Segment Information (Financial Information With Respect To Business Segments) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Segment revenues	$ 1,101.0	$ 799.4	$ 2,774.5	$ 2,305.5
Intersegment revenues	(1.2)	(1.1)	(3.7)	(3.4)
Total segment revenues	1,099.8	798.3	2,770.8	2,302.1
Net realized investment gains	8.2	5.7	24.9	16.8
Total revenues	1,108.0	804.0	2,795.7	2,318.9
Net investment income	67.8	61.3	189.2	184.2
Interest expense on debt	(17.4)	(11.8)	(38.6)	(32.8)
Segment income (loss) before income taxes	(25.0)	67.4	(43.2)	118.7
Costs related to acquired businesses	(1.9)		(15.7)
Loss on derivative instruments	(6.6)		(11.3)
Net loss from retirement of debt	(0.1)		(2.3)
Net foreign exchange gains	6.7		6.7
Income (loss) before income taxes	(18.7)	73.1	(40.9)	135.5
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Segment revenues	449.4	397.2	1,336.6	1,099.3
GAAP underwriting income (loss)	(56.7)	2.3	(132.0)	(22.5)
Net investment income	34.0	32.3	101.6	96.6
Other income	0.6	0.5	2.0	0.7
Total, Segment income (loss) before income taxes	(22.1)	35.1	(28.4)	74.8
Personal Lines [Member]
Segment Reporting Information [Line Items]
Segment revenues	389.9	397.1	1,164.8	1,190.3
GAAP underwriting income (loss)	(28.9)	15.2	(69.8)	(8.7)
Net investment income	22.7	25.4	68.5	76.6
Other income	1.4	2.4	3.7	6.1
Total, Segment income (loss) before income taxes	(4.8)	43.0	2.4	74.0
Chaucer [Member]
Segment Reporting Information [Line Items]
Segment revenues	257.5		257.5
GAAP underwriting income (loss)	12.1		12.1
Net investment income	8.6		8.6
Other net expenses	(0.8)		(0.8)
Total, Segment income (loss) before income taxes	19.9		19.9
Property And Casualty [Member]
Segment Reporting Information [Line Items]
Total, Segment income (loss) before income taxes	(7.6)	79.2	(4.6)	151.5
Other Property And Casualty [Member]
Segment Reporting Information [Line Items]
Segment revenues	4.2	5.1	15.6	15.9
GAAP underwriting income (loss)		0.4	0.1	0.7
Net investment income	2.5	3.6	10.5	11.0
Other net expenses	(3.1)	(2.9)	(9.1)	(9.0)
Total, Segment income (loss) before income taxes	$ (0.6)	$ 1.1	$ 1.5	$ 2.7