Investments (Details 4) (USD $) In Millions, unless otherwise specified	Jun. 30, 2011 Securities Policies D Companies Agreements Markets Subsidiaries Transactions FiscalQuarters Counterparties Entities	Dec. 31, 2010 Securities Policies
Less than 12 months
Fair value	$ 1,676.1	$ 2,833.2
Unrealized loss	(59.9)	(117.6)
12 months or more
Fair Value	21.8	71.1
Unrealized loss	(24.1)	(16.3)
Total
Fair value	1,697.9	2,904.3
Unrealized loss	(84.0)	(133.9)
Number of securities
Less than 12 months	264	406
12 months or more	13	19
Total	277	425
Number of securities with OTTI
Less than 12 months	5	10
12 months or more	3	3
Total	8	13
Number of securities with unrealized losses greater than 10% of book value for 12 months or more	5
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	23.3
U.S. government and agencies
Less than 12 months
Fair value	54.4	20.5
Unrealized loss	(0.2)	(0.4)
Total
Fair value	54.4	20.5
Unrealized loss	(0.2)	(0.4)
Obligations of state and political subdivisions
Less than 12 months
Fair value	851.4	1,694.5
Unrealized loss	(16.0)	(58.9)
12 months or more
Fair Value	9.5	23.5
Unrealized loss	(0.8)	(3.1)
Total
Fair value	860.9	1,718.0
Unrealized loss	(16.8)	(62.0)
Corporate securities
Less than 12 months
Fair value	355.8	403.6
Unrealized loss	(8.9)	(12.8)
Total
Fair value	355.8	403.6
Unrealized loss	(8.9)	(12.8)
RMBS
Less than 12 months
Fair value	218.4	148.4
Unrealized loss	(27.9)	(32.8)
12 months or more
Fair Value	10.1	45.3
Unrealized loss	(23.3)	(13.1)
Total
Fair value	228.5	193.7
Unrealized loss	(51.2)	(45.9)
CMBS
Less than 12 months
Fair value	72.3	92.6
Unrealized loss	(0.4)	(1.4)
Total
Fair value	72.3	92.6
Unrealized loss	(0.4)	(1.4)
Asset-backed securities
Less than 12 months
Fair value	43.7	228.3
Unrealized loss	(5.5)	(5.1)
12 months or more
Fair Value	2.2	2.3
Unrealized loss	0	(0.1)
Total
Fair value	45.9	230.6
Unrealized loss	(5.5)	(5.2)
Foreign government securities
Less than 12 months
Fair value	80.1	245.3
Unrealized loss	(1.0)	(6.2)
Total
Fair value	80.1	245.3
Unrealized loss	$ (1.0)	$ (6.2)