Investing Activities - Schedule of Available-for-sale Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Fair Value and Cost of Securities:
Total	$ 3,207.2		$ 2,798.1
Gross Unrealized Gains	120.3	[1]	219.0	[1]
Gross Unrealized Losses	(75.4)	[1]	(44.8)	[1]
Fair Value	3,252.1		2,972.3
OTTI Recognized in AOCI	(13.7)	[2]	(14.6)	[2]
U.S. government and agency
Fair Value and Cost of Securities:
Total	69.6		35.8
Gross Unrealized Gains	3.7	[1]	4.8	[1]
Gross Unrealized Losses	(0.8)	[1]	(0.4)	[1]
Fair Value	72.5		40.2
OTTI Recognized in AOCI	0	[2]	0	[2]
State and political subdivision
Fair Value and Cost of Securities:
Total	143.3		120.5
Gross Unrealized Gains	5.8	[1]	10.9	[1]
Gross Unrealized Losses	(4.8)	[1]	(0.5)	[1]
Fair Value	144.3		130.9
OTTI Recognized in AOCI	(0.2)	[2]	(0.2)	[2]
Foreign government
Fair Value and Cost of Securities:
Total	56.4		44.4
Gross Unrealized Gains	3.0	[1]	7.4	[1]
Gross Unrealized Losses	(0.2)	[1]	0	[1]
Fair Value	59.2		51.8
OTTI Recognized in AOCI	0	[2]	0	[2]
Corporate
Fair Value and Cost of Securities:
Total	2,096.6		1,776.9
Gross Unrealized Gains	78.4	[1]	144.6	[1]
Gross Unrealized Losses	(51.9)	[1]	(24.0)	[1]
Fair Value	2,123.1		1,897.5
OTTI Recognized in AOCI	(1.5)	[2]	(1.5)	[2]
Commercial mortgage-backed (“CMBS”)
Fair Value and Cost of Securities:
Total	241.9		229.6
Gross Unrealized Gains	13.9	[1]	26.0	[1]
Gross Unrealized Losses	(1.5)	[1]	(1.6)	[1]
Fair Value	254.3		254.0
OTTI Recognized in AOCI	(0.5)	[2]	(0.6)	[2]
Residential mortgage-backed (“RMBS”)
Fair Value and Cost of Securities:
Total	431.0		412.6
Gross Unrealized Gains	10.1	[1]	17.5	[1]
Gross Unrealized Losses	(8.5)	[1]	(7.6)	[1]
Fair Value	432.6		422.5
OTTI Recognized in AOCI	(8.5)	[2]	(9.0)	[2]
CDO/CLO
Fair Value and Cost of Securities:
Total	66.4		59.2
Gross Unrealized Gains	1.7	[1]	1.8	[1]
Gross Unrealized Losses	(1.4)	[1]	(4.2)	[1]
Fair Value	66.7		56.8
OTTI Recognized in AOCI	(3.0)	[2]	(3.3)	[2]
Other asset-backed
Fair Value and Cost of Securities:
Total	102.0		119.1
Gross Unrealized Gains	3.7	[1]	6.0	[1]
Gross Unrealized Losses	(6.3)	[1]	(6.5)	[1]
Fair Value	99.4		118.6
OTTI Recognized in AOCI	$ 0	[2]	$ 0	[2]

[1] Net unrealized investment gains and losses on securities classified as available-for-sale and certain other assets are included in our balance sheets as a component of AOCI.

[2] Represents the amount of non-credit OTTI losses recognized in AOCI excluding net unrealized gains or losses subsequent to the date of impairment. The table above presents the special category of AOCI for debt securities that are other-than-temporarily impaired when the impairment loss has been split between the credit loss component (in earnings) and the non-credit component.