Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Assets (Details) (Level 3, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
U.S. government and agency
Level 3 Financial Assets:
Balance, beginning of period	$ 80.2	$ 58.8
Purchases	0	7.9
Sales	(0.4)	(2.1)
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	1.3	1.2
Total	81.1	65.8
State and political subdivision
Level 3 Financial Assets:
Balance, beginning of period	193.3	133.8
Purchases	6.7	1.3
Sales	(0.8)	(0.4)
Transfers into Level 3	0	0
Transfers out of Level 3	(2.1)	0
Realized and unrealized gains (losses) included in income [1]	0	0
Unrealized gains (losses) included in OCI	4.1	3.1
Total	201.2	137.8
Foreign government
Level 3 Financial Assets:
Balance, beginning of period	16.1	3.3
Purchases	0	0
Sales	0	0
Transfers into Level 3	0.3	0
Transfers out of Level 3	(6.5)	0
Realized and unrealized gains (losses) included in income [1]	0	0
Unrealized gains (losses) included in OCI	0.6	0.1
Total	10.5	3.4
Corporate
Level 3 Financial Assets:
Balance, beginning of period	1,417.2	1,091.9
Purchases	113.0	81.8
Sales	(30.7)	(15.1)
Transfers into Level 3	18.8	0
Transfers out of Level 3	(92.8)	(62.9)
Realized and unrealized gains (losses) included in income [1]	0.4	0
Unrealized gains (losses) included in OCI	26.6	26.6
Total	1,452.5	1,122.3
Commercial mortgage-backed (“CMBS”)
Level 3 Financial Assets:
Balance, beginning of period	64.6	31.9
Purchases	0	0
Sales	0	(0.2)
Transfers into Level 3	0	0
Transfers out of Level 3	(57.7)	(25.5)
Realized and unrealized gains (losses) included in income [1]	0	0
Unrealized gains (losses) included in OCI	0	0.2
Total	6.9	6.4
Residential mortgage-backed (“RMBS”)
Level 3 Financial Assets:
Balance, beginning of period	151.6	176.1
Purchases	0.3	0.3
Sales	(4.8)	(6.0)
Transfers into Level 3	12.0	0
Transfers out of Level 3	0	0
Realized and unrealized gains (losses) included in income [1]	(0.1)	0.2
Unrealized gains (losses) included in OCI	(0.7)	0.8
Total	158.3	171.4
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Level 3 Financial Assets:
Balance, beginning of period	83.5	70.9
Purchases	17.0	18.0
Sales	(5.4)	(2.8)
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	1.2	(0.4)
Total	96.5	86.2
Other asset-backed (“ABS”)
Level 3 Financial Assets:
Balance, beginning of period	77.9	82.3
Purchases	0	0
Sales	(0.9)	(2.9)
Transfers into Level 3	0	0
Transfers out of Level 3	(5.9)	0
Realized and unrealized gains (losses) included in income [1]	0	0.1
Unrealized gains (losses) included in OCI	0.1	(0.6)
Total	71.2	78.9
Debt securities
Level 3 Financial Assets:
Balance, beginning of period	2,084.4	1,649.0
Purchases	137.0	109.3
Sales	(43.0)	(29.5)
Transfers into Level 3	31.1	0
Transfers out of Level 3	(165.0)	(88.4)
Realized and unrealized gains (losses) included in income [1]	0.5	0.8
Unrealized gains (losses) included in OCI	33.2	31.0
Total	2,078.2	1,672.2
Available-for-sale equity securities
Level 3 Financial Assets:
Balance, beginning of period	28.7	11.2
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	0
Unrealized gains (losses) included in OCI	0.8	(11.2)
Total	29.5	0
Fair value investments
Level 3 Financial Assets:
Balance, beginning of period	33.7	35.6
Purchases	0.1	0
Sales	(0.7)	(0.7)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Realized and unrealized gains (losses) included in income [1]	0.8	0.3
Unrealized gains (losses) included in OCI	0	0
Total	33.9	35.2
Total assets
Level 3 Financial Assets:
Balance, beginning of period	2,146.8	1,695.8
Purchases	137.1	109.3
Sales	(43.7)	(30.2)
Transfers into Level 3	31.1	0
Transfers out of Level 3	(165.0)	(88.4)
Realized and unrealized gains (losses) included in income [1]	1.3	1.1
Unrealized gains (losses) included in OCI	34.0	19.8
Total	$ 2,141.6	$ 1,707.4