Segment Results for Reportable Segments and Corporate Activities (Detail) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Segment Reporting Disclosure [Line Items]
Gross premiums written	$ 1,176,400,000		$ 270,400,000		$ 3,078,800,000		$ 880,600,000
Net premiums written	1,042,500,000		189,900,000		2,700,400,000		601,100,000
Net premiums earned	1,092,775,000		190,209,000		2,622,914,000		555,067,000
Net loss and LAE	672,984,000		121,777,000		1,587,815,000		315,418,000
Commissions, brokerage and other underwriting expenses	253,756,000		68,239,000		592,809,000		198,899,000
Underwriting profit (loss)	166,000,000	[1]	200,000	[1]	442,300,000	[1]	40,800,000	[1]
Net investment income	90,514,000		22,050,000		234,568,000		82,174,000
Net realized capital gains	12,388,000		22,706,000		119,829,000		63,888,000
OTTI losses			(2,756,000)		(2,907,000)		(2,756,000)
Gain on bargain purchase					494,940,000
Other income	33,843,000		321,000		43,434,000		1,454,000
Other operating expenses	47,432,000		1,757,000		97,910,000		18,998,000
Corporate administration	9,304,000		2,649,000		67,691,000		14,030,000
Amortization of intangible assets	73,356,000		838,000		213,393,000		2,516,000
Interest expense	21,764,000		4,329,000		46,584,000		13,049,000
Earnings before income taxes	150,924,000		32,941,000		906,576,000		136,917,000
Reinsurance Segment
Segment Reporting Disclosure [Line Items]
Gross premiums written	886,500,000				2,096,900,000
Net premiums written	842,900,000				2,022,100,000
Net premiums earned	881,100,000				2,017,500,000
Net loss and LAE	562,500,000				1,271,000,000
Commissions, brokerage and other underwriting expenses	181,100,000				377,300,000
Underwriting profit (loss)	137,500,000	[1]			369,200,000	[1]
Reinsurance Segment | Property
Segment Reporting Disclosure [Line Items]
Gross premiums written	281,900,000				660,800,000
Net premiums written	248,000,000				608,600,000
Net premiums earned	264,700,000				603,600,000
Net loss and LAE	103,700,000				215,500,000
Commissions, brokerage and other underwriting expenses	55,400,000				121,100,000
Underwriting profit (loss)	105,600,000	[1]			267,000,000	[1]
Reinsurance Segment | Casualty & Other
Segment Reporting Disclosure [Line Items]
Gross premiums written	604,600,000	[2]			1,436,100,000	[2]
Net premiums written	594,900,000	[2]			1,413,500,000	[2]
Net premiums earned	616,400,000	[2]			1,413,900,000	[2]
Net loss and LAE	458,800,000	[2]			1,055,500,000	[2]
Commissions, brokerage and other underwriting expenses	125,700,000	[2]			256,200,000	[2]
Underwriting profit (loss)	31,900,000	[1],[2]			102,200,000	[1],[2]
Insurance Segment
Segment Reporting Disclosure [Line Items]
Gross premiums written	295,800,000		270,400,000		994,800,000		880,600,000
Net premiums written	199,600,000		189,900,000		678,300,000		601,100,000
Net premiums earned	211,700,000		190,200,000		605,400,000		555,100,000
Net loss and LAE	110,500,000		121,800,000		316,800,000		315,400,000
Commissions, brokerage and other underwriting expenses	72,700,000		68,200,000		215,500,000		198,900,000
Underwriting profit (loss)	28,500,000	[1]	200,000	[1]	73,100,000	[1]	40,800,000	[1]
Insurance Segment | RSUI Group, Inc
Segment Reporting Disclosure [Line Items]
Gross premiums written	248,900,000		230,200,000		861,400,000		763,100,000
Net premiums written	155,100,000		151,800,000		551,700,000		489,200,000
Net premiums earned	169,800,000		152,200,000		487,600,000		438,200,000
Net loss and LAE	82,900,000		101,000,000		239,800,000		236,300,000
Commissions, brokerage and other underwriting expenses	46,100,000		44,100,000		136,700,000		126,300,000
Underwriting profit (loss)	40,800,000	[1]	7,100,000	[1]	111,100,000	[1]	75,600,000	[1]
Insurance Segment | Capitol Transamerica Corporation and Platte River Insurance Company
Segment Reporting Disclosure [Line Items]
Gross premiums written	40,600,000		38,800,000		120,800,000		115,000,000
Net premiums written	38,400,000		36,600,000		114,100,000		108,300,000
