Segments of Business, Revenue (Detail) (USD $) In Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Segment Reporting Information [Line Items]
Net premiums earned	$ 183,878		$ 188,809		$ 364,858		$ 383,509
Net investment income	28,545		32,694		60,124		64,123
Net realized capital gains	6,490		33,308		41,182		59,775
Other than temporary impairment losses			(5,703)				(6,780)
Other income	248		1,501		1,133		1,634
Total revenues	219,161		250,609		467,297		502,261
Alleghany Insurance Holdings, LLC
Segment Reporting Information [Line Items]
Net premiums earned	183,900		188,800		364,800		383,600
Net investment income	31,000		33,000		61,200		66,300
Net realized capital gains	6,500		32,700		41,200		55,500
Other than temporary impairment losses			(5,700)	[1]			(6,800)	[1]
Other income	100		200		300		300
Total revenues	221,500		249,000		467,500		498,900
Alleghany Insurance Holdings, LLC | RSUI Group, Inc
Segment Reporting Information [Line Items]
Net premiums earned	144,400		146,300		286,000		296,600
Alleghany Insurance Holdings, LLC | Capitol Transamerica Corporation And Platte River Insurance Company
Segment Reporting Information [Line Items]
Net premiums earned	37,900		41,500		77,100		82,100
Alleghany Insurance Holdings, LLC | Pacific Compensation Corporation
Segment Reporting Information [Line Items]
Net premiums earned	1,600		1,000		1,700		4,900
Corporate activities
Segment Reporting Information [Line Items]
Net investment income	(2,400)	[2]	(300)	[2]	(1,000)	[2]	(2,200)	[2]
Net realized capital gains			600				4,300
Other income	$ 100		$ 1,300		$ 800		$ 1,300

[1] Reflects impairment charges for unrealized losses related to AIHL's investment portfolio that were deemed to be other-than-temporary. See Note 7(c)

[2] Includes $7.1 million and $1.6 million of Alleghany's equity share of losses in Homesite for the six months ended June 30, 2011 and 2010, respectively, and $2.2 million and $2.8 million of Alleghany's equity share of losses in ORX for the six months ended June 30, 2011 and 2010, respectively.