Fair Value Measurements (Details 2) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Available-for-sale | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	$ 301.0	$ 324.6	$ 313.0	$ 449.7
Total realized/unrealized gains (losses) included in net income, assets	(1.8)	(4.4)	(2.8)	(7.8)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.6	1.8	(0.4)	5.3
Net purchases, sales, issuances and settlements, assets	(43.1)	(7.7)	(98.4)	(49.3)
Transfers into Level 3, assets	65.6	22.6	134.5	46.5
Transfers out of Level 3, assets	(48.7)	(15.0)	(72.3)	(122.5)
Ending balance, assets	273.6	321.9	273.6	321.9
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(1.9)	(0.2)	(2.1)	(5.1)
Gross purchases, sales, issuances and settlements
Purchases, assets	29.7	13.7	33.9	29.9
Sales, assets	(37.3)	(20.0)	(83.0)	(50.5)
Settlements, assets	(35.5)	(1.4)	(49.3)	(28.7)
Net purchases, sales, issuances and settlements, assets	(43.1)	(7.7)	(98.4)	(49.3)
Available-for-sale | Non-U.S. governments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	40.4	36.8	44.3	22.9
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	(0.8)	0.3	(0.5)
Net purchases, sales, issuances and settlements, assets	5.2	2.5	1.0	2.2
Transfers into Level 3, assets				14.5
Ending balance, assets	44.7	39.6	44.7	39.6
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	5.5	2.8	5.5	6.7
Sales, assets			(3.9)	(3.9)
Settlements, assets	(0.3)	(0.3)	(0.6)	(0.6)
Net purchases, sales, issuances and settlements, assets	5.2	2.5	1.0	2.2
Available-for-sale | States and political subdivisions
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	1.9		1.9
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(0.1)		(0.1)
Net purchases, sales, issuances and settlements, assets	(0.1)		(0.1)
Ending balance, assets	1.7		1.7
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	167.6	202.7	174.5	297.0
Total realized/unrealized gains (losses) included in net income, assets	(1.7)	(2.8)	(4.8)	(5.4)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.4	7.0	(9.7)	9.0
Net purchases, sales, issuances and settlements, assets	(24.8)	(14.9)	(42.4)	(31.5)
Transfers into Level 3, assets	53.1	22.6	100.6	26.0
Transfers out of Level 3, assets	(26.5)	(15.0)	(50.1)	(95.5)
Ending balance, assets	168.1	199.6	168.1	199.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(1.9)	(0.1)	(2.0)	(2.7)
Gross purchases, sales, issuances and settlements
Purchases, assets	7.2	5.8	11.4	18.1
Sales, assets	(22.3)	(20.0)	(31.7)	(46.6)
Settlements, assets	(9.7)	(0.7)	(22.1)	(3.0)
Net purchases, sales, issuances and settlements, assets	(24.8)	(14.9)	(42.4)	(31.5)
Available-for-sale | Collateralized debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	75.2	79.0	77.6	102.5
Total realized/unrealized gains (losses) included in net income, assets		(0.1)	2.1	(0.2)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.4	(6.1)	7.5	(3.0)
Net purchases, sales, issuances and settlements, assets	(23.0)	4.8	(56.0)	5.3
Transfers into Level 3, assets	10.3		31.7
Transfers out of Level 3, assets	(22.2)		(22.2)	(27.0)
Ending balance, assets	40.7	77.6	40.7	77.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets		(0.1)		(0.2)
Gross purchases, sales, issuances and settlements
Purchases, assets	17.0	5.1	17.0	5.1
Sales, assets	(15.0)		(47.4)
Settlements, assets	(25.0)	(0.3)	(25.6)	0.2
Net purchases, sales, issuances and settlements, assets	(23.0)	4.8	(56.0)	5.3
Available-for-sale | Other debt obligations
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	15.9	6.1	14.7	27.3
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	0.7	0.6	2.4	(0.7)
Net purchases, sales, issuances and settlements, assets	(0.4)	(0.1)	(0.9)	(25.3)
Transfers into Level 3, assets	2.2		2.2	6.0
Ending balance, assets	18.4	5.1	18.4	5.1
Changes in unrealized gains (losses) included in net income relating to positions still held, assets				(2.2)
Gross purchases, sales, issuances and settlements
Settlements, assets	(0.4)	(0.1)	(0.9)	(25.3)
Net purchases, sales, issuances and settlements, assets	(0.4)	(0.1)	(0.9)	(25.3)
Available-for-sale | Equity securities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	16.1	17.5	15.3	18.0
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.8	(0.4)	1.6	(0.9)
Ending balance, assets	16.9	17.1	16.9	17.1
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.1)		(0.1)
