Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Assets (Details) (Level 3, Fair Value, Measurements, Nonrecurring, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
U.S. government and agency
Level 3 Financial Assets:
Balance, beginning of period	$ 25.9	$ 31.7
Purchases	5.5	0
Sales	(0.1)	(2.5)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Realized and unrealized gains (losses) included in income [1]	0	0
Unrealized gains (losses) included in OCI	0.1	(2.5)
Total	31.4	26.7
State and political subdivision
Level 3 Financial Assets:
Balance, beginning of period	116.1	48.7
Purchases	0	14.9
Sales	(0.2)	0
Transfers into Level 3	0	12.9
Transfers out of Level 3	0	0
Realized and unrealized gains (losses) included in income [1]	0	0
Unrealized gains (losses) included in OCI	(17.7)	2.2
Total	98.2	78.7
Foreign government
Level 3 Financial Assets:
Balance, beginning of period	8.0	3.1
Purchases	0	3.5
Sales	0	0
Transfers into Level 3	1.6	0
Transfers out of Level 3	0	0
Realized and unrealized gains (losses) included in income [1]	0	0
Unrealized gains (losses) included in OCI	(0.1)	0.3
Total	9.5	6.9
Corporate
Level 3 Financial Assets:
Balance, beginning of period	801.5	513.8
Purchases	84.2	64.9
Sales	(2.5)	(1.6)
Transfers into Level 3	4.9	20.0
Transfers out of Level 3	(8.9)	(17.3)
Realized and unrealized gains (losses) included in income [1]	0	0
Unrealized gains (losses) included in OCI	24.0	(2.0)
Total	903.2	577.8
Commercial mortgage-backed (CMBS)
Level 3 Financial Assets:
Balance, beginning of period	19.3	35.4
Purchases	0	0
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	(1.5)	(14.1)
Realized and unrealized gains (losses) included in income [1]	0	(0.1)
Unrealized gains (losses) included in OCI	0.9	(0.8)
Total	18.7	20.4
Residential mortgage-backed (RMBS)
Level 3 Financial Assets:
Balance, beginning of period	222.6	268.0
Purchases	0.3	0.4
Sales	(7.1)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Realized and unrealized gains (losses) included in income [1]	(0.2)	(0.5)
Unrealized gains (losses) included in OCI	(1.3)	(12.5)
Total	214.3	255.4
CDO/CLO
Level 3 Financial Assets:
Balance, beginning of period	56.8	62.5
Purchases	10.9	0
Sales	(1.4)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Realized and unrealized gains (losses) included in income [1]	(0.3)	0
Unrealized gains (losses) included in OCI	(3.8)	2.4
Total	62.2	64.9
Other asset-backed
Level 3 Financial Assets:
Balance, beginning of period	80.1	81.2
Purchases	0	7.0
Sales	(1.4)	(0.4)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Realized and unrealized gains (losses) included in income [1]	0	0.1
Unrealized gains (losses) included in OCI	1.2	(9.2)
Total	79.9	78.7
Debt Securities
Level 3 Financial Assets:
Balance, beginning of period	1,330.3	1,044.4
Purchases	100.9	90.7
Sales	(12.7)	(4.5)
Transfers into Level 3	6.5	32.9
Transfers out of Level 3	(10.4)	(31.4)
Realized and unrealized gains (losses) included in income [1]	(0.5)	(0.5)
Unrealized gains (losses) included in OCI	3.3	(22.1)
Total	1,417.4	1,109.5
Related party reinsurance derivative
Level 3 Financial Assets:
Balance, beginning of period	0	3.5
Purchases	0	0
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Realized and unrealized gains (losses) included in income [1]	0	(5.3)
Unrealized gains (losses) included in OCI	0	0
Total	0	(1.8)
Fair value investments
Level 3 Financial Assets:
Balance, beginning of period	22.7	27.5
Purchases	9.8	0
Sales	(1.3)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Realized and unrealized gains (losses) included in income [1]	0.2	(7.1)
Unrealized gains (losses) included in OCI	0	0
Total	31.4	20.4
Total assets
Level 3 Financial Assets:
Balance, beginning of period	1,353.0	1,075.4
Purchases	110.7	90.7
Sales	(14.0)	(4.5)
Transfers into Level 3	6.5	32.9
Transfers out of Level 3	(10.4)	(31.4)
Realized and unrealized gains (losses) included in income [1]	(0.3)	(12.9)
Unrealized gains (losses) included in OCI	3.3	(22.1)
Total	$ 1,448.8	$ 1,128.1