Investments (Details 4) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011 Security	Dec. 31, 2010 Security
Less than 12 months
Fair value	$ 431.5	$ 2,828.2
Unrealized loss	(76.8)	(116.9)
12 months or more
Fair Value	85.9	63.1
Unrealized loss	(41.8)	(15.4)
Total
Fair value	517.4	2,891.3
Unrealized loss	(118.6)	(132.3)
Number of securities
Less than 12 months (in securities)	72	405
12 months or more (in securities)	54	18
Total (in securities)	126	423
Number of securities with OTTI
Less than 12 months (in securities)	6	10
12 months or more (in securities)	4	3
Total (in securities)	10	13
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	40.8
U.S. government and agencies
Less than 12 months
Fair value	3.8	20.5
Unrealized loss	(0.1)	(0.4)
Total
Fair value	3.8	20.5
Unrealized loss	(0.1)	(0.4)
Obligations of state and political subdivisions
Less than 12 months
Fair value	17.0	1,694.5
Unrealized loss	0	(58.9)
12 months or more
Fair Value	20.6	23.5
Unrealized loss	(0.5)	(3.1)
Total
Fair value	37.6	1,718.0
Unrealized loss	(0.5)	(62.0)
Corporate securities
Less than 12 months
Fair value	79.9	403.6
Unrealized loss	(2.3)	(12.8)
12 months or more
Fair Value	3.1
Unrealized loss	(0.1)
Total
Fair value	83.0	403.6
Unrealized loss	(2.4)	(12.8)
RMBS
Less than 12 months
Fair value	186.6	143.4
Unrealized loss	(68.2)	(32.1)
12 months or more
Fair Value	36.5	37.3
Unrealized loss	(22.1)	(12.2)
Total
Fair value	223.1	180.7
Unrealized loss	(90.3)	(44.3)
CMBS
Less than 12 months
Fair value	2.8	92.6
Unrealized loss	0	(1.4)
Total
Fair value	2.8	92.6
Unrealized loss	0	(1.4)
Asset-backed securities
Less than 12 months
Fair value		228.3
Unrealized loss		(5.1)
12 months or more
Fair Value	25.7	2.3
Unrealized loss	(19.1)	(0.1)
Total
Fair value	25.7	230.6
Unrealized loss	(19.1)	(5.2)
Foreign government securities
Less than 12 months
Fair value	141.4	245.3
Unrealized loss	(6.2)	(6.2)
Total
Fair value	141.4	245.3
Unrealized loss	$ (6.2)	$ (6.2)