Net premiums earned	37,000,000		36,700,000		107,300,000		113,900,000
Net loss and LAE	23,100,000		19,000,000		67,000,000		60,600,000
Commissions, brokerage and other underwriting expenses	19,700,000		17,600,000		58,500,000		54,200,000
Underwriting profit (loss)	(5,800,000)	[1]	100,000	[1]	(18,200,000)	[1]	(900,000)	[1]
Insurance Segment | Pacific Compensation Corporation
Segment Reporting Disclosure [Line Items]
Gross premiums written	6,300,000		1,400,000		12,600,000		2,500,000
Net premiums written	6,100,000		1,500,000		12,500,000		3,600,000
Net premiums earned	4,900,000		1,300,000		10,500,000		3,000,000
Net loss and LAE	4,500,000		1,800,000		10,000,000		18,500,000
Commissions, brokerage and other underwriting expenses	6,900,000		6,500,000		20,300,000		18,400,000
Underwriting profit (loss)	(6,500,000)	[1]	(7,000,000)	[1]	(19,800,000)	[1]	(33,900,000)	[1]
Operating Segments
Segment Reporting Disclosure [Line Items]
Gross premiums written	1,182,300,000		270,400,000		3,091,700,000		880,600,000
Net premiums written	1,042,500,000		189,900,000		2,700,400,000		601,100,000
Net premiums earned	1,092,800,000		190,200,000		2,622,900,000		555,100,000
Net loss and LAE	673,000,000		121,800,000		1,587,800,000		315,400,000
Commissions, brokerage and other underwriting expenses	253,800,000		68,200,000		592,800,000		198,900,000
Underwriting profit (loss)	166,000,000	[1]	200,000	[1]	442,300,000	[1]	40,800,000	[1]
Net investment income	90,100,000		29,700,000		226,000,000		90,900,000
Net realized capital gains	12,400,000		22,100,000		81,200,000		63,300,000
OTTI losses			(2,800,000)		(2,900,000)		(2,800,000)
Other income	24,200,000		300,000		25,200,000		600,000
Other operating expenses	36,300,000		700,000		73,900,000		16,100,000
Amortization of intangible assets	73,300,000		800,000		213,400,000		2,500,000
Interest expense	12,500,000				28,200,000		100,000
Earnings before income taxes	170,600,000		48,000,000		456,300,000		174,100,000
Corporate activities
Segment Reporting Disclosure [Line Items]
Gross premiums written	(5,900,000)	[3]			(12,900,000)	[3]
Net investment income	400,000	[3]	(7,600,000)	[3]	8,600,000	[3]	(8,700,000)	[3]
Net realized capital gains			600,000	[3]	38,600,000	[3]	600,000	[3]
Gain on bargain purchase					494,900,000	[3]
Other income	9,600,000	[3]			18,200,000	[3]	900,000	[3]
Other operating expenses	11,100,000	[3]	1,100,000	[3]	24,000,000	[3]	2,900,000	[3]
Corporate administration	9,300,000	[3]	2,700,000	[3]	67,700,000	[3]	14,100,000	[3]
Interest expense	9,300,000	[3]	4,300,000	[3]	18,300,000	[3]	13,000,000	[3]
Earnings before income taxes	$ (19,700,000)	[3]	$ (15,100,000)	[3]	$ 450,300,000	[3]	$ (37,200,000)	[3]

[1] Represents net premiums earned less net loss and LAE and commissions, brokerage and other underwriting expenses, all as determined in accordance with GAAP, and does not include net investment income, net realized capital gains, OTTI losses, other income, other operating expenses, amortization of intangible assets or interest expense. Underwriting profit does not replace earnings before income taxes determined in accordance with GAAP as a measure of profitability. Rather, Alleghany believes that underwriting profit enhances the understanding of its segments' operating results by highlighting net earnings attributable to their underwriting performance. Earnings before income taxes (a GAAP measure) may show a profit despite an underlying underwriting loss. Where underwriting losses persist over extended periods, a reinsurance or insurance company's ability to continue as an ongoing concern may be at risk. Therefore, Alleghany views underwriting profit as an important measure in the overall evaluation of performance.

[2] Casualty & other primarily consists of assumed: D&O liability; errors and omissions liability; general liability; medical malpractice; ocean marine and aviation; auto liability; accident and health; surety; and credit.

[3] Includes elimination of minor reinsurance activity between segments.