Segment Results for Reportable Segments and Other Activities (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,489,600	$ 1,258,900
Net premiums written		1,322,200	1,091,100
Net premiums earned		1,221,565	1,017,171
Net loss and LAE		664,644	546,916
Commissions, brokerage and other underwriting expenses		406,733	343,609
Underwriting profit (loss)	[1]	150,200	126,700
Net investment income		104,863	113,382
Net realized capital gains		35,893	43,151
Other than temporary impairment losses		(20,759)	(52,281)
Other revenue		137,388	36,200
Other operating expenses		162,164	67,405
Corporate administration		9,734	12,678
Amortization of intangible assets		3,085	(1,660)
Interest expense		20,269	23,092
Earnings (losses) before income taxes		212,321	165,583
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		1,496,300	1,266,400
Net premiums written		1,322,200	1,091,100
Net premiums earned		1,221,500	1,017,200
Net loss and LAE		664,600	546,900
Commissions, brokerage and other underwriting expenses		406,700	343,600
Underwriting profit (loss)	[1]	150,200	126,700
Net investment income		102,800	110,800
Net realized capital gains		39,400	48,800
Other than temporary impairment losses		(20,800)	(52,300)
Other revenue		200	1,400
Other operating expenses		22,000	26,600
Corporate administration		200	300
Amortization of intangible assets		(1,000)	(1,800)
Interest expense		6,900	9,700
Earnings (losses) before income taxes		243,700	200,600
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,142,400	902,200
Net premiums written		1,055,900	828,800
Net premiums earned		939,000	746,900
Net loss and LAE		526,100	406,400
Commissions, brokerage and other underwriting expenses		317,400	256,700
Underwriting profit (loss)	[1]	95,500	83,800
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		383,200	271,600
Net premiums written		309,500	210,100
Net premiums earned		253,100	215,600
Net loss and LAE		96,100	63,200
Commissions, brokerage and other underwriting expenses		77,900	66,900
Underwriting profit (loss)	[1]	79,100	85,500
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	759,200	630,600
Net premiums written	[2]	746,400	618,700
Net premiums earned	[2]	685,900	531,300
Net loss and LAE	[2]	430,000	343,200
Commissions, brokerage and other underwriting expenses	[2]	239,500	189,800
Underwriting profit (loss)	[1],[2]	16,400	(1,700)
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		353,900	364,200
Net premiums written		266,300	262,300
Net premiums earned		282,500	270,300
Net loss and LAE		138,500	140,500
Commissions, brokerage and other underwriting expenses		89,300	86,900
Underwriting profit (loss)	[1]	54,700	42,900
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		256,900	288,200
Net premiums written		173,600	190,500
Net premiums earned		192,300	203,100
Net loss and LAE		85,100	100,800
Commissions, brokerage and other underwriting expenses		53,200	55,300
Underwriting profit (loss)	[1]	54,000	47,000
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		62,900	54,500
Net premiums written		58,900	50,600
Net premiums earned		55,400	47,600
Net loss and LAE		26,900	24,400
Commissions, brokerage and other underwriting expenses		26,400	22,800
Underwriting profit (loss)	[1]	2,100	400
Operating Segments | Insurance Segment | Pacific Comp
Segment Reporting Information [Line Items]
Gross premiums written		34,100	21,500
Net premiums written		33,800	21,200
Net premiums earned		34,800	19,600
Net loss and LAE		26,500	15,300
Commissions, brokerage and other underwriting expenses		9,700	8,800
Underwriting profit (loss)	[1]	(1,400)	(4,500)
Other activities | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Net investment income		(100)	1,000
Net realized capital gains		(100)	(100)
Other revenue		136,900	34,600
Other operating expenses		139,500	40,200
Amortization of intangible assets		4,100	100
Interest expense		300	400
Earnings (losses) before income taxes		(7,200)	(5,200)
Other activities | Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written	[3]	(6,700)	(7,500)
Net investment income	[3]	2,200	1,600
Net realized capital gains	[3]	(3,400)	(5,600)
Other revenue	[3]	300	200
Other operating expenses	[3]	700	600
Corporate administration	[3]	9,500	12,400
Interest expense	[3]	13,100	13,000
Earnings (losses) before income taxes	[3]	$ (24,200)	$ (29,800)

[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 (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.

[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.