Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Gross premiums written		$ 2,147,000	$ 2,050,400
Net premiums written		1,792,800	1,754,800
Net premiums earned		1,605,458	1,604,763
Net loss and LAE		941,787	1,112,070
Commissions, brokerage and other underwriting expenses		477,165	476,036
Underwriting (loss) profit	[1]	186,500	16,700
Net investment income		113,476	153,484
Change in the fair value of equity securities		(138,777)	112,728
Net realized capital gains		(10,003)	12,931
Change in allowance for credit losses on available for sale securities		(582)	1,972
Product and service revenues		1,149,162	767,835
Other operating expenses		1,028,892	723,889
Corporate administration		10,723	9,558
Amortization of intangible assets		14,082	11,478
Interest expense		32,110	23,752
Earnings before income taxes		213,975	296,930
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		2,158,000	2,061,300
Net premiums written		1,792,800	1,754,800
Net premiums earned		1,605,500	1,604,800
Net loss and LAE		941,800	1,112,100
Commissions, brokerage and other underwriting expenses		477,200	476,000
Underwriting (loss) profit	[1]	186,500	16,700
Net investment income		108,500	123,100
Change in the fair value of equity securities		(154,100)	85,300
Net realized capital gains		(10,000)	11,400
Change in allowance for credit losses on available for sale securities		(600)	1,900
Product and service revenues		1,129,400	767,700
Other operating expenses		1,026,900	723,600
Corporate administration		(0)	(500)
Amortization of intangible assets		14,100	11,500
Interest expense		12,400	10,600
Earnings before income taxes		206,300	260,900
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,528,400	1,538,200
Net premiums written		1,358,000	1,394,900
Net premiums earned		1,162,900	1,229,900
Net loss and LAE		723,000	848,900
Commissions, brokerage and other underwriting expenses		368,300	376,000
Underwriting (loss) profit	[1]	71,600	5,000
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		454,400	565,300
Net premiums written		346,600	459,000
Net premiums earned		291,500	371,300
Net loss and LAE		162,100	323,500
Commissions, brokerage and other underwriting expenses		86,700	109,200
Underwriting (loss) profit	[1]	42,700	(61,400)
Operating Segments | Reinsurance Segment | Casualty & Specialty
Segment Reporting Information [Line Items]
Gross premiums written	[2]	1,074,000	972,900
Net premiums written	[2]	1,011,400	935,900
Net premiums earned	[2]	871,400	858,600
Net loss and LAE	[2]	560,900	525,400
Commissions, brokerage and other underwriting expenses	[2]	281,600	266,800
Underwriting (loss) profit	[1],[2]	28,900	66,400
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		629,600	523,100
Net premiums written		434,800	359,900
Net premiums earned		442,600	374,900
Net loss and LAE		218,800	263,200
Commissions, brokerage and other underwriting expenses		108,900	100,000
Underwriting (loss) profit	[1]	114,900	11,700
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		517,400	420,900
Net premiums written		342,100	272,500
Net premiums earned		346,400	283,600
Net loss and LAE		159,400	208,200
Commissions, brokerage and other underwriting expenses		72,200	64,100
Underwriting (loss) profit	[1]	114,800	11,300
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		112,200	102,200
Net premiums written		92,700	87,400
Net premiums earned		96,200	91,300
Net loss and LAE		59,400	55,000
Commissions, brokerage and other underwriting expenses		36,700	35,900
Underwriting (loss) profit	[1]	100	400
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		2,158,000	2,061,300
Net premiums written		1,792,800	1,754,800
Net premiums earned		1,605,500	1,604,800
Net loss and LAE		941,800	1,112,100
Commissions, brokerage and other underwriting expenses		477,200	476,000
Underwriting (loss) profit	[1]	186,500	16,700
Net investment income		108,500	123,100
Change in the fair value of equity securities		(154,100)	85,300
Net realized capital gains		(16,100)	10,500
Change in allowance for credit losses on available for sale securities		(600)	1,900
Product and service revenues		8,000	8,600
Other operating expenses		13,600	14,400
Corporate administration		(0)	(500)
Amortization of intangible assets		400	200
Interest expense		6,700	6,800
Earnings before income taxes		111,500	225,200
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	0
Change in the fair value of equity securities		0	0
Net realized capital gains		6,100	900
Change in allowance for credit losses on available for sale securities		0	0
Product and service revenues		1,121,400	759,100
Other operating expenses		1,013,300	709,200
Corporate administration		0	0
Amortization of intangible assets		13,700	11,300
Interest expense		5,700	3,800
Earnings before income taxes		94,800	35,700
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written		(11,000)	(10,900)
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		5,000	30,400
Change in the fair value of equity securities		15,200	27,400
Net realized capital gains		0	1,500
Change in allowance for credit losses on available for sale securities		0	100
Product and service revenues		19,800	100
Other operating expenses		1,900	300
Corporate administration		10,700	10,100
Amortization of intangible assets		0	0
Interest expense		19,700	13,100
Earnings before income taxes		$ 7,700	$ 36,000

[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, product and service revenues, 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.