Investing Activities - Schedule of Temporarily Impaired Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value | Less than 12 months
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	$ 0	$ 0
State and political subdivision	4.8	4.8
Foreign government	1.0	0
Corporate	180.0	114.5
CMBS	4.5	0.7
RMBS	20.6	24.1
CDO/CLO	5.7	0
Other asset-backed	3.5	0.9
Total temporarily impaired securities	220.1	145.0
Below investment grade	6.0	7.8
Fair Value | Greater than 12 months
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	3.5	3.5
State and political subdivision	0	1.0
Foreign government	0	0
Corporate	65.1	64.7
CMBS	7.4	10.6
RMBS	47.1	52.6
CDO/CLO	32.9	39.0
Other asset-backed	12.4	10.7
Total temporarily impaired securities	168.4	182.1
Below investment grade	66.8	63.4
Fair Value | Total
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	3.5	3.5
State and political subdivision	4.8	5.8
Foreign government	1.0	0
Corporate	245.1	179.2
CMBS	11.9	11.3
RMBS	67.7	76.7
CDO/CLO	38.6	39.0
Other asset-backed	15.9	11.6
Total temporarily impaired securities	388.5	327.1
Below investment grade	72.8	71.2
Unrealized Losses | Less than 12 months
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0	0
State and political subdivision	0.3	0.3
Foreign government	0	0
Corporate	3.0	1.7
CMBS	0	0.1
RMBS	0.3	0.2
CDO/CLO	0.1	0
Other asset-backed	0.3	0
Total temporarily impaired securities	4.0	2.3
Below investment grade	0.2	0.5
Number of securities	72	48
Unrealized Losses | Greater than 12 months
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0.4	0.4
State and political subdivision	0	0.2
Foreign government	0	0
Corporate	20.0	22.3
CMBS	0.6	1.5
RMBS	5.2	7.4
CDO/CLO	2.5	4.2
Other asset-backed	5.6	6.5
Total temporarily impaired securities	34.3	42.5
Below investment grade	20.5	25.4
Number of securities	79	89
Unrealized Losses | Total
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0.4	0.4
State and political subdivision	0.3	0.5
Foreign government	0	0
Corporate	23.0	24.0
CMBS	0.6	1.6
RMBS	5.5	7.6
CDO/CLO	2.6	4.2
Other asset-backed	5.9	6.5
Total temporarily impaired securities	38.3	44.8
Below investment grade	$ 20.7	$ 25.9
Number of securities	151	137