Financial Information with Respect to Business Segments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Segment revenues									$ 4,239.1	$ 3,598.6	$ 2,841.0
Intersegment revenues									(6.5)	(5.2)	(4.6)
Total segment revenues									4,567.1	3,903.5	3,122.5
Net realized investment gains									23.6	28.1	29.7
Total revenues	1,184.0	1,157.7	1,127.2	1,121.8	1,135.9	1,108.0	853.9	833.8	4,590.7	3,931.6	3,152.2
Total, Segment income (loss) before income taxes									75.1	71.8	222.2
Interest expense									(61.9)	(55.0)	(44.3)
Segment income (loss) before income taxes									13.2	16.8	177.9
Net realized investment gains									23.6	28.1	29.7
Net loss from retirement of debt									(5.1)	(2.3)	(2.0)
Net costs related to acquired businesses									(2.6)	(16.4)
Loss on derivative instruments										(11.3)
Net foreign exchange gains (losses)									(0.4)	6.7
Income (loss) before income taxes									28.7	21.6	205.6
Net investment income									276.6	258.2	247.2
Commercial Lines
Segment Reporting Information [Line Items]
Total, Segment income (loss) before income taxes									(80.3)	16.3	106.4
GAAP underwriting income (loss)									(224.2)	(123.0)	(25.4)
Net investment income									142.4	136.5	129.9
Other income									1.5	2.8	1.9
Personal Lines
Segment Reporting Information [Line Items]
Total, Segment income (loss) before income taxes									25.5	23.1	112.3
GAAP underwriting income (loss)									(67.3)	(72.6)	0.9
Net investment income									86.5	92.1	102.9
Other income									6.3	3.6	8.5
Chaucer
Segment Reporting Information [Line Items]
Total, Segment income (loss) before income taxes									136.8	32.9
GAAP underwriting income (loss)									93.5	11.9
Net investment income									40.2	16.9
Other income (expenses)									3.1	4.1
Other Property And Casualty
Segment Reporting Information [Line Items]
Total, Segment income (loss) before income taxes									(6.9)	(0.5)	3.5
GAAP underwriting income (loss)									(3.5)	(0.3)	0.8
Net investment income									7.2	12.7	14.4
Other net expenses									(10.6)	(12.9)	(11.7)
Earned Premiums
Segment Reporting Information [Line Items]
Segment revenues									4,573.6	3,908.7	3,127.1
Earned Premiums | Commercial Lines
Segment Reporting Information [Line Items]
Segment revenues									1,966.2	1,798.9	1,522.3
Earned Premiums | Personal Lines
Segment Reporting Information [Line Items]
Segment revenues									1,560.0	1,555.9	1,583.8
Earned Premiums | Chaucer
Segment Reporting Information [Line Items]
Segment revenues									1,030.0	534.1
Earned Premiums | Other Property And Casualty
Segment Reporting Information [Line Items]
Segment revenues									$ 17.4	$ 19.8	$ 21.0