Segment Information (Financial Information With Respect To Business Segments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Segment revenues	$ 1,120.3	$ 831.8
Intersegment revenues	(1.6)	(1.3)
Total segment revenues	1,118.7	830.5
Net realized investment gains	3.1	3.3
Total revenues	1,121.8	833.8
Net investment income	68.8	60.4
Total, Segment income (loss) before income taxes	85.7	49.8
Interest expense on debt	(16.2)	(10.4)
Segment income before income taxes	69.5	39.4
Loss from retirement of debt		(2.5)
Costs related to acquired businesses	(1.5)	(2.7)
Net foreign exchange losses	(0.3)
Income before income taxes	70.8	37.5
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Segment revenues	475.7	439.3
GAAP underwriting income	(2.5)	(16.0)
Net investment income	35.7	33.6
Other income	0.7	0.6
Total, Segment income (loss) before income taxes	33.9	18.2
Personal Lines [Member]
Segment Reporting Information [Line Items]
Segment revenues	388.4	386.7
GAAP underwriting income	4.2	6.4
Net investment income	21.6	22.7
Other income	1.7	1.2
Total, Segment income (loss) before income taxes	27.5	30.3
Chaucer [Member]
Segment Reporting Information [Line Items]
Segment revenues	251.7
GAAP underwriting income	14.7
Net investment income	9.4
Other income	1.4
Total, Segment income (loss) before income taxes	25.5
Other Property And Casualty [Member]
Segment Reporting Information [Line Items]
Segment revenues	4.5	5.8
GAAP underwriting income	(0.4)	0.1
Net investment income	2.1	4.1
Other net expenses	(2.9)	(2.9)
Total, Segment income (loss) before income taxes	$ (1.2)	$ 1.3