Investing Activities - Schedule of Available-for-sale Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Fair Value and Cost of Securities:
Total	$ 3,950.0		$ 3,402.6
Gross Unrealized Gains	164.0	[1]	105.6	[1]
Gross Unrealized Losses	(37.8)	[1]	(81.9)	[1]
Fair Value	4,076.2		3,426.3
OTTI Recognized in AOCI	(13.4)	[2]	(13.7)	[2]
U.S. government and agency
Fair Value and Cost of Securities:
Total	70.3		66.1
Gross Unrealized Gains	4.8	[1]	3.3	[1]
Gross Unrealized Losses	0	[1]	(0.8)	[1]
Fair Value	75.1		68.6
OTTI Recognized in AOCI	0	[2]	0	[2]
State and political subdivision
Fair Value and Cost of Securities:
Total	191.3		156.2
Gross Unrealized Gains	9.6	[1]	3.8	[1]
Gross Unrealized Losses	(2.5)	[1]	(5.4)	[1]
Fair Value	198.4		154.6
OTTI Recognized in AOCI	(0.2)	[2]	(0.2)	[2]
Foreign government
Fair Value and Cost of Securities:
Total	64.1		63.1
Gross Unrealized Gains	4.4	[1]	2.7	[1]
Gross Unrealized Losses	(0.2)	[1]	(0.3)	[1]
Fair Value	68.3		65.5
OTTI Recognized in AOCI	0	[2]	0	[2]
Corporate
Fair Value and Cost of Securities:
Total	2,613.9		2,194.9
Gross Unrealized Gains	108.3	[1]	69.5	[1]
Gross Unrealized Losses	(24.0)	[1]	(57.1)	[1]
Fair Value	2,698.2		2,207.3
OTTI Recognized in AOCI	(1.5)	[2]	(1.5)	[2]
Commercial mortgage-backed (“CMBS”)
Fair Value and Cost of Securities:
Total	237.3		241.9
Gross Unrealized Gains	17.8	[1]	13.0	[1]
Gross Unrealized Losses	(0.2)	[1]	(1.2)	[1]
Fair Value	254.9		253.7
OTTI Recognized in AOCI	(0.4)	[2]	(0.4)	[2]
Residential mortgage-backed (“RMBS”)
Fair Value and Cost of Securities:
Total	595.9		513.7
Gross Unrealized Gains	14.7	[1]	7.9	[1]
Gross Unrealized Losses	(6.2)	[1]	(11.8)	[1]
Fair Value	604.4		509.8
OTTI Recognized in AOCI	(8.6)	[2]	(8.6)	[2]
Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)
Fair Value and Cost of Securities:
Total	93.3		70.6
Gross Unrealized Gains	0.7	[1]	1.7	[1]
Gross Unrealized Losses	(1.1)	[1]	(1.4)	[1]
Fair Value	92.9		70.9
OTTI Recognized in AOCI	(2.7)	[2]	(3.0)	[2]
Other asset-backed (“ABS”)
Fair Value and Cost of Securities:
Total	83.9		96.1
Gross Unrealized Gains	3.7	[1]	3.7	[1]
Gross Unrealized Losses	(3.6)	[1]	(3.9)	[1]
Fair Value	84.0		95.9
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 (separate category of AOCI).