Financial Information with Respect to Business Segments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Segment revenues	$ 1,132.1	$ 841.7	$ 2,252.4	$ 1,673.5
Intersegment revenues	(1.5)	(1.2)	(3.1)	(2.5)
Total segment revenues	1,130.6	840.5	2,249.3	1,671.0
Net realized investment gains	(3.4)	13.4	(0.3)	16.7
Total revenues	1,127.2	853.9	2,249.0	1,687.7
Total, Segment income (loss) before income taxes	31.0	(47.3)	116.7	2.5
Interest expense	(15.9)	(10.8)	(32.1)	(21.2)
Segment income (loss) before income taxes	15.1	(58.1)	84.6	(18.7)
Net realized investment gains (losses)	(3.4)	13.4	(0.3)	16.7
Net gain (loss) from retirement of debt		0.3		(2.2)
Costs related to acquired businesses	(0.9)	(11.1)	(2.4)	(13.8)
Loss on derivative instruments		(4.7)		(4.7)
Net foreign exchange gains	0.3
Income (loss) before income taxes	11.1	(60.2)	81.9	(22.7)
Net investment income	68.5	61.0	137.3	121.4
Commercial Lines
Segment Reporting Information [Line Items]
Segment revenues	487.3	447.9	963.0	887.2
Total, Segment income (loss) before income taxes	(9.4)	(25.3)	24.5	(7.1)
GAAP underwriting income (loss)	(44.7)	(60.1)	(47.2)	(76.1)
Net investment income	35.0	34.0	70.7	67.6
Other income	0.3	0.8	1.0	1.4
Personal Lines
Segment Reporting Information [Line Items]
Segment revenues	388.8	388.2	777.2	774.9
Total, Segment income (loss) before income taxes	12.3	(22.8)	39.8	7.5
GAAP underwriting income (loss)	(10.4)	(47.0)	(6.2)	(40.6)
Net investment income	21.3	23.1	42.9	45.8
Other income	1.4	1.1	3.1	2.3
Chaucer
Segment Reporting Information [Line Items]
Segment revenues	251.8		503.5
Total, Segment income (loss) before income taxes	29.8		55.3
GAAP underwriting income (loss)	19.2		33.9
Net investment income	10.2		19.6
Other income	0.4		1.8
Other Property And Casualty
Segment Reporting Information [Line Items]
Segment revenues	4.2	5.6	8.7	11.4
Total, Segment income (loss) before income taxes	(1.7)	0.8	(2.9)	2.1
GAAP underwriting income (loss)	(0.8)		(1.2)	0.1
Net investment income	1.9	3.9	4.0	8.0
Other net expenses	$ (2.8)	$ (3.1)	$ (5.7)	$ (6.0)