Investing Activities - Schedule of Temporariliy Impaired Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value | Less than 12 months
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	$ 8.7	$ 14.5
State and political subdivision	14.1	65.3
Foreign government	2.2	17.0
Corporate	66.4	806.5
CMBS	0	39.4
RMBS	6.9	243.5
CDO/CLO	13.0	30.5
Other ABS	1.9	7.9
Total temporarily impaired securities	113.2	1,224.6
Below investment grade	10.0	33.7
Fair Value | Greater than 12 months
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0	0
State and political subdivision	23.7	4.0
Foreign government	0	0
Corporate	416.2	120.5
CMBS	5.4	3.0
RMBS	111.9	40.3
CDO/CLO	28.7	24.1
Other ABS	7.7	8.0
Total temporarily impaired securities	593.6	199.9
Below investment grade	18.9	12.1
Fair Value | Total
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	8.7	14.5
State and political subdivision	37.8	69.3
Foreign government	2.2	17.0
Corporate	482.6	927.0
CMBS	5.4	42.4
RMBS	118.8	283.8
CDO/CLO	41.7	54.6
Other ABS	9.6	15.9
Total temporarily impaired securities	706.8	1,424.5
Below investment grade	28.9	45.8
Unrealized Losses | Less than 12 months
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0	0.8
State and political subdivision	0.1	4.4
Foreign government	0	0.3
Corporate	1.6	38.3
CMBS	0	1.1
RMBS	0.1	8.5
CDO/CLO	0	0.3
Other ABS	0	0.1
Total temporarily impaired securities	1.8	53.8
Below investment grade	0.2	1.9
Number of securities	38	325
Unrealized Losses | Greater than 12 months
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0	0
State and political subdivision	2.3	1.0
Foreign government	0	0
Corporate	18.3	18.8
CMBS	0.1	0.1
RMBS	5.8	3.3
CDO/CLO	0.8	1.1
Other ABS	3.6	3.8
Total temporarily impaired securities	30.9	28.1
Below investment grade	1.5	1.7
Number of securities	163	79
Unrealized Losses | Total
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0	0.8
State and political subdivision	2.4	5.4
Foreign government	0	0.3
Corporate	19.9	57.1
CMBS	0.1	1.2
RMBS	5.9	11.8
CDO/CLO	0.8	1.4
Other ABS	3.6	3.9
Total temporarily impaired securities	32.7	81.9
Below investment grade	$ 1.7	$ 3.6
Number of securities	201	404