Trading | Fixed maturities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	169.5	206.2	166.8	220.8
Total realized/unrealized gains (losses) included in net income, assets	1.2	(5.1)	3.9	(1.5)
Net purchases, sales, issuances and settlements, assets		(24.9)		(43.1)
Transfers into Level 3, assets		9.5		9.5
Ending balance, assets	170.7	185.7	170.7	185.7
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	1.1	(1.1)	3.9	(3.6)
Gross purchases, sales, issuances and settlements
Sales, assets				(0.9)
Settlements, assets		(24.9)		(42.2)
Net purchases, sales, issuances and settlements, assets		(24.9)		(43.1)
Derivative assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	67.2	47.3	75.1	60.2
Total realized/unrealized gains (losses) included in net income, assets	(10.8)	10.8	(22.3)	(3.8)
Net purchases, sales, issuances and settlements, assets	0.5	0.7	4.1	2.4
Ending balance, assets	56.9	58.8	56.9	58.8
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(10.6)	11.8	(22.9)	(2.8)
Gross purchases, sales, issuances and settlements
Purchases, assets	0.5	0.7	7.2	3.2
Sales, assets			(3.1)	(0.8)
Net purchases, sales, issuances and settlements, assets	0.5	0.7	4.1	2.4
Other investments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	110.9	89.8	113.9	97.5
Total realized/unrealized gains (losses) included in net income, assets	5.9	0.2	5.3	(0.7)
Net purchases, sales, issuances and settlements, assets	(0.3)	(6.4)	(2.7)	(13.2)
Ending balance, assets	116.5	83.6	116.5	83.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	5.9	0.2	5.3	(0.7)
Gross purchases, sales, issuances and settlements
Purchases, assets	2.4		2.6
Settlements, assets	(2.7)	(6.4)	(5.3)	(13.2)
Net purchases, sales, issuances and settlements, assets	(0.3)	(6.4)	(2.7)	(13.2)
Separate account assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	4,659.2	4,280.3	4,616.0	4,198.2
Total realized/unrealized gains (losses) included in net income, assets	173.6	126.3	292.6	212.4
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.1	0.2	(0.1)	0.3
Net purchases, sales, issuances and settlements, assets	2.6	32.5	(74.4)	29.8
Transfers into Level 3, assets	6.4	1.3	7.8	1.6
Transfers out of Level 3, assets	(0.2)	(0.1)	(0.2)	(1.8)
Ending balance, assets	4,841.7	4,440.5	4,841.7	4,440.5
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	173.5	126.9	290.3	203.6
Gross purchases, sales, issuances and settlements
Purchases, assets	43.1	41.0	109.6	168.5
Sales, assets	(31.9)	(28.7)	(168.7)	(119.0)
Issuances, assets	(1.7)	(11.4)	(6.2)	(146.3)
Settlements, assets	(6.9)	31.6	(9.1)	126.6
Net purchases, sales, issuances and settlements, assets	2.6	32.5	(74.4)	29.8
Investment-type insurance contracts
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(117.8)	(129.0)	(170.5)	(195.8)
Total realized/unrealized gains (losses) included in net income, liabilities	41.9	(46.4)	93.6	22.4
Net purchases, sales, issuances and settlements, liabilities	1.7	(4.0)	2.7	(6.0)
Ending balance, liabilities	(74.2)	(179.4)	(74.2)	(179.4)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	41.5	(47.6)	92.1	21.2
Gross purchases, sales, issuances and settlements
Issuances, liabilities	0.5	(4.8)	0.2	(8.1)
Settlements, liabilities	1.2	0.8	2.5	2.1
Net purchases, sales, issuances and settlements, liabilities	1.7	(4.0)	2.7	(6.0)
Derivative liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(75.6)	(142.3)	(102.6)	(177.1)
Total realized/unrealized gains (losses) included in net income, liabilities	4.5	(12.6)	30.3	12.8
Total realized/unrealized gains (losses) included in other comprehensive income, liabilities	(0.8)	(1.1)	(0.5)	0.2
Net purchases, sales, issuances and settlements, liabilities	1.9	8.9	2.8	17.0
Ending balance, liabilities	(70.0)	(147.1)	(70.0)	(147.1)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	4.4	(13.4)	29.6	13.7
Gross purchases, sales, issuances and settlements
Purchases, liabilities	(1.5)	(1.0)	(3.1)	(1.7)
Sales, liabilities	3.4	9.9	5.9	18.7
Net purchases, sales, issuances and settlements, liabilities	1.9	8.9	2.8	17.0
Other liabilities
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(54.2)	(40.7)	(39.6)	(24.2)
Total realized/unrealized gains (losses) included in net income, liabilities	(5.4)	7.1	(20.0)	(9.4)
Ending balance, liabilities	(59.6)	(33.6)	(59.6)	(33.6)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	$ (5.4)	$ 7.2	$ (20.0)	$ (9.4)