Results for Reportable Segments and Other Activities (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Gross premiums written		$ 5,767,100	$ 5,122,200	$ 5,096,600
Net premiums written		5,091,800	4,489,200	4,497,500
Net premiums earned		4,975,777	4,230,286	4,410,647
Net loss and LAE		2,917,166	2,339,790	2,494,565
Commissions, brokerage and other underwriting expenses	[1]	1,657,251	1,423,889	1,421,306
Underwriting profit (loss)	[2]	401,300	466,600	494,800
Net investment income		438,455	438,817	459,876
Net realized capital gains		63,205	213,897	247,058
Other than temporary impairment losses		(45,165)	(133,868)	(36,294)
Other revenue		698,747	250,346	150,522
Other operating expenses		765,226	342,361	252,673
Corporate administration		42,960	46,503	47,054
Amortization of intangible assets		19,012	(2,211)	(5,750)
Interest expense		81,599	91,778	90,052
Earnings (losses) before income taxes		647,805	757,368	931,909
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		5,793,000	5,150,200	5,125,400
Net premiums written		5,091,800	4,489,200	4,497,500
Net premiums earned		4,975,800	4,230,300	4,410,600
Net loss and LAE		2,917,200	2,339,800	2,494,500
Commissions, brokerage and other underwriting expenses	[1]	1,657,300	1,423,900	1,421,300
Underwriting profit (loss)	[2]	401,300	466,600	494,800
Net investment income		433,100	427,600	448,900
Net realized capital gains		159,900	242,600	230,000
Other than temporary impairment losses		(45,200)	(125,500)	(36,300)
Other revenue		4,400	6,500	4,000
Other operating expenses		80,600	80,400	85,700
Corporate administration		1,000	900	1,300
Amortization of intangible assets		(3,100)	(5,300)	(6,100)
Interest expense		27,200	38,300	46,800
Earnings (losses) before income taxes		847,800	903,500	1,013,700
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		4,330,300	3,662,100	3,600,100
Net premiums written		3,969,400	3,387,300	3,410,100
Net premiums earned		3,845,000	3,115,500	3,330,700
Net loss and LAE		2,285,400	1,718,700	1,909,200
Commissions, brokerage and other underwriting expenses	[1]	1,299,000	1,069,800	1,076,500
Underwriting profit (loss)	[2]	260,600	327,000	345,000
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		1,515,500	1,171,900	1,205,400
Net premiums written		1,237,200	953,600	1,073,400
Net premiums earned		1,168,000	887,400	1,048,600
Net loss and LAE		578,400	292,100	423,200
Commissions, brokerage and other underwriting expenses	[1]	376,200	295,600	319,300
Underwriting profit (loss)	[2]	213,400	299,700	306,100
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[3]	2,814,800	2,490,200	2,394,700
Net premiums written	[3]	2,732,200	2,433,700	2,336,700
Net premiums earned	[3]	2,677,000	2,228,100	2,282,100
Net loss and LAE	[3]	1,707,000	1,426,600	1,486,000
Commissions, brokerage and other underwriting expenses	[1],[3]	922,800	774,200	757,200
Underwriting profit (loss)	[2],[3]	47,200	27,300	38,900
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,462,700	1,488,100	1,525,300
Net premiums written		1,122,400	1,101,900	1,087,400
Net premiums earned		1,130,800	1,114,800	1,079,900
Net loss and LAE		631,800	621,100	585,300
Commissions, brokerage and other underwriting expenses	[1]	358,300	354,100	344,800
Underwriting profit (loss)	[2]	140,700	139,600	149,800
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		1,056,400	1,148,400	1,242,100
Net premiums written		734,100	779,400	825,500
Net premiums earned		754,500	809,800	828,200
Net loss and LAE		403,800	428,800	427,300
Commissions, brokerage and other underwriting expenses	[1]	212,300	222,900	220,800
Underwriting profit (loss)	[2]	138,400	158,100	180,100
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		266,500	236,600	212,700
Net premiums written		250,000	220,600	192,400
Net premiums earned		237,500	205,000	184,400
Net loss and LAE		125,300	115,700	103,000
Commissions, brokerage and other underwriting expenses	[1]	107,300	94,300	92,000
Underwriting profit (loss)	[2]	4,900	(5,000)	(10,600)
Operating Segments | Insurance Segment | Pacific Comp
Segment Reporting Information [Line Items]
Gross premiums written		139,800	103,100	70,500
Net premiums written		138,300	101,900	69,500
Net premiums earned		138,800	100,000	67,300
Net loss and LAE		102,700	76,600	55,000
Commissions, brokerage and other underwriting expenses	[1]	38,700	36,900	32,000
Underwriting profit (loss)	[2]	(2,600)	(13,500)	(19,700)
Other activities | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Net investment income		(2,300)	5,400	2,800
Net realized capital gains		(86,000)	(25,600)	2,800
Other revenue		687,100	241,000	145,600
Other operating expenses		680,500	259,300	164,500
Amortization of intangible assets		22,100	3,100	400
Interest expense		1,900	1,500	900
Earnings (losses) before income taxes		(105,700)	(43,100)	(14,600)
Other activities | Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written	[4]	(25,900)	(28,000)	(28,800)
Net investment income	[4]	7,700	5,800	8,200
Net realized capital gains	[4]	(10,700)	(3,100)	14,300
Other than temporary impairment losses	[4]		(8,400)
Other revenue	[4]	7,300	2,900	900
Other operating expenses	[4]	4,100	2,600	2,500
Corporate administration	[4]	42,000	45,600	45,800
Interest expense	[4]	52,500	52,000	42,300
Earnings (losses) before income taxes	[4]	$ (94,300)	$ (103,000)	$ (67,200)

[1] Includes amortization associated with deferred acquisition costs of $1,253.2 million, $1,024.5 million and $1,042.0 million for the years ended December 31, 2016, 2015 and 2014, respectively.

[2] Underwriting profit 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 revenue, other operating expenses, corporate administration, 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 an 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.

[3] Primarily consists of the following assumed reinsurance lines of business: directors' and officers' liability; errors and omissions liability; general liability; medical malpractice; ocean marine and aviation; auto liability; accident and health; surety; and credit.

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