Information Related to Reportable Segment (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Revenues
Net premiums earned									$ 747,639,000		$ 768,134,000		$ 845,015,000
Net investment income									108,910,000		125,012,000		101,949,000
Net realized capital gains									127,141,000		97,374,000		320,389,000
Other than temporary impairment losses									(3,607,000)		(12,356,000)		(85,916,000)
Other income									1,754,000		7,188,000		2,955,000
Total revenues	282,000,000	232,500,000	219,200,000	248,100,000	232,900,000	250,200,000	250,600,000	251,700,000	981,837,000		985,352,000		1,184,392,000
Net investment income									108,910,000		125,012,000		101,949,000
Net realized capital gains									127,141,000		97,374,000		320,389,000
Other than temporary impairment losses									(3,607,000)		(12,356,000)		(85,916,000)
Other income									1,754,000		7,188,000		2,955,000
Earnings before income taxes									190,837,000		277,371,000		395,380,000
Corporate administration and other expenses									41,007,000		28,854,000		26,938,000
Interest expense									17,426,000		4,698,000		633,000
Total assets	6,478,089,000				6,431,699,000				6,478,089,000		6,431,699,000		6,192,800,000
Capital expenditures									6,411,000		6,607,000		5,539,000
Depreciation and amortization									27,045,000		33,816,000		32,358,000
AIHL insurance group
Revenues
Net premiums earned									747,600,000		768,100,000		845,000,000
Net investment income									117,400,000		128,900,000		116,700,000
Net realized capital gains									79,700,000		92,900,000		119,800,000
Other than temporary impairment losses									(3,600,000)	[1]	(12,300,000)	[1]	(85,900,000)	[1]
Other income									700,000		600,000		1,300,000
Total revenues									941,800,000		978,200,000		996,900,000
Underwriting profit (loss)									49,500,000		130,900,000		129,200,000
Net investment income									117,400,000		128,900,000		116,700,000
Net realized capital gains									79,700,000		92,900,000		119,800,000
Other than temporary impairment losses									(3,600,000)	[1]	(12,300,000)	[1]	(85,900,000)	[1]
Other income, less other expenses									(29,000,000)		(33,800,000)		(42,200,000)
Other income									700,000		600,000		1,300,000
Earnings before income taxes									214,000,000		306,600,000		237,600,000
Total assets	5,706,300,000				5,546,700,000				5,706,300,000		5,546,700,000		5,659,200,000
Capital expenditures									6,300,000		6,600,000		5,500,000
Depreciation and amortization									26,000,000		29,800,000		30,500,000
AIHL insurance group | RSUI Group, Inc
Revenues
Net premiums earned									593,800,000	[2]	593,600,000	[2]	633,400,000	[2]
Underwriting profit (loss)									107,800,000	[3]	159,900,000	[3]	189,800,000	[3]
AIHL insurance group | Capitol Transamerica Corporation And Platte River Insurance Company
Revenues
Net premiums earned									149,300,000		164,300,000		166,700,000
Underwriting profit (loss)									(6,700,000)	[3]	1,500,000	[3]	10,100,000	[3]
AIHL insurance group | Pacific Compensation Corporation
Revenues
Net premiums earned									4,500,000		10,200,000		44,900,000
Underwriting profit (loss)									(51,600,000)	[3]	(30,500,000)	[3]	(70,700,000)	[3]
Corporate activities
Revenues
Net investment income									(8,500,000)	[4]	(3,900,000)	[4]	(14,800,000)	[4]
Net realized capital gains									47,500,000	[5]	4,500,000	[5]	200,600,000	[5]
Other income									1,000,000	[6]	6,600,000	[6]	1,700,000	[6]
Net investment income									(8,500,000)	[4]	(3,900,000)	[4]	(14,800,000)	[4]
Net realized capital gains									47,500,000	[5]	4,500,000	[5]	200,600,000	[5]
Other income									1,000,000	[6]	6,600,000	[6]	1,700,000	[6]
Corporate administration and other expenses									45,900,000		31,400,000		29,100,000
Interest expense									17,300,000		5,000,000		600,000
Total assets	771,800,000				885,000,000				771,800,000		885,000,000		533,600,000
Capital expenditures									100,000
Depreciation and amortization									$ 1,000,000		$ 4,000,000		$ 1,900,000

[1] Reflects impairment charges for unrealized losses related to AIHL's investment portfolio that were deemed to be other than temporary. See Note 3.

[2] RSUI's 2011 net premiums earned includes immaterial amounts arising from reinsurance assumed from insurance companies domiciled outside the U.S. (specifically, in France and the United Kingdom).

[3] Represents net premiums earned less loss and LAE and commission, brokerage and other underwriting expenses, all as determined in accordance with GAAP, does not include net investment income, net realized capital gains, other-than-temporary impairment losses, other income or other expenses. Commissions, brokerage and other underwriting expenses represent commission and brokerage expenses and that portion of salaries, administration and other operating expenses attributable primarily to underwriting activities, whereas the remainder constitutes other expenses.

[4] Includes $20.2 million, $3.2 million and $1.1 million of Alleghany's equity in losses of Homesite for 2011, 2010 and 2009, respectively. Also includes $1.6 million, $2.0 million and $21.9 million of Alleghany's equity in losses of ORX for 2011, 2010 and 2009, respectively. See Notes 4(b) and 4(c).

[5] For 2011, primarily reflects net realized capital gains from the sale of shares of common stock of Exxon Mobil Corporation. For 2009, primarily reflects net realized capital gains from the sale of shares of Burlington Northern common stock.

[6] Includes Alleghany Properties revenue.