Investing Activities - Schedule of Temporariliy Impaired Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value [Member] | Less than 12 months [Member]
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	$ 4.9	$ 0
State and political subdivision	39.7	4.8
Foreign government	3.3	0
Corporate	783.5	114.5
CMBS	32.8	0.7
RMBS	155.2	24.1
CDO/CLO	15.9	0
Other asset-backed	15.6	0.9
Total temporarily impaired securities	1,050.9	145.0
Below investment grade	32.5	7.8
Fair Value [Member] | Greater than 12 months [Member]
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	67.6	64.7
CMBS	7.0	10.6
RMBS	40.7	52.6
CDO/CLO	30.1	39.0
Other asset-backed	7.1	10.7
Total temporarily impaired securities	156.0	182.1
Below investment grade	49.2	63.4
Fair Value [Member] | Total [Member]
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	8.4	3.5
State and political subdivision	39.7	5.8
Foreign government	3.3	0
Corporate	851.1	179.2
CMBS	39.8	11.3
RMBS	195.9	76.7
CDO/CLO	46.0	39.0
Other asset-backed	22.7	11.6
Total temporarily impaired securities	1,206.9	327.1
Below investment grade	81.7	71.2
Unrealized Losses [Member] | Less than 12 months [Member]
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0.1	0
State and political subdivision	3.3	0.3
Foreign government	0.2	0
Corporate	33.7	1.7
CMBS	1.2	0.1
RMBS	5.3	0.2
CDO/CLO	0.5	0
Other asset-backed	0.5	0
Total temporarily impaired securities	44.8	2.3
Below investment grade	0.9	0.5
Number of securities	290	48
Unrealized Losses [Member] | Greater than 12 months [Member]
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0.5	0.4
State and political subdivision	0	0.2
Foreign government	0	0
Corporate	17.5	22.3
CMBS	0.3	1.5
RMBS	3.4	7.4
CDO/CLO	1.7	4.2
Other asset-backed	5.0	6.5
Total temporarily impaired securities	28.4	42.5
Below investment grade	17.0	25.4
Number of securities	71	89
Unrealized Losses [Member] | Total [Member]
Schedule of Available-for-sale Securities [Line Items]
U.S. government and agency	0.6	0.4
State and political subdivision	3.3	0.5
Foreign government	0.2	0
Corporate	51.2	24.0
CMBS	1.5	1.6
RMBS	8.7	7.6
CDO/CLO	2.2	4.2
Other asset-backed	5.5	6.5
Total temporarily impaired securities	73.2	44.8
Below investment grade	$ 17.9	$ 25.9
Number of securities	361	137