Investing Activities Schedule of Available-for-sale Securities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 3,402.6					$ 2,798.1
Gross Unrealized Gains	105.6	[1]				219.0	[1]
Gross Unrealized Losses	(81.9)	[1]				(44.8)	[1]
Available-for-sale debt securities, at fair value	3,426.3		3,252.1	3,162.2	3,054.7	2,972.3
OTTI Recognized in AOCI	(13.7)	[2]				(14.6)	[3]
U.S. government and agency
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	66.1					35.8
Gross Unrealized Gains	3.3	[1]				4.8	[1]
Gross Unrealized Losses	(0.8)	[1]				(0.4)	[1]
Available-for-sale debt securities, at fair value	68.6					40.2
OTTI Recognized in AOCI	0	[2]				0	[3]
State and political subdivision
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	156.2					120.5
Gross Unrealized Gains	3.8	[1]				10.9
Gross Unrealized Losses	(5.4)	[1]				(0.5)	[1]
Available-for-sale debt securities, at fair value	154.6					130.9
OTTI Recognized in AOCI	(0.2)	[2]				(0.2)	[3]
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	63.1					44.4
Gross Unrealized Gains	2.7	[1]				7.4	[1]
Gross Unrealized Losses	(0.3)	[1]				0	[1]
Available-for-sale debt securities, at fair value	65.5					51.8
OTTI Recognized in AOCI	0	[2]				0	[3]
Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,194.9					1,776.9
Gross Unrealized Gains	69.5	[1]				144.6	[1]
Gross Unrealized Losses	(57.1)	[1]				(24.0)	[1]
Available-for-sale debt securities, at fair value	2,207.3					1,897.5
OTTI Recognized in AOCI	(1.5)	[2]				(1.5)	[3]
Commercial mortgage-backed (“CMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	241.9					229.6
Gross Unrealized Gains	13.0	[1]				26.0	[1]
Gross Unrealized Losses	(1.2)	[1]				(1.6)	[1]
Available-for-sale debt securities, at fair value	253.7					254.0
OTTI Recognized in AOCI	(0.4)	[2]				(0.6)	[3]
Residential mortgage-backed (“RMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	513.7					412.6
Gross Unrealized Gains	7.9	[1]				17.5	[1]
Gross Unrealized Losses	(11.8)	[1]				(7.6)	[1]
Available-for-sale debt securities, at fair value	509.8					422.5
OTTI Recognized in AOCI	(8.6)	[2]				(9.0)	[3]
CDO/CLO
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	70.6					59.2
Gross Unrealized Gains	1.7	[1]				1.8	[1]
Gross Unrealized Losses	(1.4)	[1]				(4.2)	[1]
Available-for-sale debt securities, at fair value	70.9					56.8
OTTI Recognized in AOCI	(3.0)	[2]				(3.3)	[3]
Other asset-backed
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	96.1					119.1
Gross Unrealized Gains	3.7	[1]				6.0	[1]
Gross Unrealized Losses	(3.9)					(6.5)	[1]
Available-for-sale debt securities, at fair value	95.9					118.6
OTTI Recognized in AOCI	$ 0	[2]				$ 0	[3]

[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.

[3] 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.