Investing Activities - Schedule of Available-for-sale Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Fair Value and Cost of Securities:
Amortized Cost	$ 3,692.6		$ 3,402.6
Gross Unrealized Gain	181.0	[1]	105.6	[1]
Gross Unrealized Loss	(32.7)	[1]	(81.9)	[1]
Fair Value	3,840.9		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	68.7		66.1
Gross Unrealized Gain	4.8	[1]	3.3	[1]
Gross Unrealized Loss	0	[1]	(0.8)	[1]
Fair Value	73.5		68.6
OTTI Recognized in AOCI	0	[2]	0	[2]
State and political subdivision
Fair Value and Cost of Securities:
Amortized Cost	165.9		156.2
Gross Unrealized Gain	8.6	[1]	3.8	[1]
Gross Unrealized Loss	(2.4)	[1]	(5.4)	[1]
Fair Value	172.1		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 Gain	5.4	[1]	2.7	[1]
Gross Unrealized Loss	0	[1]	(0.3)	[1]
Fair Value	70.2		65.5
OTTI Recognized in AOCI	0	[2]	0	[2]
Corporate
Fair Value and Cost of Securities:
Amortized Cost	2,403.0		2,194.9
Gross Unrealized Gain	120.1	[1]	69.5	[1]
Gross Unrealized Loss	(19.9)	[1]	(57.1)	[1]
Fair Value	2,503.2		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	239.1		241.9
Gross Unrealized Gain	20.9	[1]	13.0	[1]
Gross Unrealized Loss	(0.1)	[1]	(1.2)	[1]
Fair Value	259.9		253.7
OTTI Recognized in AOCI	(0.4)	[2]	(0.4)	[2]
Residential mortgage-backed (“RMBS”)
Fair Value and Cost of Securities:
Amortized Cost	569.9		513.7
Gross Unrealized Gain	15.5	[1]	7.9	[1]
Gross Unrealized Loss	(5.9)	[1]	(11.8)	[1]
Fair Value	579.5		509.8
OTTI Recognized in AOCI	(8.6)	[2]	(8.6)	[2]
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Fair Value and Cost of Securities:
Amortized Cost	96.3		70.6
Gross Unrealized Gain	1.4	[1]	1.7	[1]
Gross Unrealized Loss	(0.8)	[1]	(1.4)	[1]
Fair Value	96.9		70.9
OTTI Recognized in AOCI	(2.7)	[2]	(3.0)	[2]
Available-for-sale debt securities
Fair Value and Cost of Securities:
Amortized Cost	84.9		96.1
Gross Unrealized Gain	4.3	[1]	3.7	[1]
Gross Unrealized Loss	(3.6)	[1]	(3.9)	[1]
Fair Value	85.6		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).