sale | Collateralized debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	68.5	40.7	37.8	77.6
Total realized/unrealized gains (losses) included in net income, assets				2.1
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.1	(0.1)	1.4	7.4
Net purchases, sales, issuances and settlements, assets	9.5		46.4	(56.0)
Transfers into Level 3, assets				31.7
Transfers out of Level 3, assets	(16.5)	(2.6)	(24.0)	(24.8)
Ending balance, assets	61.6	38.0	61.6	38.0
Gross purchases, sales, issuances and settlements
Purchases, assets	9.7		61.3	17.0
Sales, assets				(47.4)
Settlements, assets	(0.2)		(14.9)	(25.6)
Net purchases, sales, issuances and settlements, assets	9.5		46.4	(56.0)
Available-for-sale | Other debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	47.1	18.4	84.1	14.7
Total realized/unrealized gains (losses) included in net income, assets		(0.3)		(0.3)
Total realized/unrealized gains (losses) included in other comprehensive income, assets		0.6	1.1	3.0
Net purchases, sales, issuances and settlements, assets	(1.4)	13.6	(9.8)	12.7
Transfers into Level 3, assets		15.0		17.2
Transfers out of Level 3, assets			(29.7)
Ending balance, assets	45.7	47.3	45.7	47.3
Changes in unrealized gains (losses) included in net income relating to positions still held, assets		(0.3)		(0.3)
Gross purchases, sales, issuances and settlements
Purchases, assets		15.0		15.0
Settlements, assets	(1.4)	(1.4)	(9.8)	(2.3)
Net purchases, sales, issuances and settlements, assets	(1.4)	13.6	(9.8)	12.7
Available-for-sale | Equity securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	17.5	16.9	16.9	15.3
Total realized/unrealized gains (losses) included in net income, assets	(0.3)		(0.3)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.2	0.3	0.6	1.9
Transfers into Level 3, assets			0.2
Ending balance, assets	17.4	17.2	17.4	17.2
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.3)		(0.3)
Trading | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	159.6	170.7	169.9	166.8
Total realized/unrealized gains (losses) included in net income, assets	3.7	2.1	9.5	6.0
Net purchases, sales, issuances and settlements, assets	(24.0)	0.1	(40.1)	0.1
Ending balance, assets	139.3	172.9	139.3	172.9
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	0.2	2.1	0.8	6.1
Gross purchases, sales, issuances and settlements
Sales, assets	(10.0)		(10.0)
Settlements, assets	(14.0)	0.1	(30.1)	0.1
Net purchases, sales, issuances and settlements, assets	(24.0)	0.1	(40.1)	0.1
Derivative assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	62.3	56.9	74.2	75.1
Total realized/unrealized gains (losses) included in net income, assets	(14.4)	1.0	(26.4)	(21.3)
Net purchases, sales, issuances and settlements, assets	9.8	(0.4)	9.9	3.7
Ending balance, assets	57.7	57.5	57.7	57.5
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(14.3)	1.2	(26.4)	(21.6)
Gross purchases, sales, issuances and settlements
Purchases, assets	9.8		9.9	7.2
Sales, assets		(0.4)		(3.5)
Net purchases, sales, issuances and settlements, assets	9.8	(0.4)	9.9	3.7
Other investments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	134.6	116.5	142.9	113.9
Total realized/unrealized gains (losses) included in net income, assets	3.8	1.8	6.6	7.1
Net purchases, sales, issuances and settlements, assets	(14.1)	22.2	(25.2)	19.5
Ending balance, assets	124.3	140.5	124.3	140.5
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	3.8	1.9	6.6	7.1
Gross purchases, sales, issuances and settlements
Purchases, assets		25.7	0.2	28.3
Settlements, assets	(14.1)	(3.5)	(25.4)	(8.8)
Net purchases, sales, issuances and settlements, assets	(14.1)	22.2	(25.2)	19.5
Separate account assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	5,533.6	4,841.7	5,265.2	4,616.0
Total realized/unrealized gains (losses) included in net income, assets	167.0	134.6	358.4	427.2
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(0.3)	0.1	(0.2)
Net purchases, sales, issuances and settlements, assets	79.0	(33.7)	161.3	(108.1)
Transfers into Level 3, assets	2.3	4.9	4.4	12.7
Transfers out of Level 3, assets	(4.2)	(6.3)	(11.7)	(6.5)
Ending balance, assets	5,777.4	4,941.3	5,777.4	4,941.3
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	163.7	122.0	359.9	406.3
Gross purchases, sales, issuances and settlements
Purchases, assets	228.5	33.6	467.2	143.2
Sales, assets	(127.3)	(54.7)	(257.5)	(223.4)
Issuances, assets	(196.5)	(1.4)	(290.1)	(7.6)
Settlements, assets	174.3	(11.2)	241.7	(20.3)
Net purchases, sales, issuances and settlements, assets	79.0	(33.7)	161.3	(108.1)
Investment-type insurance contracts
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(37.6)	(74.2)	(6.9)	(170.5)
Total realized/unrealized gains (losses) included in net income, liabilities	(62.8)	17.6	(109.9)	111.2
Net purchases, sales, issuances and settlements, liabilities	6.3	7.3	22.7	10.0
Ending balance, liabilities	(94.1)	(49.3)	(94.1)	(49.3)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(56.1)	19.8	(109.9)	109.2
Gross purchases, sales, issuances and settlements
Issuances, liabilities	4.2	5.4	18.7	5.6
Settlements, liabilities	2.1	1.9	4.0	4.4
Net purchases, sales, issuances and settlements, liabilities	6.3	7.3	22.7	10.0
Derivative liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(24.7)	(70.0)	(39.6)	(102.6)
Total realized/unrealized gains (losses) included in net income, liabilities	(13.3)	14.1	0.5	44.4
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities	(0.6)	0.3	0.7	(0.2)
Net purchases, sales, issuances and settlements, liabilities	0.8	5.6	0.6	8.4
Ending balance, liabilities	(37.8)	(50.0)	(37.8)	(50.0)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(14.5)	14.2	(3.7)	43.8
Gross purchases, sales, issuances and settlements
Purchases, liabilities	(0.6)		(0.8)	(3.1)
Sales, liabilities	1.4	5.6	1.4	11.5
Net purchases, sales, issuances and settlements, liabilities	0.8	5.6	0.6	8.4
Other liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(79.9)	(59.6)	(73.9)	(39.6)
Total realized/unrealized gains (losses) included in net income, liabilities	9.9	(7.5)	3.9	(27.5)
Net purchases, sales, issuances and settlements, liabilities	9.0		9.0
Ending balance, liabilities	(61.0)	(67.1)	(61.0)	(67.1)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	9.8	(7.6)	4.5	(27.5)
Gross purchases, sales, issuances and settlements
Sales, liabilities	9.0		9.0
Net purchases, sales, issuances and settlements, liabilities	$ 9.0		$ 9.0