Investments (Details 4) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012 Security	Dec. 31, 2011 Security
Less than 12 months
Fair value	$ 481.8	$ 431.5
Unrealized loss	(62.8)	(76.8)
12 months or more
Fair Value	93.3	85.9
Unrealized loss	(64.2)	(41.8)
Total
Fair value	575.1	517.4
Unrealized loss	(127.0)	(118.6)
Number of securities
Less than 12 months (in securities)	77	72
12 months or more (in securities)	18	54
Total (in securities)	95	126
Number of securities with OTTI
Less than 12 months (in securities)	4	6
12 months or more (in securities)	6	4
Total (in securities)	10	10
Number of securities with unrealized losses greater than 10% of book value for 12 months or more	10
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	63.3
U.S. government and agencies
Less than 12 months
Fair value	71.4	3.8
Unrealized loss	0	(0.1)
12 months or more
Fair Value	0	0
Unrealized loss	0	0
Total
Fair value	71.4	3.8
Unrealized loss	0	(0.1)
Obligations of state and political subdivisions
Less than 12 months
Fair value	99.9	17.0
Unrealized loss	(0.6)	0
12 months or more
Fair Value	3.5	20.6
Unrealized loss	0	(0.5)
Total
Fair value	103.4	37.6
Unrealized loss	(0.6)	(0.5)
Corporate securities
Less than 12 months
Fair value	40.7	79.9
Unrealized loss	(0.1)	(2.3)
12 months or more
Fair Value	0	3.1
Unrealized loss	0	(0.1)
Total
Fair value	40.7	83.0
Unrealized loss	(0.1)	(2.4)
RMBS
Less than 12 months
Fair value	168.0	186.6
Unrealized loss	(51.7)	(68.2)
12 months or more
Fair Value	67.3	36.5
Unrealized loss	(42.7)	(22.1)
Total
Fair value	235.3	223.1
Unrealized loss	(94.4)	(90.3)
CMBS
Less than 12 months
Fair value	1.4	2.8
Unrealized loss	0	0
12 months or more
Fair Value	0	0
Unrealized loss	0	0
Total
Fair value	1.4	2.8
Unrealized loss	0	0
Asset-backed securities
Less than 12 months
Fair value	33.9	0
Unrealized loss	(8.2)	0
12 months or more
Fair Value	22.5	25.7
Unrealized loss	(21.5)	(19.1)
Total
Fair value	56.4	25.7
Unrealized loss	(29.7)	(19.1)
Foreign government securities
Less than 12 months
Fair value	66.5	141.4
Unrealized loss	(2.2)	(6.2)
12 months or more
Fair Value	0	0
Unrealized loss	0	0
Total
Fair value	66.5	141.4
Unrealized loss	$ (2.2)	$ (6.2)