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				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
U.S. government and agency
Level 3 Financial Assets:
Balance, beginning of period	$ 65.8	[1]	$ 31.4	[1]	$ 58.8	[1]	$ 25.9	[1]
Purchases	0	[1]	18.2	[1]	7.9	[1]	23.7	[1]
Sales	(0.9)	[1]	(1.3)	[1]	(2.9)	[1]	(1.4)	[1]
Transfers into Level 3	0	[1]	0	[1]	0	[1]	0	[1]
Transfers out of Level 3	0	[1]	0	[1]	0	[1]	0	[1]
Realized and unrealized gains (losses) included in income	0	[1],[2]	0	[1],[2]	0	[1],[2]	0	[1],[2]
Unrealized gains (losses) included in OCI	(0.6)	[1]	(1.2)	[1]	0.5	[1]	(1.1)	[1]
Total	64.3	[1]	47.1	[1]	64.3	[1]	47.1	[1]
State and political subdivision
Level 3 Financial Assets:
Balance, beginning of period	137.8		98.2		133.8		116.1
Purchases	4.6		13.8		5.9		13.8
Sales	(0.5)		(0.4)		(0.9)		(0.6)
Transfers into Level 3	0		0		0		0
Transfers out of Level 3	0		0		0		0
Realized and unrealized gains (losses) included in income	0	[2]	0	[2]	0	[2]	0	[2]
Unrealized gains (losses) included in OCI	2.7		(7.3)		5.8		(25.0)
Total	144.6		104.3		144.6		104.3
Foreign government
Level 3 Financial Assets:
Balance, beginning of period	3.4		9.5		3.3		8.0
Purchases	0		0		0		0
Sales	0		0		0		0
Transfers into Level 3	0		0		0		1.6
Transfers out of Level 3	0		(5.8)		0		(6.1)
Realized and unrealized gains (losses) included in income	0	[2]	0	[2]	0	[2]	0	[2]
Unrealized gains (losses) included in OCI	0.1		(0.4)		0.2		(0.2)
Total	3.5		3.3		3.5		3.3
Corporate
Level 3 Financial Assets:
Balance, beginning of period	1,122.3		903.2		1,102.3		801.5
Purchases	23.9		96.0		104.6		184.7
Sales	(3.7)		(3.4)		(7.2)		(5.1)
Transfers into Level 3	2.5		30.4		2.4		31.9
Transfers out of Level 3	(2.7)		(2.1)		(65.5)		(10.9)
Realized and unrealized gains (losses) included in income	0	[2]	0	[2]	0	[2]	0	[2]
Unrealized gains (losses) included in OCI	2.8		(68.2)		8.5		(46.2)
Total	1,145.1		955.9		1,145.1		955.9
Commercial mortgage-backed (“CMBS”)
Level 3 Financial Assets:
Balance, beginning of period	6.4		18.7		31.9		19.3
Purchases	0		15.6		0		15.6
Sales	(0.2)		0		(0.4)		0
Transfers into Level 3	0		0		0		0
Transfers out of Level 3	0		0		(25.5)		(1.5)
Realized and unrealized gains (losses) included in income	0	[2]	(0.1)	[2]	0	[2]	(0.1)	[2]
Unrealized gains (losses) included in OCI	(3.3)		(0.7)		(3.1)		0.2
Total	2.9		33.5		2.9		33.5
Residential mortgage-backed (“RMBS”)
Level 3 Financial Assets:
Balance, beginning of period	170.9		214.3		175.9		222.6
Purchases	0.3		0.3		0.6		0.6
Sales	(5.8)		(10.5)		(8.2)		(14.2)
Transfers into Level 3	0		0		0		0
Transfers out of Level 3	(4.5)		0		(4.3)		0
Realized and unrealized gains (losses) included in income	(0.1)	[2]	0.1	[2]	(0.1)	[2]	(0.1)	[2]
Unrealized gains (losses) included in OCI	0.2		(1.7)		(2.9)		(6.4)
Total	161.0		202.5		161.0		202.5
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Level 3 Financial Assets:
Balance, beginning of period	86.2		62.2		70.9		56.8
Purchases	22.0		11.8		34.9		22.7
Sales	(1.9)		(5.6)		(3.1)		(4.8)
Transfers into Level 3	0		0		0		0
Transfers out of Level 3	0		0		0		0
Realized and unrealized gains (losses) included in income	0	[2]	0	[2]	0.2	[2]	(0.3)	[2]
Unrealized gains (losses) included in OCI	(9.4)		(0.8)		(6.0)		(6.8)
Total	96.9		67.6		96.9		67.6
Available-for-sale debt securities
Level 3 Financial Assets:
Balance, beginning of period	78.9		79.9		82.3		80.1
Purchases	0		0.5		0		0.5
Sales	(1.7)		(2.4)		(2.0)		(3.3)
Transfers into Level 3	0		0		0		0
Transfers out of Level 3	0		0		0		0
Realized and unrealized gains (losses) included in income	0	[2]	0	[2]	0	[2]	0	[2]
Unrealized gains (losses) included in OCI	(2.5)		(4.1)		(5.6)		(3.4)
Total	74.7		73.9		74.7		73.9
Debt Securities
Level 3 Financial Assets:
Balance, beginning of period	1,671.7		1,417.4		1,659.2		1,330.3
Purchases	50.8		156.2		153.9		261.6
Sales	(14.7)		(23.6)		(24.7)		(29.4)
Transfers into Level 3	2.5		30.4		2.4		33.5
Transfers out of Level 3	(7.2)		(7.9)		(95.3)		(18.5)
Realized and unrealized gains (losses) included in income	(0.1)	[2]	0	[2]	0.1	[2]	(0.5)	[2]
Unrealized gains (losses) included in OCI	(10.0)		(84.4)		(2.6)		(88.9)
Total	1,693.0		1,488.1		1,693.0		1,488.1
Fair value investments
Level 3 Financial Assets:
Balance, beginning of period	35.2		31.4		35.6		22.7
Purchases	0		4.8		0		14.6
Sales	(0.8)		(1.0)		(1.4)		(1.5)
Transfers into Level 3	0		0		0		0
Transfers out of Level 3	0		0		0		0
Realized and unrealized gains (losses) included in income	0.2	[2]	(0.8)	[2]	0.4	[2]	(1.4)	[2]
Unrealized gains (losses) included in OCI	0		0		0		0
Total	34.6		34.4		34.6		34.4
Total assets
Level 3 Financial Assets:
Balance, beginning of period	1,706.9		1,448.8		1,694.8		1,353.0
Purchases	50.8		161.0		153.9		276.2
Sales	(15.5)		(24.6)		(26.1)		(30.9)
Transfers into Level 3	2.5		30.4		2.4		33.5
Transfers out of Level 3	(7.2)		(7.9)		(95.3)		(18.5)
Realized and unrealized gains (losses) included in income	0.1	[2]	(0.8)	[2]	0.5	[2]	(1.9)	[2]
Unrealized gains (losses) included in OCI	(10.0)		(84.4)		(2.6)		(88.9)
Total	$ 1,727.6		$ 1,522.5		$ 1,727.6		$ 1,522.5

[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.