Segment Information (Financial Information With Respect to Business Segments) (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Property and Casualty	$ 841.7	$ 769.3	$ 1,673.5	$ 1,506.1
Intersegment revenues	(1.2)	(1.2)	(2.5)	(2.3)
Total segment revenues	840.5	768.1	1,671.0	1,503.8
Net realized investment gains	13.4	0.2	16.7	11.1
Total revenues	853.9	768.3	1,687.7	1,514.9
Net investment income	61.0	61.8	121.4	122.9
Interest expense on debt	(10.8)	(11.7)	(21.2)	(21.0)
Segment (loss) income before federal income taxes	(57.6)	2.9	(18.2)	51.3
Costs related to acquired businesses	(11.1)		(13.8)
Loss on derivative instruments	(4.7)		(4.7)
Gain (loss) from retirement of debt	0.3		(2.2)
(Loss) income before federal income taxes	(59.7)	3.1	(22.2)	62.4
Personal Lines [Member]
Property and Casualty	388.2	397.9	774.9	793.2
GAAP underwriting loss	(47.1)	(30.6)	(40.9)	(23.9)
Net investment income	23.1	25.6	45.8	51.2
Other income	1.1	1.5	2.3	3.7
Segment (loss) income	(22.9)	(3.5)	7.2	31.0
Commercial Lines [Member]
Property and Casualty	447.9	365.9	887.2	702.1
GAAP underwriting loss	(59.5)	(16.0)	(75.3)	(24.8)
Net investment income	34.0	32.3	67.6	64.3
Other income	0.8	0.6	1.4	0.2
Segment (loss) income	(24.7)	16.9	(6.3)	39.7
Property and Casualty [Member]
Segment (loss) income	(46.8)	14.6	3.0	72.3
Other Property and Casualty [Member]
Property and Casualty	5.6	5.5	11.4	10.8
GAAP underwriting loss			0.1	0.3
Net investment income	3.9	3.9	8.0	7.4
Segment (loss) income	0.8	1.2	2.1	1.6
Other net expenses	$ (3.1)	$ (2.7)	$ (6.0)	$ (6.1)