Results for Reportable Segments and Other Activities (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,420,800	$ 1,489,600
Net premiums written		1,233,100	1,322,200
Net premiums earned		1,209,188	1,221,565
Net loss and LAE		699,305	664,644
Commissions, brokerage and other underwriting expenses		408,515	406,733
Underwriting profit (loss)	[1]	101,400	150,200
Net investment income		115,538	104,863
Net realized capital gains		59,651	35,893
Other than temporary impairment losses		(3,217)	(20,759)
Other revenue		151,292	137,388
Other operating expenses		175,138	162,164
Corporate administration		16,885	9,734
Amortization of intangible assets		3,764	3,085
Interest expense		20,935	20,269
Earnings (losses) before income taxes		207,910	212,321
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		1,426,400	1,496,300
Net premiums written		1,233,100	1,322,200
Net premiums earned		1,209,200	1,221,500
Net loss and LAE		699,300	664,600
Commissions, brokerage and other underwriting expenses		408,500	406,700
Underwriting profit (loss)	[1]	101,400	150,200
Net investment income		112,200	102,800
Net realized capital gains		60,200	39,400
Other than temporary impairment losses		(3,200)	(20,800)
Other revenue		2,500	200
Other operating expenses		23,200	22,000
Corporate administration		800	200
Amortization of intangible assets		(500)	(1,000)
Interest expense		6,800	6,900
Earnings (losses) before income taxes		242,800	243,700
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,055,700	1,142,400
Net premiums written		949,400	1,055,900
Net premiums earned		926,100	939,000
Net loss and LAE		557,100	526,100
Commissions, brokerage and other underwriting expenses		315,600	317,400
Underwriting profit (loss)	[1]	53,400	95,500
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		375,400	383,200
Net premiums written		285,200	309,500
Net premiums earned		286,800	253,100
Net loss and LAE		121,800	96,100
Commissions, brokerage and other underwriting expenses		90,800	77,900
Underwriting profit (loss)	[1]	74,200	79,100
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	680,300	759,200
Net premiums written	[2]	664,200	746,400
Net premiums earned	[2]	639,300	685,900
Net loss and LAE	[2]	435,300	430,000
Commissions, brokerage and other underwriting expenses	[2]	224,800	239,500
Underwriting profit (loss)	[1],[2]	(20,800)	16,400
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		370,700	353,900
Net premiums written		283,700	266,300
Net premiums earned		283,100	282,500
Net loss and LAE		142,200	138,500
Commissions, brokerage and other underwriting expenses		92,900	89,300
Underwriting profit (loss)	[1]	48,000	54,700
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		264,400	256,900
Net premiums written		182,600	173,600
Net premiums earned		182,700	192,300
Net loss and LAE		80,600	85,100
Commissions, brokerage and other underwriting expenses		54,800	53,200
Underwriting profit (loss)	[1]	47,300	54,000
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		65,600	62,900
Net premiums written		61,000	58,900
Net premiums earned		62,100	55,400
Net loss and LAE		33,100	26,900
Commissions, brokerage and other underwriting expenses		27,700	26,400
Underwriting profit (loss)	[1]	1,300	2,100
Operating Segments | Insurance Segment | Pacific Comp
Segment Reporting Information [Line Items]
Gross premiums written		40,700	34,100
Net premiums written		40,100	33,800
Net premiums earned		38,300	34,800
Net loss and LAE		28,500	26,500
Commissions, brokerage and other underwriting expenses		10,400	9,700
Underwriting profit (loss)	[1]	(600)	(1,400)
Other activities | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Net investment income		400	(100)
Net realized capital gains		(100)	(100)
Other revenue		147,200	136,900
Other operating expenses		150,500	139,500
Amortization of intangible assets		4,300	4,100
Interest expense		900	300
Earnings (losses) before income taxes		(8,200)	(7,200)
Other activities | Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written	[3]	(5,600)	(6,700)
Net investment income	[3]	2,900	2,200
Net realized capital gains	[3]	(500)	(3,400)
Other revenue	[3]	1,600	300
Other operating expenses	[3]	1,400	700
Corporate administration	[3]	16,100	9,500
Interest expense	[3]	13,200	13,100
Earnings (losses) before income taxes	[3]	$ (26,700)	$ (24,200)

[1] 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 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.

[2] 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.

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