Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,770,600	$ 1,606,600
Net premiums written		1,533,700	1,385,700
Net premiums earned		1,444,420	1,297,309
Net loss and LAE		1,027,444	792,498
Commissions, brokerage and other underwriting expenses		441,396	420,126
Underwriting (loss) profit	[1]	(24,400)	84,700
Net investment income		111,982	123,049
Change in the fair value of equity securities		(522,976)	392,353
Net realized capital gains		13,026	4,442
Credit losses for available for sale securities		(31,375)
Credit losses for available for sale securities			(10,020)
Noninsurance revenue		467,849	514,123
Other operating expenses		466,661	502,418
Corporate administration		(14,307)	19,369
Amortization of intangible assets		9,568	6,900
Interest expense		18,356	23,531
(Losses) earnings before income taxes		(466,192)	556,414
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		1,778,900	1,613,400
Net premiums written		1,533,700	1,385,700
Net premiums earned		1,444,400	1,297,300
Net loss and LAE		1,027,400	792,500
Commissions, brokerage and other underwriting expenses		441,400	420,100
Underwriting (loss) profit	[1]	(24,400)	84,700
Net investment income		111,500	119,200
Change in the fair value of equity securities		(521,600)	389,100
Net realized capital gains		50,000	4,400
Credit losses for available for sale securities		(30,700)
Credit losses for available for sale securities			(10,000)
Noninsurance revenue		464,700	510,400
Other operating expenses		460,600	495,100
Corporate administration		(2,000)	1,100
Amortization of intangible assets		9,600	6,900
Interest expense		10,500	11,000
(Losses) earnings before income taxes		(429,200)	583,700
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,327,500	1,231,600
Net premiums written		1,211,000	1,107,600
Net premiums earned		1,133,100	1,029,200
Net loss and LAE		862,200	653,200
Commissions, brokerage and other underwriting expenses		349,800	336,000
Underwriting (loss) profit	[1]	(78,900)	40,000
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		412,600	415,100
Net premiums written		312,900	316,200
Net premiums earned		321,400	308,900
Net loss and LAE		247,900	162,600
Commissions, brokerage and other underwriting expenses		105,400	103,700
Underwriting (loss) profit	[1]	(31,900)	42,600
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	914,900	816,500
Net premiums written	[2]	898,100	791,400
Net premiums earned	[2]	811,700	720,300
Net loss and LAE	[2]	614,300	490,600
Commissions, brokerage and other underwriting expenses	[2]	244,400	232,300
Underwriting (loss) profit	[1],[2]	(47,000)	(2,600)
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		451,400	381,800
Net premiums written		322,700	278,100
Net premiums earned		311,300	268,100
Net loss and LAE		165,200	139,300
Commissions, brokerage and other underwriting expenses		91,600	84,100
Underwriting (loss) profit	[1]	54,500	44,700
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		365,900	299,000
Net premiums written		244,500	201,400
Net premiums earned		229,400	192,400
Net loss and LAE		116,000	94,300
Commissions, brokerage and other underwriting expenses		57,800	53,500
Underwriting (loss) profit	[1]	55,600	44,600
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		85,500	82,800
Net premiums written		78,200	76,700
Net premiums earned		81,900	75,700
Net loss and LAE		49,200	45,000
Commissions, brokerage and other underwriting expenses		33,800	30,600
Underwriting (loss) profit	[1]	(1,100)	100
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,778,900	1,613,400
Net premiums written		1,533,700	1,385,700
Net premiums earned		1,444,400	1,297,300
Net loss and LAE		1,027,400	792,500
Commissions, brokerage and other underwriting expenses		441,400	420,100
Underwriting (loss) profit	[1]	(24,400)	84,700
Net investment income		109,700	117,900
Change in the fair value of equity securities		(521,600)	389,100
Net realized capital gains		45,100	4,100
Credit losses for available for sale securities		(30,700)
Credit losses for available for sale securities			(10,000)
Noninsurance revenue		7,400	5,400
Other operating expenses		9,900	29,700
Corporate administration		(2,000)	1,100
Amortization of intangible assets		400	200
Interest expense		6,700	6,700
(Losses) earnings before income taxes		(429,500)	553,500
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Net investment income		1,800	1,300
Net realized capital gains		4,900	300
Noninsurance revenue		457,300	505,000
Other operating expenses		450,700	465,400
Amortization of intangible assets		9,200	6,700
Interest expense		3,800	4,300
(Losses) earnings before income taxes		300	30,200
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written		(8,300)	(6,800)
Net investment income		500	3,800
Change in the fair value of equity securities		(1,400)	3,300
Net realized capital gains		(37,000)
Credit losses for available for sale securities		(700)
Noninsurance revenue		3,200	3,700
Other operating expenses		6,100	7,300
Corporate administration		(12,300)	18,300
Interest expense		7,800	12,500
(Losses) earnings before income taxes		$ (37,000)	$ (27,300)

[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, credit losses for 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.