Financial Information with Respect to Business Segments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Net realized investment gains	$ 8.1	$ 3.1
Total revenues	1,180.3	1,121.8
Operating income before interest expense and income taxes	102.2	85.7
Interest on debt	(14.7)	(16.2)
Operating income before income taxes	87.5	69.5
Net realized investment gains	8.1	3.1
Net loss from repayment of advances	(7.8)
Net benefit (costs) related to acquired businesses	0.4	(1.5)
Net foreign exchange gains (losses)	0.7	(0.3)
Income before income taxes	88.9	70.8
Net investment income	67.3	68.8
Commercial Lines
Segment Reporting Information [Line Items]
Operating income before interest expense and income taxes	33.0	33.9
GAAP underwriting income (loss)	(3.1)	(2.5)
Net investment income	36.1	35.7
Other income		0.7
Personal Lines
Segment Reporting Information [Line Items]
Operating income before interest expense and income taxes	30.5	27.5
GAAP underwriting income (loss)	10.1	4.2
Net investment income	19.0	21.6
Other income	1.4	1.7
Chaucer
Segment Reporting Information [Line Items]
Operating income before interest expense and income taxes	40.9	25.5
GAAP underwriting income (loss)	33.0	14.7
Net investment income	10.5	9.4
Other income (loss)	(2.6)	1.4
Other
Segment Reporting Information [Line Items]
Operating income before interest expense and income taxes	(2.2)	(1.2)
GAAP underwriting income (loss)	(1.1)	(0.4)
Net investment income	1.7	2.1
Other net expenses	(2.8)	(2.9)
Earned Premiums
Segment Reporting Information [Line Items]
Operating revenues	1,172.2	1,118.7
Earned Premiums | Commercial Lines
Segment Reporting Information [Line Items]
Operating revenues	514.7	475.7
Earned Premiums | Personal Lines
Segment Reporting Information [Line Items]
Operating revenues	391.1	388.4
Earned Premiums | Chaucer
Segment Reporting Information [Line Items]
Operating revenues	263.9	251.7
Earned Premiums | Other
Segment Reporting Information [Line Items]
Operating revenues	$ 2.5	$ 2.9