Segment Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Disclosure [Line Items]
Gross premiums written		$ 1,168,000	$ 1,269,300	$ 3,735,800	$ 3,962,400
Net premiums written		1,013,200	1,127,500	3,250,400	3,471,000
Net premiums earned		1,049,071	1,136,215	3,140,965	3,289,156
Net loss and LAE		594,729	623,132	1,737,100	1,863,504
Commissions, brokerage and other underwriting expenses		360,203	372,291	1,067,766	1,056,131
Underwriting profit (loss)	[1]	94,200	140,800	336,100	369,500
Net investment income		118,159	118,284	334,628	342,961
Net realized capital gains		29,202	59,368	158,513	197,728
Other than temporary impairment losses		(52,680)	(720)	(112,278)	(6,872)
Other income		45,285	38,769	125,270	106,801
Other operating expenses		68,453	59,622	199,670	179,200
Corporate administration		10,922	9,455	33,441	31,774
Amortization of intangible assets		(1,104)	(1,142)	(3,815)	(3,879)
Interest expense		22,691	22,671	69,158	66,414
Earnings (losses) before income taxes		133,143	265,887	543,778	736,630
Operating Segments
Segment Reporting Disclosure [Line Items]
Gross premiums written		1,174,300	1,276,900	3,756,000	3,982,500
Net premiums written		1,013,200	1,127,500	3,250,400	3,471,000
Net premiums earned		1,049,100	1,136,200	3,141,000	3,289,200
Net loss and LAE		594,700	623,100	1,737,100	1,863,600
Commissions, brokerage and other underwriting expenses		360,200	372,300	1,067,800	1,056,100
Underwriting profit (loss)	[1]	94,200	140,800	336,100	369,500
Net investment income		113,400	113,000	327,400	334,700
Net realized capital gains		28,200	56,300	163,200	173,200
Other than temporary impairment losses		(44,300)	(700)	(103,900)	(6,900)
Other income		300	900	2,800	2,800
Other operating expenses		16,700	18,400	60,200	63,600
Corporate administration		200	100	600	400
Amortization of intangible assets		(1,200)	(1,200)	(4,100)	(4,200)
Interest expense		9,700	12,200	29,000	36,700
Earnings (losses) before income taxes		166,400	280,800	639,900	776,800
Operating Segments | Reinsurance Segment
Segment Reporting Disclosure [Line Items]
Gross premiums written		830,000	922,700	2,622,400	2,811,100
Net premiums written		753,100	866,700	2,412,400	2,636,500
Net premiums earned		766,900	859,200	2,313,400	2,484,500
Net loss and LAE		438,600	474,400	1,273,900	1,421,000
Commissions, brokerage and other underwriting expenses		271,500	284,600	805,900	798,500
Underwriting profit (loss)	[1]	56,800	100,200	233,600	265,000
Operating Segments | Reinsurance Segment | Property
Segment Reporting Disclosure [Line Items]
Gross premiums written		258,000	324,800	832,000	933,700
Net premiums written		200,600	286,100	663,100	808,000
Net premiums earned		207,100	278,200	659,400	771,400
Net loss and LAE		85,200	113,400	215,600	296,500
Commissions, brokerage and other underwriting expenses		70,400	87,500	214,100	233,600
Underwriting profit (loss)	[1]	51,500	77,300	229,700	241,300
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Disclosure [Line Items]
Gross premiums written	[2]	572,000	597,900	1,790,400	1,877,400
Net premiums written	[2]	552,500	580,600	1,749,300	1,828,500
Net premiums earned	[2]	559,800	581,000	1,654,000	1,713,100
Net loss and LAE	[2]	353,400	361,000	1,058,300	1,124,500
Commissions, brokerage and other underwriting expenses	[2]	201,100	197,100	591,800	564,900
Underwriting profit (loss)	[1],[2]	5,300	22,900	3,900	23,700
Operating Segments | Insurance Segment
Segment Reporting Disclosure [Line Items]
Gross premiums written		344,300	354,200	1,133,600	1,171,400
Net premiums written		260,100	260,800	838,000	834,500
Net premiums earned		282,200	277,000	827,600	804,700
Net loss and LAE		156,100	148,700	463,200	442,600
Commissions, brokerage and other underwriting expenses		88,700	87,700	261,900	257,600
Underwriting profit (loss)	[1]	37,400	40,600	102,500	104,500
Operating Segments | Insurance Segment | RSUI
Segment Reporting Disclosure [Line Items]
Gross premiums written		255,500	282,000	883,400	961,700
Net premiums written		176,800	193,400	601,100	641,300
Net premiums earned		202,400	210,900	607,000	620,500
Net loss and LAE		105,700	106,900	320,800	324,400
Commissions, brokerage and other underwriting expenses		55,400	56,500	165,900	165,800
Underwriting profit (loss)	[1]	41,300	47,500	120,300	130,300
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Disclosure [Line Items]
Gross premiums written		60,500	52,800	176,300	158,800
Net premiums written		55,200	48,200	163,800	143,000
Net premiums earned		52,400	48,000	149,900	136,100
Net loss and LAE		29,700	25,800	88,100	79,500
Commissions, brokerage and other underwriting expenses		24,000	23,000	68,500	68,100
Underwriting profit (loss)	[1]	(1,300)	(800)	(6,700)	(11,500)
Operating Segments | Insurance Segment | Pacific Comp
Segment Reporting Disclosure [Line Items]
Gross premiums written		28,300	19,400	73,900	50,900
Net premiums written		28,100	19,200	73,100	50,200
Net premiums earned		27,400	18,100	70,700	48,100
Net loss and LAE		20,700	16,000	54,300	38,700
Commissions, brokerage and other underwriting expenses		9,300	8,200	27,500	23,700
Underwriting profit (loss)	[1]	(2,600)	(6,100)	(11,100)	(14,300)
Corporate activities
Segment Reporting Disclosure [Line Items]
Gross premiums written	[3]	(6,300)	(7,600)	(20,200)	(20,100)
Net investment income	[3]	4,800	5,300	7,200	8,300
Net realized capital gains	[3]	1,000	3,100	(4,700)	24,500
Other than temporary impairment losses	[3]	(8,400)		(8,400)
Other income	[3]	45,000	37,900	122,500	104,000
Other operating expenses	[3]	51,800	41,200	139,500	115,600
Corporate administration	[3]	10,700	9,400	32,800	31,400
Amortization of intangible assets	[3]	100	100	300	300
Interest expense	[3]	13,100	10,500	40,100	29,700
Earnings (losses) before income taxes	[3]	$ (33,300)	$ (14,900)	$ (96,100)	$ (40,200)

[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 income, 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.