Financial Information with Respect to Business Segments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Segment revenues	$ 1,154.3	$ 1,101.0	$ 3,406.7	$ 2,774.5
Intersegment revenues	(1.6)	(1.2)	(4.7)	(3.7)
Total segment revenues	1,152.7	1,099.8	3,402.0	2,770.8
Net realized investment gains	5.0	8.2	4.7	24.9
Total revenues	1,157.7	1,108.0	3,406.7	2,795.7
Total, Segment income (loss) before income taxes	62.9	(8.2)	179.6	(5.7)
Interest expense	(14.6)	(17.4)	(46.7)	(38.6)
Segment income (loss) before income taxes	48.3	(25.6)	132.9	(44.3)
Net realized investment gains	5.0	8.2	4.7	24.9
Net loss from retirement of debt		(0.1)		(2.3)
Net costs related to acquired businesses	0.2	(1.9)	(2.2)	(15.7)
Loss on derivative instruments		(6.6)		(11.3)
Net foreign exchange gains (losses)	(0.3)	6.7	(0.3)	6.7
Income (loss) before income taxes	53.2	(19.3)	135.1	(42.0)
Net investment income	69.2	67.8	206.5	189.2
Commercial Lines
Segment Reporting Information [Line Items]
Segment revenues	498.9	449.4	1,461.9	1,336.6
Total, Segment income (loss) before income taxes	8.1	(22.5)	32.6	(29.6)
GAAP underwriting income (loss)	(27.8)	(57.1)	(75.0)	(133.2)
Net investment income	35.6	34.0	106.3	101.6
Other income	0.3	0.6	1.3	2.0
Personal Lines
Segment Reporting Information [Line Items]
Segment revenues	391.6	389.9	1,168.8	1,164.8
Total, Segment income (loss) before income taxes	14.7	(5.0)	54.5	2.5
GAAP underwriting income (loss)	(8.4)	(29.1)	(14.6)	(69.7)
Net investment income	21.6	22.7	64.5	68.5
Other income	1.5	1.4	4.6	3.7
Chaucer
Segment Reporting Information [Line Items]
Segment revenues	259.2	257.5	762.7	257.5
Total, Segment income (loss) before income taxes	42.0	19.9	97.3	19.9
GAAP underwriting income (loss)	30.7	12.1	64.6	12.1
Net investment income	10.2	8.6	29.8	8.6
Other income (expenses)	1.1	(0.8)	2.9	(0.8)
Other Property And Casualty
Segment Reporting Information [Line Items]
Segment revenues	4.6	4.2	13.3	15.6
Total, Segment income (loss) before income taxes	(1.9)	(0.6)	(4.8)	1.5
GAAP underwriting income (loss)	(1.1)		(2.3)	0.1
Net investment income	1.7	2.5	5.7	10.5
Other net expenses	$ (2.5)	$ (3.1)	$ (8.2)	$ (9.1)