Investing Activities - Schedule of Temporarily Impaired Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Less than 12 months | Fair Value
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	$ 13.4	$ 14.5
State and political subdivision	47.3	65.3
Foreign government	12.6	17.0
Corporate	516.8	806.5
CMBS	9.5	39.4
RMBS	135.0	243.5
CDO/CLO	32.1	30.5
Other asset-backed	0	7.9
Total temporarily impaired securities	766.7	1,224.6
Below investment grade	15.4	33.7
Less than 12 months | Unrealized Losses
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	(0.3)	(0.8)
State and political subdivision	(2.5)	(4.4)
Foreign government	(0.1)	(0.3)
Corporate	(15.3)	(38.3)
CMBS	(0.5)	(1.1)
RMBS	(5.4)	(8.5)
CDO/CLO	(0.3)	(0.3)
Other asset-backed	0	(0.1)
Total temporarily impaired securities	(24.4)	(53.8)
Below investment grade	(0.6)	(1.9)
Number of securities	206	325
Greater than 12 months | Fair Value
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0	0
State and political subdivision	4.1	4.0
Foreign government	0	0
Corporate	192.9	120.5
CMBS	0.7	3.0
RMBS	37.6	40.3
CDO/CLO	25.4	24.1
Other asset-backed	8.2	8.0
Total temporarily impaired securities	268.9	199.9
Below investment grade	10.4	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	(0.9)	(1.0)
Foreign government	0	0
Corporate	(19.5)	(18.8)
CMBS	0	(0.1)
RMBS	(3.3)	(3.3)
CDO/CLO	(0.9)	(1.1)
Other asset-backed	(3.6)	(3.8)
Total temporarily impaired securities	(28.2)	(28.1)
Below investment grade	(1.4)	(1.7)
Number of securities	94	79
Total | Fair Value
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	13.4	14.5
State and political subdivision	51.4	69.3
Foreign government	12.6	17.0
Corporate	709.7	927.0
CMBS	10.2	42.4
RMBS	172.6	283.8
CDO/CLO	57.5	54.6
Other asset-backed	8.2	15.9
Total temporarily impaired securities	1,035.6	1,424.5
Below investment grade	25.8	45.8
Total | Unrealized Losses
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	(0.3)	(0.8)
State and political subdivision	(3.4)	(5.4)
Foreign government	(0.1)	(0.3)
Corporate	(34.8)	(57.1)
CMBS	(0.5)	(1.2)
RMBS	(8.7)	(11.8)
CDO/CLO	(1.2)	(1.4)
Other asset-backed	(3.6)	(3.9)
Total temporarily impaired securities	(52.6)	(81.9)
Below investment grade	$ (2.0)	$ (3.6)
Number of securities	300	404