Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Assets (Details) (Level 3, Fair Value, Measurements, Nonrecurring, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013		Dec. 31, 2011
U.S. government and agency
($ in millions)
Balance, beginning of period	$ 58.8	[1]	$ 25.9	[1]
Purchases	7.9	[1]	5.5	[1]
Sales	(2.1)	[1]	(0.1)	[1]
Transfers into Level 3	0	[1]	0	[1]
Transfers out of Level 3	0	[1]	0	[1]
Realized and unrealized gains (losses) included in income [1]	0	[1],[2]	0	[1],[2]
Unrealized gains (losses) included in OCI	1.2	[1]	0.1	[1]
Total	65.8	[1]	31.4	[1]
State and political subdivision
($ in millions)
Balance, beginning of period	133.8				116.1
Purchases	1.3		0
Sales	(0.4)		(0.2)
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	[2]
Unrealized gains (losses) included in OCI	3.1		(17.7)
Total	137.8		98.2		116.1
Foreign government
($ in millions)
Balance, beginning of period	3.3				8.0
Purchases	0		0
Sales	0		0
Transfers into Level 3	0		1.6
Transfers out of Level 3	0		0
Realized and unrealized gains (losses) included in income [1]	0	[2]	0	[2]
Unrealized gains (losses) included in OCI	0.1		(0.1)
Total	3.4		9.5		8.0
Corporate
($ in millions)
Balance, beginning of period	1,102.3				801.5
Purchases	81.8		84.2
Sales	(4.1)		(2.5)
Transfers into Level 3	0		4.9
Transfers out of Level 3	(62.9)		(8.9)
Realized and unrealized gains (losses) included in income [1]	0	[2]	0	[2]
Unrealized gains (losses) included in OCI	5.2		24.0
Total	1,122.3		903.2		801.5
Commercial mortgage-backed (“CMBS”)
($ in millions)
Balance, beginning of period	31.9				19.3
Purchases	0		0
Sales	(0.2)		0
Transfers into Level 3	0		0
Transfers out of Level 3	(25.5)		(1.5)
Realized and unrealized gains (losses) included in income [1]	0	[2]	0	[2]
Unrealized gains (losses) included in OCI	0.2		0.9
Total	6.4		18.7		19.3
Residential mortgage-backed (“RMBS”)
($ in millions)
Balance, beginning of period	175.9				222.6
Purchases	0.3		0.3
Sales	(3.7)		(7.1)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Realized and unrealized gains (losses) included in income [1]	(0.1)	[2]	(0.2)	[2]
Unrealized gains (losses) included in OCI	(1.5)		(1.3)
Total	170.9		214.3		222.6
Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)
($ in millions)
Balance, beginning of period	70.9				56.8
Purchases	18.0		10.9
Sales	(1.4)		(1.4)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Realized and unrealized gains (losses) included in income [1]	0.2	[2]	(0.3)	[2]
Unrealized gains (losses) included in OCI	(1.5)		(3.8)
Total	86.2		62.2		56.8
Other asset-backed
($ in millions)
Balance, beginning of period	82.3				80.1
Purchases	0		0
Sales	(0.6)		(1.4)
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	[2]
Unrealized gains (losses) included in OCI	(2.8)		1.2
Total	78.9		79.9		80.1
Debt Securities
($ in millions)
Balance, beginning of period	1,659.2				1,330.3
Purchases	109.3		100.9
Sales	(12.5)		(12.7)
Transfers into Level 3	0		6.5
Transfers out of Level 3	(88.4)		(10.4)
Realized and unrealized gains (losses) included in income [1]	0.1	[2]	(0.5)	[2]
Unrealized gains (losses) included in OCI	4.0		3.3
Total	1,671.7		1,417.4		1,330.3
Fair value investments
($ in millions)
Balance, beginning of period	35.6				22.7
Purchases	0		9.8
Sales	(0.7)		(1.3)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Realized and unrealized gains (losses) included in income [1]	0.3	[2]	0.2	[2]
Unrealized gains (losses) included in OCI	0		0
Total	35.2		31.4		22.7
Total assets
($ in millions)
Balance, beginning of period	1,694.8				1,353.0
Purchases	109.3		110.7
Sales	(13.2)		(14.0)
Transfers into Level 3	0		6.5
Transfers out of Level 3	(88.4)		(10.4)
Realized and unrealized gains (losses) included in income [1]	0.4	[2]	(0.3)	[2]
Unrealized gains (losses) included in OCI	4.0		3.3
Total	$ 1,706.9		$ 1,448.8		$ 1,353.0

[1] Includes securities whose underlying collateral is an obligation of a U.S. government entity.

[2] Reflected in realized investment gains and losses for all assets except fair value investments which are included in net investment income.