Investing Activities - Schedule of Temporariliy Impaired Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Less than 12 months | Fair Value
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	$ 0	$ 14.5
State and political subdivision	7.0	65.3
Foreign government	6.8	17.0
Corporate	356.5	806.5
CMBS	4.4	39.4
RMBS	72.3	243.5
CDO/CLO	47.4	30.5
Other ABS	6.1	7.9
Total temporarily impaired securities	500.5	1,224.6
Below investment grade	40.4	33.7
Less than 12 months | Unrealized Losses
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0	(0.8)
State and political subdivision	(0.4)	(4.4)
Foreign government	(0.2)	(0.3)
Corporate	(5.2)	(38.3)
CMBS	(0.1)	(1.1)
RMBS	(0.3)	(8.5)
CDO/CLO	(0.5)	(0.3)
Other ABS	0	(0.1)
Total temporarily impaired securities	(6.7)	(53.8)
Below investment grade	(1.1)	(1.9)
Number of securities	144	325
Greater than 12 months | Fair Value
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	8.7	0
State and political subdivision	35.0	4.0
Foreign government	0	0
Corporate	379.7	120.5
CMBS	5.3	3.0
RMBS	107.8	40.3
CDO/CLO	26.5	24.1
Other ABS	7.7	8.0
Total temporarily impaired securities	570.7	199.9
Below investment grade	19.7	12.1
Greater than 12 months | Unrealized Losses
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0	0
State and political subdivision	(2.1)	(1.0)
Foreign government	0	0
Corporate	(18.8)	(18.8)
CMBS	(0.1)	(0.1)
RMBS	(5.9)	(3.3)
CDO/CLO	(0.6)	(1.1)
Other ABS	(3.6)	(3.8)
Total temporarily impaired securities	(31.1)	(28.1)
Below investment grade	(2.1)	(1.7)
Number of securities	159	79
Total | Fair Value
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	8.7	14.5
State and political subdivision	42.0	69.3
Foreign government	6.8	17.0
Corporate	736.2	927.0
CMBS	9.7	42.4
RMBS	180.1	283.8
CDO/CLO	73.9	54.6
Other ABS	13.8	15.9
Total temporarily impaired securities	1,071.2	1,424.5
Below investment grade	60.1	45.8
Total | Unrealized Losses
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0	(0.8)
State and political subdivision	(2.5)	(5.4)
Foreign government	(0.2)	(0.3)
Corporate	(24.0)	(57.1)
CMBS	(0.2)	(1.2)
RMBS	(6.2)	(11.8)
CDO/CLO	(1.1)	(1.4)
Other ABS	(3.6)	(3.9)
Total temporarily impaired securities	(37.8)	(81.9)
Below investment grade	$ (3.2)	$ (3.6)
Number of securities	303	404