Investing Activities - Schedule of Temporariliy Impaired Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Less than 12 months | Fair Value
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	$ 13.6	$ 0
State and political subdivision	48.9	4.8
Foreign government	17.3	0
Corporate	773.8	114.5
CMBS	31.2	0.7
RMBS	121.1	24.1
CDO/CLO	16.1	0
Other asset-backed	12.5	0.9
Total temporarily impaired securities	1,034.5	145.0
Below investment grade	40.8	7.8
Less than 12 months | Unrealized Losses
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	(0.4)	0
State and political subdivision	(3.9)	(0.3)
Foreign government	(0.2)	0
Corporate	(33.6)	(1.7)
CMBS	(1.1)	(0.1)
RMBS	(5.1)	(0.2)
CDO/CLO	(0.2)	0
Other asset-backed	(0.6)	0
Total temporarily impaired securities	(45.1)	(2.3)
Below investment grade	(2.2)	(0.5)
Number of securities	284	48
Greater than 12 months | Fair Value
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	3.5	3.5
State and political subdivision	4.2	1.0
Foreign government	0	0
Corporate	71.7	64.7
CMBS	5.4	10.6
RMBS	39.9	52.6
CDO/CLO	24.7	39.0
Other asset-backed	6.2	10.7
Total temporarily impaired securities	155.6	182.1
Below investment grade	27.5	63.4
Greater than 12 months | Unrealized Losses
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	(0.4)	(0.4)
State and political subdivision	(0.9)	(0.2)
Foreign government	0	0
Corporate	(18.3)	(22.3)
CMBS	(0.4)	(1.5)
RMBS	(3.4)	(7.4)
CDO/CLO	(1.2)	(4.2)
Other asset-backed	(5.7)	(6.5)
Total temporarily impaired securities	(30.3)	(42.5)
Below investment grade	(9.1)	(25.4)
Number of securities	69	89
Total | Fair Value
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	17.1	3.5
State and political subdivision	53.1	5.8
Foreign government	17.3	0
Corporate	845.5	179.2
CMBS	36.6	11.3
RMBS	161.0	76.7
CDO/CLO	40.8	39.0
Other asset-backed	18.7	11.6
Total temporarily impaired securities	1,190.1	327.1
Below investment grade	68.3	71.2
Total | Unrealized Losses
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	(0.8)	(0.4)
State and political subdivision	(4.8)	(0.5)
Foreign government	(0.2)	0
Corporate	(51.9)	(24.0)
CMBS	(1.5)	(1.6)
RMBS	(8.5)	(7.6)
CDO/CLO	(1.4)	(4.2)
Other asset-backed	(6.3)	(6.5)
Total temporarily impaired securities	(75.4)	(44.8)
Below investment grade	$ (11.3)	$ (25.9)
Number of securities	353	137