10. Fair Value of Financial Instruments (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
U.S. government and agency [Member]
Level 3 Financial Assets:
Fair Value, Total assets, Balance, beginning of period	$ 31.7	$ 24.0
Purchases	3.2	10.0
Sales	(6.7)	(2.2)
Transfers into Level 3		2.0
Transfers out of Level 3
Realized (gains) losses included in income
Unrealized (gains) losses included in other comprehensive loss	(2.3)	(2.1)
Fair Value, Total assets, Balance	25.9	31.7
State and political subdivision [Member]
Level 3 Financial Assets:
Fair Value, Total assets, Balance, beginning of period	48.7	13.3
Purchases	41.9	31.4
Sales	(0.8)
Transfers into Level 3	12.9
Transfers out of Level 3
Realized (gains) losses included in income
Unrealized (gains) losses included in other comprehensive loss	13.4	4.0
Fair Value, Total assets, Balance	58.0	48.7
Foreign government [Member]
Level 3 Financial Assets:
Fair Value, Total assets, Balance, beginning of period	3.1	1.4
Purchases	3.5	0.9
Sales
Transfers into Level 3		1.0
Transfers out of Level 3
Realized (gains) losses included in income
Unrealized (gains) losses included in other comprehensive loss	1.4	(0.2)
Fair Value, Total assets, Balance	8.0	3.1
Corporate [Member]
Level 3 Financial Assets:
Fair Value, Total assets, Balance, beginning of period	513.8	337.3
Purchases	297.1	185.7
Sales	(16.2)	(18.0)
Transfers into Level 3	21.0	(21.6)
Transfers out of Level 3	(14.2)	(9.1)
Realized (gains) losses included in income		(0.4)
Unrealized (gains) losses included in other comprehensive loss		(3.3)
Fair Value, Total assets, Balance	649.6	513.8
Commercial mortgage-backed (CMBS) [Member]
Level 3 Financial Assets:
Fair Value, Total assets, Balance, beginning of period	35.4	13.4
Purchases		17.6
Sales	(0.6)	(0.8)
Transfers into Level 3	1.1	4.3
Transfers out of Level 3	(17.1)
Realized (gains) losses included in income		0.2
Unrealized (gains) losses included in other comprehensive loss	0.5	1.1
Fair Value, Total assets, Balance	19.3	35.4
Residential mortgage-backed (RMBS) [Member]
Level 3 Financial Assets:
Fair Value, Total assets, Balance, beginning of period	268.0	216.1
Purchases	1.6	107.7
Sales	(38.9)	(47.1)
Transfers into Level 3
Transfers out of Level 3
Realized (gains) losses included in income	2.0	(1.6)
Unrealized (gains) losses included in other comprehensive loss	(10.1)	(7.1)
Fair Value, Total assets, Balance	222.6	268.0
CDO/CLO [Member]
Level 3 Financial Assets:
Fair Value, Total assets, Balance, beginning of period	62.5	59.6
Purchases	2.8	8.1
Sales	(5.3)	(1.1)
Realized (gains) losses included in income	(0.3)	(0.2)
Unrealized (gains) losses included in other comprehensive loss	(2.9)	(3.9)
Fair Value, Total assets, Balance	3.4	62.5
Other asset-backed [Member]
Level 3 Financial Assets:
Fair Value, Total assets, Balance, beginning of period	81.2	57.7
Purchases	9.5	33.1
Sales	(7.6)	(8.9)
Transfers into Level 3		0.1
Transfers out of Level 3	(1.8)	(4.0)
Unrealized (gains) losses included in other comprehensive loss	(1.2)	3.2
Fair Value, Total assets, Balance	2.8	81.2
Inter-company reinsurance derivative asset
Level 3 Financial Assets:
Fair Value, Total assets, Balance, beginning of period	3.5	(5.5)
Purchases
Sales
Transfers into Level 3
Transfers out of Level 3
Realized (gains) losses included in income	(3.5)	9.0
Unrealized (gains) losses included in other comprehensive loss
Fair Value, Total assets, Balance		3.5
Fair value option investments [Member]
Level 3 Financial Assets:
Fair Value, Total assets, Balance, beginning of period	27.5	13.8
Purchases	5.7	19.2
Sales	(11.5)	(3.1)
Realized (gains) losses included in income	1.0	(2.4)
Fair Value, Total assets, Balance	22.7	27.5
Total assets [Member]
Level 3 Financial Assets:
Fair Value, Total assets, Balance, beginning of period	1,075.4	731.1
Purchases	365.3	413.7
Sales	(87.6)	(81.2)
Transfers into Level 3	35.0	29.0
Transfers out of Level 3	(33.1)	(13.1)
Realized (gains) losses included in income	(0.8)	4.2
Unrealized (gains) losses included in other comprehensive loss	(1.2)	(8.3)
Fair Value, Total assets, Balance	$ 1,353.0	$ 1,075.4