Results for Reportable Segments and Other Activities (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Gross premiums written		$ 5,696,900	$ 5,767,100	$ 5,122,200
Net premiums written		4,965,900	5,091,800	4,489,200
Net premiums earned		4,954,990	4,975,777	4,230,286
Net loss and LAE		3,620,197	2,917,166	2,339,790
Commissions, brokerage and other underwriting expenses	[1]	1,651,177	1,657,251	1,423,889
Underwriting (loss) profit	[2]	(316,400)	401,300	466,600
Net investment income		451,016	438,455	438,817
Net realized capital gains		107,222	63,205	213,897
Other than temporary impairment losses		(16,871)	(45,165)	(133,868)
Other revenue		928,298	698,747	250,346
Other operating expenses		967,104	765,226	342,361
Corporate administration		46,998	42,960	46,503
Amortization of intangible assets		19,419	19,012	(2,211)
Interest expense		83,070	81,599	91,778
(Losses) earnings before income taxes		36,690	647,805	757,368
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		5,720,200	5,793,000	5,150,200
Net premiums written		4,965,900	5,091,800	4,489,200
Net premiums earned		4,955,000	4,975,800	4,230,300
Net loss and LAE		3,620,200	2,917,200	2,339,800
Commissions, brokerage and other underwriting expenses	[1]	1,651,200	1,657,300	1,423,900
Underwriting (loss) profit	[2]	(316,400)	401,300	466,600
Net investment income		434,600	433,100	427,600
Net realized capital gains		85,700	159,900	242,600
Other than temporary impairment losses		(16,900)	(45,200)	(125,500)
Other revenue		15,500	4,400	6,500
Other operating expenses		82,800	80,600	80,400
Corporate administration		1,700	1,000	900
Amortization of intangible assets		(1,500)	(3,100)	(5,300)
Interest expense		26,900	27,200	38,300
(Losses) earnings before income taxes		92,600	847,800	903,500
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		4,210,600	4,330,300	3,662,100
Net premiums written		3,810,100	3,969,400	3,387,300
Net premiums earned		3,808,700	3,845,000	3,115,500
Net loss and LAE		2,785,400	2,285,400	1,718,700
Commissions, brokerage and other underwriting expenses	[1]	1,286,700	1,299,000	1,069,800
Underwriting (loss) profit	[2]	(263,400)	260,600	327,000
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		1,557,800	1,515,500	1,171,900
Net premiums written		1,233,100	1,237,200	953,600
Net premiums earned		1,181,900	1,168,000	887,400
Net loss and LAE		1,080,100	578,400	292,100
Commissions, brokerage and other underwriting expenses	[1]	383,400	376,200	295,600
Underwriting (loss) profit	[2]	(281,600)	213,400	299,700
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[3]	2,652,800	2,814,800	2,490,200
Net premiums written	[3]	2,577,000	2,732,200	2,433,700
Net premiums earned	[3]	2,626,800	2,677,000	2,228,100
Net loss and LAE	[3]	1,705,300	1,707,000	1,426,600
Commissions, brokerage and other underwriting expenses	[1],[3]	903,300	922,800	774,200
Underwriting (loss) profit	[2],[3]	18,200	47,200	27,300
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,509,600	1,462,700	1,488,100
Net premiums written		1,155,800	1,122,400	1,101,900
Net premiums earned		1,146,300	1,130,800	1,114,800
Net loss and LAE		834,800	631,800	621,100
Commissions, brokerage and other underwriting expenses	[1]	364,500	358,300	354,100
Underwriting (loss) profit	[2]	(53,000)	140,700	139,600
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		1,056,800	1,056,400	1,148,400
Net premiums written		724,400	734,100	779,400
Net premiums earned		721,700	754,500	809,800
Net loss and LAE		569,900	403,800	428,800
Commissions, brokerage and other underwriting expenses	[1]	208,900	212,300	222,900
Underwriting (loss) profit	[2]	(57,100)	138,400	158,100
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		290,200	266,500	236,600
Net premiums written		271,200	250,000	220,600
Net premiums earned		260,900	237,500	205,000
Net loss and LAE		143,900	125,300	115,700
Commissions, brokerage and other underwriting expenses	[1]	112,700	107,300	94,300
Underwriting (loss) profit	[2]	4,300	4,900	(5,000)
Operating Segments | Insurance Segment | Pacific Comp
Segment Reporting Information [Line Items]
Gross premiums written		162,600	139,800	103,100
Net premiums written		160,200	138,300	101,900
Net premiums earned		163,700	138,800	100,000
Net loss and LAE		121,000	102,700	76,600
Commissions, brokerage and other underwriting expenses	[1]	42,900	38,700	36,900
Underwriting (loss) profit	[2]	(200)	(2,600)	(13,500)
Other activities | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Net investment income		2,700	(2,300)	5,400
Net realized capital gains		18,200	(86,000)	(25,600)
Other revenue		906,900	687,100	241,000
Other operating expenses		881,000	680,500	259,300
Amortization of intangible assets		20,900	22,100	3,100
Interest expense		4,000	1,900	1,500
(Losses) earnings before income taxes		21,900	(105,700)	(43,100)
Other activities | Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written	[4]	(23,300)	(25,900)	(28,000)
Net investment income	[4]	13,700	7,700	5,800
Net realized capital gains	[4]	3,300	(10,700)	(3,100)
Other than temporary impairment losses	[4]			(8,400)
Other revenue	[4]	5,900	7,300	2,900
Other operating expenses	[4]	3,300	4,100	2,600
Corporate administration	[4]	45,300	42,000	45,600
Interest expense	[4]	52,100	52,500	52,000
(Losses) earnings before income taxes	[4]	$ (77,800)	$ (94,300)	$ (103,000)

[1] Includes amortization associated with deferred acquisition costs of $1,261.7 million, $1,253.2 million and $1,024.5 million for the years ended December 31, 2017, 2016 and 2015, 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 reinsurance and insurance 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.