Segments of Business, Revenue (Detail) (USD $) In Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Segment Reporting Information [Line Items]
Net premiums earned	$ 190,209		$ 190,632		$ 555,067		$ 574,141
Net investment income	22,050		29,424		82,174		93,547
Net realized capital gains	22,706		27,248		63,888		87,023
Other than temporary impairment losses	(2,756)		(2,453)		(2,756)		(9,233)
Other income	321		5,312		1,454		6,946
Total revenues	232,530		250,163		699,827		752,424
Alleghany Insurance Holdings, LLC
Segment Reporting Information [Line Items]
Net premiums earned	190,200		190,600		555,100		574,100
Net investment income	29,700		30,500		90,900		96,900
Net realized capital gains	22,100		27,200		63,300		82,600
Other than temporary impairment losses	(2,800)	[1]	(2,500)	[1]	(2,800)	[1]	(9,200)	[1]
Other income	300		200		600		400
Total revenues	239,500		246,000		707,100		744,800
Alleghany Insurance Holdings, LLC | RSUI Group, Inc
Segment Reporting Information [Line Items]
Net premiums earned	152,200		148,500		438,200		445,100
Alleghany Insurance Holdings, LLC | Capitol Transamerica Corporation And Platte River Insurance Company
Segment Reporting Information [Line Items]
Net premiums earned	36,700		42,000		113,900		124,100
Alleghany Insurance Holdings, LLC | Pacific Compensation Corporation
Segment Reporting Information [Line Items]
Net premiums earned	1,300		100		3,000		4,900
Corporate activities
Segment Reporting Information [Line Items]
Net investment income	(7,600)	[2]	(1,100)	[2]	(8,700)	[2]	(3,300)	[2]
Net realized capital gains	600		100		600		4,400
Other income			$ 5,200		$ 900		$ 6,500

[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 $16.7 million and $5.2 million of Alleghany's equity share of losses in Homesite for the nine months ended September 30, 2011 and 2010, respectively, and $3.4 million and $0.5 million of Alleghany's equity share of losses in ORX for the nine months ended September 30, 2011 and 2010, respectively.