Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,458,700	$ 1,420,800
Net premiums written		1,253,300	1,233,100
Net premiums earned		1,207,856	1,209,188
Net loss and LAE		670,578	699,305
Commissions, brokerage and other underwriting expenses		406,295	408,515
Underwriting (loss) profit	[1]	130,900	101,400
Net investment income		124,126	115,538
Change in the fair value of equity securities		(42,649)
Net realized capital gains		44,505	59,651
Other than temporary impairment losses		(511)	(3,217)
Noninsurance revenue		251,627	151,292
Other operating expenses		264,897	175,138
Corporate administration		7,785	16,885
Amortization of intangible assets		5,264	3,764
Interest expense		21,531	20,935
(Losses) earnings before income taxes		208,604	207,910
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		1,464,700	1,426,400
Net premiums written		1,253,300	1,233,100
Net premiums earned		1,207,800	1,209,200
Net loss and LAE		670,600	699,300
Commissions, brokerage and other underwriting expenses		406,300	408,500
Underwriting (loss) profit	[1]	130,900	101,400
Net investment income		120,300	112,600
Change in the fair value of equity securities		(13,900)
Net realized capital gains		44,500	60,100
Other than temporary impairment losses		(500)	(3,200)
Noninsurance revenue		248,400	147,200
Other operating expenses		257,000	164,700
Corporate administration		(300)	800
Amortization of intangible assets		5,300	3,800
Interest expense		8,000	7,700
(Losses) earnings before income taxes		259,700	241,100
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,115,600	1,055,700
Net premiums written		997,000	949,400
Net premiums earned		960,200	926,100
Net loss and LAE		539,400	557,100
Commissions, brokerage and other underwriting expenses		322,000	315,600
Underwriting (loss) profit	[1]	98,800	53,400
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		397,200	375,400
Net premiums written		305,100	285,200
Net premiums earned		275,100	286,800
Net loss and LAE		96,600	121,800
Commissions, brokerage and other underwriting expenses		93,100	90,800
Underwriting (loss) profit	[1]	85,400	74,200
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	718,400	680,300
Net premiums written	[2]	691,900	664,200
Net premiums earned	[2]	685,100	639,300
Net loss and LAE	[2]	442,800	435,300
Commissions, brokerage and other underwriting expenses	[2]	228,900	224,800
Underwriting (loss) profit	[1],[2]	13,400	(20,800)
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		349,100	370,700
Net premiums written		256,300	283,700
Net premiums earned		247,600	283,100
Net loss and LAE		131,200	142,200
Commissions, brokerage and other underwriting expenses		84,300	92,900
Underwriting (loss) profit	[1]	32,100	48,000
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		274,600	264,400
Net premiums written		187,000	182,600
Net premiums earned		180,900	182,700
Net loss and LAE		96,900	80,600
Commissions, brokerage and other underwriting expenses		53,500	54,800
Underwriting (loss) profit	[1]	30,500	47,300
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		74,500	65,600
Net premiums written		69,300	61,000
Net premiums earned		66,700	62,100
Net loss and LAE		34,300	33,100
Commissions, brokerage and other underwriting expenses		30,800	27,700
Underwriting (loss) profit	[1]	1,600	1,300
Operating Segments | Insurance Segment | Pacific Comp
Segment Reporting Information [Line Items]
Gross premiums written			40,700
Net premiums written			40,100
Net premiums earned			38,300
Net loss and LAE			28,500
Commissions, brokerage and other underwriting expenses			10,400
Underwriting (loss) profit	[1]		(600)
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,464,700	1,426,400
Net premiums written		1,253,300	1,233,100
Net premiums earned		1,207,800	1,209,200
Net loss and LAE		670,600	699,300
Commissions, brokerage and other underwriting expenses		406,300	408,500
Underwriting (loss) profit	[1]	130,900	101,400
Net investment income		118,400	112,200
Change in the fair value of equity securities		(13,900)
Net realized capital gains		43,800	60,200
Other than temporary impairment losses		(500)	(3,200)
Noninsurance revenue		5,700	2,500
Other operating expenses		17,000	23,200
Corporate administration		(300)	800
Amortization of intangible assets		(100)	(500)
Interest expense		6,700	6,800
(Losses) earnings before income taxes		261,100	242,800
Operating Segments | Alleghany Capital
Segment Reporting Information [Line Items]
Net investment income		1,900	400
Net realized capital gains		700	(100)
Noninsurance revenue		242,700	144,700
Other operating expenses		240,000	141,500
Amortization of intangible assets		5,400	4,300
Interest expense		1,300	900
(Losses) earnings before income taxes		(1,400)	(1,700)
Corporate Activities
Segment Reporting Information [Line Items]
Gross premiums written	[3]	(6,000)	(5,600)
Net investment income	[3]	3,800	2,900
Change in the fair value of equity securities	[3]	(28,600)
Net realized capital gains	[3]		(500)
Noninsurance revenue	[3]	3,200	4,100
Other operating expenses	[3]	7,900	10,400
Corporate administration	[3]	8,100	16,100
Interest expense	[3]	13,500	13,200
(Losses) earnings before income taxes	[3]	$ (51,100)	$ (33,200)

[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, change in the fair value of equity securities, net realized capital gains, OTTI losses, noninsurance 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.

[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.