Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,721,200	$ 1,670,800	$ 3,491,800	$ 3,277,400
Net premiums written		1,478,000	1,446,500	3,011,700	2,832,100
Net premiums earned		1,394,868	1,356,008	2,839,288	2,653,317
Net loss and LAE		1,024,362	796,803	2,051,806	1,589,301
Commissions, brokerage and other underwriting expenses		410,017	444,316	851,413	864,442
Underwriting (loss) profit	[1]	(39,500)	114,900	(63,900)	199,600
Net investment income		118,691	142,745	230,673	265,794
Change in the fair value of equity securities		242,186	143,660	(280,790)	536,013
Net realized capital gains		(38,100)	12,354	(25,074)	16,796
Change in allowance for credit losses on available for sale securities		17,021		(14,354)
Change in allowance for credit losses on available for sale securities					(10,020)
Noninsurance revenue		492,701	603,906	960,550	1,118,029
Other operating expenses		510,306	581,474	976,967	1,083,892
Corporate administration		18,579	25,967	4,272	45,336
Amortization of intangible assets		12,093	8,795	21,661	15,695
Interest expense		22,194	25,129	40,550	48,660
(Losses) earnings before income taxes		229,816	376,189	(236,376)	932,603
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		1,729,800	1,676,900	3,508,700	3,290,300
Net premiums written		1,478,000	1,446,500	3,011,700	2,832,100
Net premiums earned		1,394,900	1,356,000	2,839,300	2,653,300
Net loss and LAE		1,024,400	796,800	2,051,800	1,589,300
Commissions, brokerage and other underwriting expenses		410,000	444,300	851,400	864,400
Underwriting (loss) profit	[1]	(39,500)	114,900	(63,900)	199,600
Net investment income		115,400	139,200	226,800	258,500
Change in the fair value of equity securities		242,200	143,300	(279,400)	532,400
Net realized capital gains		6,300	12,400	56,500	16,800
Change in allowance for credit losses on available for sale securities		16,600		(14,100)
Change in allowance for credit losses on available for sale securities					(10,000)
Noninsurance revenue		491,500	600,300	956,100	1,110,700
Other operating expenses		506,900	574,500	967,400	1,069,600
Corporate administration		500	1,800	(1,600)	2,800
Amortization of intangible assets		12,100	8,800	21,700	15,700
Interest expense		11,100	11,500	21,600	22,600
(Losses) earnings before income taxes		301,900	413,500	(127,100)	997,300
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,153,900	1,210,700	2,481,400	2,442,300
Net premiums written		1,086,500	1,112,900	2,297,500	2,220,500
Net premiums earned		1,071,000	1,074,900	2,204,100	2,104,100
Net loss and LAE		778,200	645,100	1,640,400	1,298,300
Commissions, brokerage and other underwriting expenses		323,100	353,900	672,900	689,900
Underwriting (loss) profit	[1]	(30,300)	75,900	(109,200)	115,900
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		413,300	381,600	825,900	796,700
Net premiums written		355,800	305,900	668,700	622,100
Net premiums earned		328,700	303,200	650,100	612,100
Net loss and LAE		273,400	136,200	521,300	298,800
Commissions, brokerage and other underwriting expenses		96,800	103,800	202,200	207,500
Underwriting (loss) profit	[1]	(41,500)	63,200	(73,400)	105,800
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	740,600	829,100	1,655,500	1,645,600
Net premiums written	[2]	730,700	807,000	1,628,800	1,598,400
Net premiums earned	[2]	742,300	771,700	1,554,000	1,492,000
Net loss and LAE	[2]	504,800	508,900	1,119,100	999,500
Commissions, brokerage and other underwriting expenses	[2]	226,300	250,100	470,700	482,400
Underwriting (loss) profit	[1],[2]	11,200	12,700	(35,800)	10,100
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		575,900	466,200	1,027,300	848,000
Net premiums written		391,500	333,600	714,200	611,600
Net premiums earned		323,900	281,100	635,200	549,200
Net loss and LAE		246,200	151,700	411,400	291,000
Commissions, brokerage and other underwriting expenses		86,900	90,400	178,500	174,500
Underwriting (loss) profit	[1]	(9,200)	39,000	45,300	83,700
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		474,700	372,400	840,600	671,400
Net premiums written		300,000	246,900	544,500	448,300
Net premiums earned		241,500	202,400	470,900	394,800
Net loss and LAE		196,000	105,600	311,900	199,900
Commissions, brokerage and other underwriting expenses		52,700	57,900	110,600	111,400
Underwriting (loss) profit	[1]	(7,200)	38,900	48,400	83,500
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		101,200	93,800	186,700	176,600
Net premiums written		91,500	86,700	169,700	163,300
Net premiums earned		82,400	78,700	164,300	154,400
Net loss and LAE		50,200	46,100	99,500	91,100
Commissions, brokerage and other underwriting expenses		34,200	32,500	67,900	63,100
Underwriting (loss) profit	[1]	(2,000)	100	(3,100)	200
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,729,800	1,676,900	3,508,700	3,290,300
Net premiums written		1,478,000	1,446,500	3,011,700	2,832,100
Net premiums earned		1,394,900	1,356,000	2,839,300	2,653,300
Net loss and LAE		1,024,400	796,800	2,051,800	1,589,300
Commissions, brokerage and other underwriting expenses		410,000	444,300	851,400	864,400
Underwriting (loss) profit	[1]	(39,500)	114,900	(63,900)	199,600
Net investment income		115,400	138,300	225,000	256,300
Change in the fair value of equity securities		242,200	143,300	(279,400)	532,400
Net realized capital gains		(10,100)	12,200	35,100	16,200
Change in allowance for credit losses on available for sale securities		16,600		(14,100)
Change in allowance for credit losses on available for sale securities					(10,000)
Noninsurance revenue		8,300	6,600	15,600	12,000
Other operating expenses		33,300	26,200	43,100	55,800
Corporate administration		500	1,800	(1,600)	2,800
Amortization of intangible assets		200	400	500	600
Interest expense		6,800	6,800	13,500	13,600
(Losses) earnings before income taxes		292,100	380,100	(137,200)	933,700
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Net investment income			900	1,800	2,200
Net realized capital gains		16,400	200	21,400	600
Noninsurance revenue		483,200	593,700	940,500	1,098,700
Other operating expenses		473,600	548,300	924,300	1,013,800
Amortization of intangible assets		11,900	8,400	21,200	15,100
Interest expense		4,300	4,700	8,100	9,000
(Losses) earnings before income taxes		9,800	33,400	10,100	63,600
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written		(8,600)	(6,100)	(16,900)	(12,900)
Net investment income		3,300	3,500	3,900	7,300
Change in the fair value of equity securities			400	(1,400)	3,600
Net realized capital gains		(44,400)		(81,600)
Change in allowance for credit losses on available for sale securities		400		(300)
Noninsurance revenue		1,200	3,600	4,500	7,300
Other operating expenses		3,400	7,000	9,600	14,300
Corporate administration		18,100	24,200	5,800	42,500
Interest expense		11,100	13,600	19,000	26,100
(Losses) earnings before income taxes		$ (72,100)	$ (37,300)	$ (109,300)	$ (64,700)

[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, change in allowance for credit losses on available for sale securities, 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 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; mortgage reinsurance; surety; and credit.