Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Gross premiums written		$ 2,050,400	$ 1,770,600
Net premiums written		1,754,800	1,533,700
Net premiums earned		1,604,763	1,444,420
Net loss and LAE		1,112,070	1,027,444
Commissions, brokerage and other underwriting expenses		476,036	441,396
Underwriting (loss) profit	[1]	16,700	(24,400)
Net investment income		153,484	111,982
Change in the fair value of equity securities		112,728	(522,976)
Net realized capital gains		12,931	13,026
Change in allowance for credit losses on available for sale securities		1,972	(31,375)
Noninsurance revenue		767,835	467,849
Other operating expenses		723,889	466,661
Corporate administration		9,558	(14,307)
Amortization of intangible assets		11,478	9,568
Interest expense		23,752	18,356
Earnings (losses) before income taxes		296,930	(466,192)
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		2,061,300	1,778,900
Net premiums written		1,754,800	1,533,700
Net premiums earned		1,604,800	1,444,400
Net loss and LAE		1,112,100	1,027,400
Commissions, brokerage and other underwriting expenses		476,000	441,400
Underwriting (loss) profit	[1]	16,700	(24,400)
Net investment income		123,100	111,500
Change in the fair value of equity securities		85,300	(521,600)
Net realized capital gains		11,400	50,000
Change in allowance for credit losses on available for sale securities		1,900	(30,700)
Noninsurance revenue		767,700	464,700
Other operating expenses		723,600	460,600
Corporate administration		(500)	(2,000)
Amortization of intangible assets		11,500	9,600
Interest expense		10,600	10,500
Earnings (losses) before income taxes		260,900	(429,200)
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,538,200	1,327,500
Net premiums written		1,394,900	1,211,000
Net premiums earned		1,229,900	1,133,100
Net loss and LAE		848,900	862,200
Commissions, brokerage and other underwriting expenses		376,000	349,800
Underwriting (loss) profit	[1]	5,000	(78,900)
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		565,300	412,600
Net premiums written		459,000	312,900
Net premiums earned		371,300	321,400
Net loss and LAE		323,500	247,900
Commissions, brokerage and other underwriting expenses		109,200	105,400
Underwriting (loss) profit	[1]	(61,400)	(31,900)
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	972,900	914,900
Net premiums written	[2]	935,900	898,100
Net premiums earned	[2]	858,600	811,700
Net loss and LAE	[2]	525,400	614,300
Commissions, brokerage and other underwriting expenses	[2]	266,800	244,400
Underwriting (loss) profit	[1],[2]	66,400	(47,000)
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		523,100	451,400
Net premiums written		359,900	322,700
Net premiums earned		374,900	311,300
Net loss and LAE		263,200	165,200
Commissions, brokerage and other underwriting expenses		100,000	91,600
Underwriting (loss) profit	[1]	11,700	54,500
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		420,900	365,900
Net premiums written		272,500	244,500
Net premiums earned		283,600	229,400
Net loss and LAE		208,200	116,000
Commissions, brokerage and other underwriting expenses		64,100	57,800
Underwriting (loss) profit	[1]	11,300	55,600
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		102,200	85,500
Net premiums written		87,400	78,200
Net premiums earned		91,300	81,900
Net loss and LAE		55,000	49,200
Commissions, brokerage and other underwriting expenses		35,900	33,800
Underwriting (loss) profit	[1]	400	(1,100)
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		2,061,300	1,778,900
Net premiums written		1,754,800	1,533,700
Net premiums earned		1,604,800	1,444,400
Net loss and LAE		1,112,100	1,027,400
Commissions, brokerage and other underwriting expenses		476,000	441,400
Underwriting (loss) profit	[1]	16,700	(24,400)
Net investment income		123,100	109,700
Change in the fair value of equity securities		85,300	(521,600)
Net realized capital gains		10,500	45,100
Change in allowance for credit losses on available for sale securities		1,900	(30,700)
Noninsurance revenue		8,600	7,400
Other operating expenses		14,400	9,900
Corporate administration		(500)	(2,000)
Amortization of intangible assets		200	400
Interest expense		6,800	6,700
Earnings (losses) before income taxes		225,200	(429,500)
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Gross premiums written		0	0
Net premiums written		0	0
Net premiums earned		0	0
Net loss and LAE		0	0
Commissions, brokerage and other underwriting expenses		0	0
Underwriting (loss) profit	[1]	0	0
Net investment income		0	1,800
Change in the fair value of equity securities		0	0
Net realized capital gains		900	4,900
Change in allowance for credit losses on available for sale securities		0	0
Noninsurance revenue		759,100	457,300
Other operating expenses		709,200	450,700
Corporate administration		0	0
Amortization of intangible assets		11,300	9,200
Interest expense		3,800	3,800
Earnings (losses) before income taxes		35,700	300
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written		(10,900)	(8,300)
Net premiums written		0	0
Net premiums earned		0	0
Net loss and LAE		0	0
Commissions, brokerage and other underwriting expenses		0	0
Underwriting (loss) profit	[1]	0	0
Net investment income		30,400	500
Change in the fair value of equity securities		27,400	(1,400)
Net realized capital gains		1,500	(37,000)
Change in allowance for credit losses on available for sale securities		100	(700)
Noninsurance revenue		100	3,200
Other operating expenses		300	6,100
Corporate administration		10,100	(12,300)
Amortization of intangible assets		0	0
Interest expense		13,100	7,800
Earnings (losses) before income taxes		$ 36,000	$ (37,000)

[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.