7. Investing Activities (Details 3) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
U.S. government and agency	$ 285.1	$ 174.1
State and political subdivision	130.9	83.5
Foreign government	51.8	31.9
Corporate	1,897.5	1,204.7
Residential mortgage-backed ("RMBS")	422.5	531.5
CDO/CLO	56.8	62.5
Other asset-backed	118.6	142.4
Fair Value [Member] | Less than 12 months [Member]
U.S. government and agency
State and political subdivision	4.8	15.4
Foreign government		7.2
Corporate	114.5	113.4
Commercial mortgage-backed ("CMBS")	0.7	50.3
Residential mortgage-backed ("RMBS")	24.1	78.0
CDO/CLO		4.8
Other asset-backed	0.9	24.4
Total temporarily impaired securities	145.0	293.5
Below investment grade	7.8	25.3
Fair Value [Member] | Greater than 12 months [Member]
U.S. government and agency	3.5	3.5
State and political subdivision	1.0	0.9
Foreign government
Corporate	64.7	69.3
Commercial mortgage-backed ("CMBS")	10.6	6.9
Residential mortgage-backed ("RMBS")	52.6	91.1
CDO/CLO	39.0	40.6
Other asset-backed	10.7	16.5
Total temporarily impaired securities	182.1	228.8
Below investment grade	63.4	74.8
Fair Value [Member] | Total [Member]
U.S. government and agency	3.5	3.5
State and political subdivision	5.8	16.3
Foreign government		7.2
Corporate	179.2	182.7
Commercial mortgage-backed ("CMBS")	11.3	57.2
Residential mortgage-backed ("RMBS")	76.7	169.1
CDO/CLO	39.0	45.4
Other asset-backed	11.6	40.9
Total temporarily impaired securities	327.1	522.3
Below investment grade	71.2	100.1
Unrealized Losses [Member] | Less than 12 months [Member]
U.S. government and agency
State and political subdivision	(0.3)
Foreign government		(0.4)
Corporate	(1.7)	(3.5)
Commercial mortgage-backed ("CMBS")	(0.1)	(1.2)
Residential mortgage-backed ("RMBS")	(0.2)	(3.3)
CDO/CLO		(0.4)
Other asset-backed		(0.2)
Total temporarily impaired securities	(2.3)	(9.0)
Below investment grade	(0.5)	(1.6)
Number of securities	48	129
Unrealized Losses [Member] | Greater than 12 months [Member]
U.S. government and agency	(0.4)	(0.5)
State and political subdivision	(0.2)	(0.5)
Foreign government
Corporate	(22.3)	(34.0)
Commercial mortgage-backed ("CMBS")	(1.5)	(2.7)
Residential mortgage-backed ("RMBS")	(7.4)	(17.1)
CDO/CLO	(4.2)	(11.8)
Other asset-backed	(6.5)	(7.1)
Total temporarily impaired securities	(42.5)	(73.7)
Below investment grade	(25.4)	(50.2)
Number of securities	89	139
Unrealized Losses [Member] | Total [Member]
U.S. government and agency	(0.4)	(0.5)
State and political subdivision	(0.5)	(0.5)
Foreign government		(0.4)
Corporate	(24.0)	(37.5)
Commercial mortgage-backed ("CMBS")	(1.6)	(3.9)
Residential mortgage-backed ("RMBS")	(7.6)	(20.4)
CDO/CLO	(4.2)	(12.2)
Other asset-backed	(6.5)	(7.3)
Total temporarily impaired securities	(44.8)	(82.7)
Below investment grade	$ (25.9)	$ (51.8)
Number of securities	137	268