Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,651,100	$ 1,475,300	$ 4,928,500	$ 4,406,700
Net premiums written		1,413,900	1,239,100	4,246,000	3,768,300
Net premiums earned		1,389,981	1,225,346	4,043,298	3,670,161
Net loss and LAE		907,736	957,703	2,497,037	2,366,491
Commissions, brokerage and other underwriting expenses		449,519	407,679	1,313,961	1,216,057
Underwriting (loss) profit	[1]	32,800	(140,000)	232,300	87,700
Net investment income		147,829	127,329	413,623	377,728
Change in the fair value of equity securities		(16,691)	370,175	519,322	512,771
Net realized capital gains		3,957	16,230	20,753	67,197
Other than temporary impairment losses		(3,597)	(3)	(13,617)	(514)
Noninsurance revenue		638,478	438,338	1,756,507	1,032,690
Other operating expenses		617,326	415,378	1,701,218	1,023,440
Corporate administration		22,276	19,094	67,612	40,998
Amortization of intangible assets		8,095	5,500	23,790	16,730
Interest expense		25,703	22,189	74,363	65,997
Earnings before income taxes		129,302	349,872	1,061,905	930,320
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		1,658,600	1,482,000	4,949,000	4,425,900
Net premiums written		1,413,900	1,239,100	4,246,000	3,768,300
Net premiums earned		1,390,000	1,225,400	4,043,300	3,670,200
Net loss and LAE		907,700	957,700	2,497,000	2,366,500
Commissions, brokerage and other underwriting expenses		449,500	407,700	1,314,000	1,216,000
Underwriting (loss) profit	[1]	32,800	(140,000)	232,300	87,700
Net investment income		146,300	123,300	404,900	365,700
Change in the fair value of equity securities		(16,700)	373,900	515,700	506,700
Net realized capital gains		3,900	16,200	20,700	67,400
Other than temporary impairment losses		(3,600)		(13,600)	(500)
Noninsurance revenue		635,500	413,700	1,746,300	995,900
Other operating expenses		610,700	406,100	1,680,300	997,600
Corporate administration		1,500	1,300	4,400	1,800
Amortization of intangible assets		8,100	5,500	23,800	16,800
Interest expense		12,000	9,300	34,500	26,600
Earnings before income taxes		165,900	364,900	1,163,300	980,100
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,243,300	1,137,300	3,685,600	3,324,600
Net premiums written		1,111,700	985,200	3,332,100	2,958,900
Net premiums earned		1,097,600	961,500	3,201,700	2,903,000
Net loss and LAE		730,700	809,000	2,028,900	1,942,000
Commissions, brokerage and other underwriting expenses		360,900	324,900	1,050,800	964,800
Underwriting (loss) profit	[1]	6,000	(172,400)	122,000	(3,800)
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		455,900	451,400	1,252,600	1,193,700
Net premiums written		339,000	331,100	961,100	912,100
Net premiums earned		327,000	326,500	939,100	893,700
Net loss and LAE		225,100	387,600	523,800	637,700
Commissions, brokerage and other underwriting expenses		108,300	113,300	315,800	301,200
Underwriting (loss) profit	[1]	(6,400)	(174,400)	99,500	(45,200)
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	787,400	685,900	2,433,000	2,130,900
Net premiums written	[2]	772,700	654,100	2,371,000	2,046,800
Net premiums earned	[2]	770,600	635,000	2,262,600	2,009,300
Net loss and LAE	[2]	505,600	421,400	1,505,100	1,304,300
Commissions, brokerage and other underwriting expenses	[2]	252,600	211,600	735,000	663,600
Underwriting (loss) profit	[1],[2]	12,400	2,000	22,500	41,400
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		415,300	344,700	1,263,400	1,101,300
Net premiums written		302,200	253,900	913,900	809,400
Net premiums earned		292,400	263,900	841,600	767,200
Net loss and LAE		177,000	148,700	468,100	424,500
Commissions, brokerage and other underwriting expenses		88,600	82,800	263,200	251,200
Underwriting (loss) profit	[1]	26,800	32,400	110,300	91,500
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		319,700	260,800	991,100	854,200
Net premiums written		213,400	176,000	661,700	579,800
Net premiums earned		208,900	190,600	603,700	556,200
Net loss and LAE		125,900	107,000	325,800	309,600
Commissions, brokerage and other underwriting expenses		55,700	52,800	167,200	160,000
Underwriting (loss) profit	[1]	27,300	30,800	110,700	86,600
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		95,600	83,900	272,300	247,100
Net premiums written		88,800	77,900	252,200	229,600
Net premiums earned		83,500	73,300	237,900	211,000
Net loss and LAE		51,100	41,700	142,300	114,900
Commissions, brokerage and other underwriting expenses		32,900	30,000	96,000	91,200
Underwriting (loss) profit	[1]	(500)	1,600	(400)	4,900
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,658,600	1,482,000	4,949,000	4,425,900
Net premiums written		1,413,900	1,239,100	4,246,000	3,768,300
Net premiums earned		1,390,000	1,225,400	4,043,300	3,670,200
Net loss and LAE		907,700	957,700	2,497,000	2,366,500
Commissions, brokerage and other underwriting expenses		449,500	407,700	1,314,000	1,216,000
Underwriting (loss) profit	[1]	32,800	(140,000)	232,300	87,700
Net investment income		145,100	122,500	401,500	362,000
Change in the fair value of equity securities		(16,700)	373,900	515,700	506,700
Net realized capital gains		4,100	16,200	20,400	66,800
Other than temporary impairment losses		(3,600)		(13,600)	(500)
Noninsurance revenue		7,500	6,200	19,500	16,700
Other operating expenses		32,500	23,600	88,300	60,600
Corporate administration		1,500	1,300	4,400	1,800
Amortization of intangible assets		400	(300)	1,000	(200)
Interest expense		6,800	6,700	20,300	20,500
Earnings before income taxes		128,000	347,500	1,061,800	956,700
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Net investment income	[3]	1,200	800	3,400	3,700
Net realized capital gains	[3]	(200)		300	600
Noninsurance revenue	[3]	628,000	407,500	1,726,800	979,200
Other operating expenses	[3]	578,200	382,500	1,592,000	937,000
Amortization of intangible assets	[3]	7,700	5,800	22,800	17,000
Interest expense	[3]	5,200	2,600	14,200	6,100
Earnings before income taxes	[3]	37,900	17,400	101,500	23,400
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written	[4]	(7,500)	(6,700)	(20,500)	(19,200)
Net investment income	[4]	1,500	4,000	8,700	12,000
Change in the fair value of equity securities	[4]		(3,700)	3,600	6,100
Net realized capital gains	[4]			100	(200)
Noninsurance revenue	[4]	3,000	24,600	10,200	36,800
Other operating expenses	[4]	6,600	9,200	20,900	25,900
Corporate administration	[4]	20,800	17,800	63,200	39,200
Interest expense	[4]	13,700	12,900	39,900	39,400
Earnings before income taxes	[4]	$ (36,600)	$ (15,000)	$ (101,400)	$ (49,800)

[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, OTTI losses, 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.

[3] Excludes certain minor, legacy investments that were previously reflected in Alleghany Capital in 2018 and prior periods, to align with management’s view of reportable segments.

[4] Includes elimination of minor reinsurance activity between segments. Also, includes certain minor, legacy investments that were previously reflected in Alleghany Capital in 2018 and prior periods.