Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013 security	Dec. 31, 2012 security
Less than 12 months
Debt Instrument [Line Items]
Number of securities	325	48
Less than 12 months | Fair Value
Debt Instrument [Line Items]
U.S. government and agency	$ 14.5	$ 0
State and political subdivision	65.3	4.8
Foreign government	17.0	0
Corporate	806.5	114.5
CMBS	39.4	0.7
RMBS	243.5	24.1
CDO/CLO	30.5	0
Other asset-backed	7.9	0.9
Total temporarily impaired securities	1,224.6	145.0
Below investment grade	33.7	7.8
Less than 12 months | Unrealized Losses
Debt Instrument [Line Items]
U.S. government and agency	(0.8)	0
State and political subdivision	(4.4)	(0.3)
Foreign government	(0.3)	0
Corporate	(38.3)	(1.7)
CMBS	(1.1)	(0.1)
RMBS	(8.5)	(0.2)
CDO/CLO	(0.3)	0
Other asset-backed	(0.1)	0
Total temporarily impaired securities	(53.8)	(2.3)
Below investment grade	(1.9)	(0.5)
Greater than 12 months
Debt Instrument [Line Items]
Number of securities	79	89
Greater than 12 months | Fair Value
Debt Instrument [Line Items]
U.S. government and agency	0	3.5
State and political subdivision	4.0	1.0
Foreign government	0	0
Corporate	120.5	64.7
CMBS	3.0	10.6
RMBS	40.3	52.6
CDO/CLO	24.1	39.0
Other asset-backed	8.0	10.7
Total temporarily impaired securities	199.9	182.1
Below investment grade	12.1	63.4
Greater than 12 months | Unrealized Losses
Debt Instrument [Line Items]
U.S. government and agency	0	(0.4)
State and political subdivision	(1.0)	(0.2)
Foreign government	0	0
Corporate	(18.8)	(22.3)
CMBS	(0.1)	(1.5)
RMBS	(3.3)	(7.4)
CDO/CLO	(1.1)	(4.2)
Other asset-backed	(3.8)	(6.5)
Total temporarily impaired securities	(28.1)	(42.5)
Below investment grade	(1.7)	(25.4)
Total
Debt Instrument [Line Items]
Number of securities	404	137
Total | Fair Value
Debt Instrument [Line Items]
U.S. government and agency	14.5	3.5
State and political subdivision	69.3	5.8
Foreign government	17.0	0
Corporate	927.0	179.2
CMBS	42.4	11.3
RMBS	283.8	76.7
CDO/CLO	54.6	39.0
Other asset-backed	15.9	11.6
Total temporarily impaired securities	1,424.5	327.1
Below investment grade	45.8	71.2
Total | Unrealized Losses
Debt Instrument [Line Items]
U.S. government and agency	(0.8)	(0.4)
State and political subdivision	(5.4)	(0.5)
Foreign government	(0.3)	0
Corporate	(57.1)	(24.0)
CMBS	(1.2)	(1.6)
RMBS	(11.8)	(7.6)
CDO/CLO	(1.4)	(4.2)
Other asset-backed	(3.9)	(6.5)
Total temporarily impaired securities	(81.9)	(44.8)
Below investment grade	$ (3.6)	$ (25.9)