Investments (Details 4) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012 Security	Dec. 31, 2011 Security
Less than 12 months
Fair value	$ 478.3	$ 431.5
Unrealized loss	(52.0)	(76.8)
12 months or more
Fair Value	82.5	85.9
Unrealized loss	(49.0)	(41.8)
Total
Fair value	560.8	517.4
Unrealized loss	(101.0)	(118.6)
Number of securities
Less than 12 months (in securities)	97	72
12 months or more (in securities)	18	54
Total (in securities)	115	126
Number of securities with OTTI
Less than 12 months (in securities)	4	6
12 months or more (in securities)	5	4
Total (in securities)	9	10
Number of securities with unrealized losses greater than 10% of book value for 12 months or more	9
Threshold for measuring unrealized losses for securities as a percent of book value	10.00%
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more	47.7
U.S. government and agencies
Less than 12 months
Fair value	7.2	3.8
Unrealized loss	(0.2)	(0.1)
Total
Fair value	7.2	3.8
Unrealized loss	(0.2)	(0.1)
Obligations of state and political subdivisions
Less than 12 months
Fair value	132.7	17.0
Unrealized loss	(1.3)	0
12 months or more
Fair Value	3.7	20.6
Unrealized loss	(0.1)	(0.5)
Total
Fair value	136.4	37.6
Unrealized loss	(1.4)	(0.5)
Corporate securities
Less than 12 months
Fair value	27.7	79.9
Unrealized loss	(0.2)	(2.3)
12 months or more
Fair Value		3.1
Unrealized loss		(0.1)
Total
Fair value	27.7	83.0
Unrealized loss	(0.2)	(2.4)
RMBS
Less than 12 months
Fair value	170.7	186.6
Unrealized loss	(36.7)	(68.2)
12 months or more
Fair Value	67.8	36.5
Unrealized loss	(42.7)	(22.1)
Total
Fair value	238.5	223.1
Unrealized loss	(79.4)	(90.3)
CMBS
Less than 12 months
Fair value	2.8	2.8
Unrealized loss	0	0
Total
Fair value	2.8	2.8
Asset-backed securities
Less than 12 months
Fair value	19.3
Unrealized loss	(9.1)
12 months or more
Fair Value	11.0	25.7
Unrealized loss	(6.2)	(19.1)
Total
Fair value	30.3	25.7
Unrealized loss	(15.3)	(19.1)
Foreign government securities
Less than 12 months
Fair value	117.9	141.4
Unrealized loss	(4.5)	(6.2)
Total
Fair value	117.9	141.4
Unrealized loss	$ (4.5)	$ (6.2)