Investing Activities Schedule of Available-for-sale Securities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,060.9				$ 3,390.6
Gross Unrealized Gains	198.8				105.9
Gross Unrealized Losses	(37.9)				(80.4)
Available-for-sale debt securities, at fair value	4,221.8	4,049.3	3,829.6	3,578.0	3,416.1	3,245.8	3,154.8	3,049.9
OTTI Recognized in AOCI	(13.0)				(13.7)
Amortized Cost	28.4				12.1
Gross Unrealized Gains	0.6				0
Gross Unrealized Losses	(0.3)				(0.9)
Fair Value	28.7	27.7	11.9	11.6	11.2	7.3	7.8	5.6
OTTI Recognizaed in AOCI	0				0
U.S. government and agency
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	82.4				66.1
Gross Unrealized Gains	6.9				3.3
Gross Unrealized Losses	0				(0.8)
Available-for-sale debt securities, at fair value	89.3				68.6
OTTI Recognized in AOCI	0				0
State and political subdivision
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	217.2				156.2
Gross Unrealized Gains	13.6				3.8
Gross Unrealized Losses	(1.6)				(5.4)
Available-for-sale debt securities, at fair value	229.2				154.6
OTTI Recognized in AOCI	(0.2)				(0.2)
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	69.2				63.1
Gross Unrealized Gains	5.4				2.7
Gross Unrealized Losses	(0.8)				(0.3)
Available-for-sale debt securities, at fair value	73.8				65.5
OTTI Recognized in AOCI	0				0
Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,730.7				2,182.9
Gross Unrealized Gains	128.4				69.6
Gross Unrealized Losses	(27.2)				(55.6)
Available-for-sale debt securities, at fair value	2,831.9				2,196.9
OTTI Recognized in AOCI	(1.5)				(1.5)
Commercial mortgage-backed (“CMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	236.2				241.9
Gross Unrealized Gains	19.4				13.0
Gross Unrealized Losses	0				(1.2)
Available-for-sale debt securities, at fair value	255.6				253.7
OTTI Recognized in AOCI	0				(0.4)
Residential mortgage-backed (“RMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	558.9				513.7
Gross Unrealized Gains	20.9				8.1
Gross Unrealized Losses	(3.4)				(11.8)
Available-for-sale debt securities, at fair value	576.4				510.0
OTTI Recognized in AOCI	(8.6)				(8.6)
Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84.2				70.6
Gross Unrealized Gains	0.4				1.7
Gross Unrealized Losses	(1.1)				(1.4)
Available-for-sale debt securities, at fair value	83.5				70.9
OTTI Recognized in AOCI	(2.7)				(3.0)
Other asset-backed (“ABS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	82.1				96.1
Gross Unrealized Gains	3.8				3.7
Gross Unrealized Losses	(3.8)				(3.9)
Available-for-sale debt securities, at fair value	82.1				95.9
OTTI Recognized in AOCI	$ 0				$ 0