Investing Activities - Schedule of Available-for-sale Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Fair Value and Cost of Securities:
Amortized Cost	$ 3,503.2		$ 3,402.6
Gross Unrealized Gains	138.5	[1]	105.6	[1]
Gross Unrealized Losses	(52.6)	[1]	(81.9)	[1]
Fair Value	3,589.1		3,426.3
OTTI Recognized in AOCI	(13.4)	[2]	(13.7)	[2]
U.S. government and agency
Fair Value and Cost of Securities:
Amortized Cost	71.3		66.1
Gross Unrealized Gains	4.1	[1]	3.3	[1]
Gross Unrealized Losses	(0.3)	[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:
Amortized Cost	160.0		156.2
Gross Unrealized Gains	5.9	[1]	3.8	[1]
Gross Unrealized Losses	(3.4)	[1]	(5.4)	[1]
Fair Value	162.5		154.6
OTTI Recognized in AOCI	(0.2)	[2]	(0.2)	[2]
Foreign government
Fair Value and Cost of Securities:
Amortized Cost	64.8		63.1
Gross Unrealized Gains	3.6	[1]	2.7	[1]
Gross Unrealized Losses	(0.1)	[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:
Amortized Cost	2,291.7		2,194.9
Gross Unrealized Gains	92.0	[1]	69.5	[1]
Gross Unrealized Losses	(34.8)	[1]	(57.1)	[1]
Fair Value	2,348.9		2,207.3
OTTI Recognized in AOCI	(1.5)	[2]	(1.5)	[2]
Commercial mortgage-backed (“CMBS”)
Fair Value and Cost of Securities:
Amortized Cost	241.8		241.9
Gross Unrealized Gains	16.3	[1]	13.0	[1]
Gross Unrealized Losses	(0.5)	[1]	(1.2)	[1]
Fair Value	257.6		253.7
OTTI Recognized in AOCI	(0.4)	[2]	(0.4)	[2]
Residential mortgage-backed (“RMBS”)
Fair Value and Cost of Securities:
Amortized Cost	500.5		513.7
Gross Unrealized Gains	10.8	[1]	7.9	[1]
Gross Unrealized Losses	(8.7)	[1]	(11.8)	[1]
Fair Value	502.6		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:
Amortized Cost	85.5		70.6
Gross Unrealized Gains	1.9	[1]	1.7	[1]
Gross Unrealized Losses	(1.2)	[1]	(1.4)	[1]
Fair Value	86.2		70.9
OTTI Recognized in AOCI	(2.7)	[2]	(3.0)	[2]
Other asset-backed
Fair Value and Cost of Securities:
Amortized Cost	87.6		96.1
Gross Unrealized Gains	3.9	[1]	3.7	[1]
Gross Unrealized Losses	(3.6)	[1]	(3.9)	[1]
Fair Value	87.9		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).