Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,670,800	$ 1,472,700	$ 3,277,400	$ 2,931,500
Net premiums written		1,446,500	1,275,800	2,832,100	2,529,200
Net premiums earned		1,356,008	1,236,959	2,653,317	2,444,815
Net loss and LAE		796,803	738,210	1,589,301	1,408,788
Commissions, brokerage and other underwriting expenses		444,316	402,083	864,442	808,378
Underwriting profit	[1]	114,900	96,700	199,600	227,600
Net investment income		142,745	126,273	265,794	250,399
Change in the fair value of equity securities		143,660	185,245	536,013	142,596
Net realized capital gains		12,354	6,462	16,796	50,967
Other than temporary impairment losses			0	(10,020)	(511)
Noninsurance revenue		603,906	342,725	1,118,029	594,352
Other operating expenses		581,474	343,165	1,083,892	608,062
Corporate administration		25,967	14,119	45,336	21,904
Amortization of intangible assets		8,795	5,966	15,695	11,230
Interest expense		25,129	22,277	48,660	43,808
Earnings before income taxes		376,189	371,844	932,603	580,448
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		1,676,900	1,479,200	3,290,300	2,943,900
Net premiums written		1,446,500	1,275,800	2,832,100	2,529,200
Net premiums earned		1,356,000	1,237,000	2,653,300	2,444,800
Net loss and LAE		796,800	738,200	1,589,300	1,408,800
Commissions, brokerage and other underwriting expenses		444,300	402,100	864,400	808,400
Underwriting profit	[1]	114,900	96,700	199,600	227,600
Net investment income		139,200	122,500	258,500	242,400
Change in the fair value of equity securities		143,300	146,800	532,400	132,700
Net realized capital gains		12,400	6,700	16,800	51,200
Other than temporary impairment losses				(10,000)	(500)
Noninsurance revenue		600,300	333,700	1,110,700	582,100
Other operating expenses		574,500	334,400	1,069,600	591,400
Corporate administration		1,800	800	2,800	500
Amortization of intangible assets		8,800	6,000	15,700	11,200
Interest expense		11,500	9,200	22,600	17,200
Earnings before income taxes		413,500	356,000	997,300	615,200
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,210,700	1,071,700	2,442,300	2,187,300
Net premiums written		1,112,900	976,600	2,220,500	1,973,700
Net premiums earned		1,074,900	981,300	2,104,100	1,941,500
Net loss and LAE		645,100	593,500	1,298,300	1,132,900
Commissions, brokerage and other underwriting expenses		353,900	318,000	689,900	640,000
Underwriting profit	[1]	75,900	69,800	115,900	168,600
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		381,600	345,100	796,700	742,300
Net premiums written		305,900	275,900	622,100	581,000
Net premiums earned		303,200	292,100	612,100	567,200
Net loss and LAE		136,200	153,500	298,800	250,100
Commissions, brokerage and other underwriting expenses		103,800	94,800	207,500	187,900
Underwriting profit	[1]	63,200	43,800	105,800	129,200
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	829,100	726,600	1,645,600	1,445,000
Net premiums written	[2]	807,000	700,700	1,598,400	1,392,700
Net premiums earned	[2]	771,700	689,200	1,492,000	1,374,300
Net loss and LAE	[2]	508,900	440,000	999,500	882,800
Commissions, brokerage and other underwriting expenses	[2]	250,100	223,200	482,400	452,100
Underwriting profit	[1],[2]	12,700	26,000	10,100	39,400
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		466,200	407,500	848,000	756,600
Net premiums written		333,600	299,200	611,600	555,500
Net premiums earned		281,100	255,700	549,200	503,300
Net loss and LAE		151,700	144,700	291,000	275,900
Commissions, brokerage and other underwriting expenses		90,400	84,100	174,500	168,400
Underwriting profit	[1]	39,000	26,900	83,700	59,000
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		372,400	318,800	671,400	593,400
Net premiums written		246,900	216,900	448,300	403,800
Net premiums earned		202,400	184,700	394,800	365,600
Net loss and LAE		105,600	105,700	199,900	202,600
Commissions, brokerage and other underwriting expenses		57,900	53,700	111,400	107,200
Underwriting profit	[1]	38,900	25,300	83,500	55,800
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		93,800	88,700	176,600	163,200
Net premiums written		86,700	82,300	163,300	151,700
Net premiums earned		78,700	71,000	154,400	137,700
Net loss and LAE		46,100	39,000	91,100	73,300
Commissions, brokerage and other underwriting expenses		32,500	30,400	63,100	61,200
Underwriting profit	[1]	100	1,600	200	3,200
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Net investment income	[3]	900	1,400	2,200	3,000
Net realized capital gains	[3]	200	(100)	600	500
Noninsurance revenue	[3]	593,700	329,000	1,098,700	571,700
Other operating expenses	[3]	548,300	314,500	1,013,800	554,400
Amortization of intangible assets	[3]	8,400	5,800	15,100	11,200
Interest expense	[3]	4,700	2,300	9,000	3,600
Earnings before income taxes	[3]	33,400	7,700	63,600	6,000
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,676,900	1,479,200	3,290,300	2,943,900
Net premiums written		1,446,500	1,275,800	2,832,100	2,529,200
Net premiums earned		1,356,000	1,237,000	2,653,300	2,444,800
Net loss and LAE		796,800	738,200	1,589,300	1,408,800
Commissions, brokerage and other underwriting expenses		444,300	402,100	864,400	808,400
Underwriting profit	[1]	114,900	96,700	199,600	227,600
Net investment income		138,300	121,100	256,300	239,400
Change in the fair value of equity securities		143,300	146,800	532,400	132,700
Net realized capital gains		12,200	6,800	16,200	50,700
Other than temporary impairment losses				(10,000)	(500)
Noninsurance revenue		6,600	4,700	12,000	10,400
Other operating expenses		26,200	19,900	55,800	37,000
Corporate administration		1,800	800	2,800	500
Amortization of intangible assets		400	200	600
Interest expense		6,800	6,900	13,600	13,600
Earnings before income taxes		380,100	348,300	933,700	609,200
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written	[4]	(6,100)	(6,500)	(12,900)	(12,400)
Net investment income	[4]	3,500	3,800	7,300	8,000
Change in the fair value of equity securities	[4]	400	38,400	3,600	9,900
Net realized capital gains	[4]		(200)		(200)
Noninsurance revenue	[4]	3,600	9,000	7,300	12,300
Other operating expenses	[4]	7,000	8,800	14,300	16,700
Corporate administration	[4]	24,200	13,300	42,500	21,400
Interest expense	[4]	13,600	13,100	26,100	26,600
Earnings before income taxes	[4]	$ (37,300)	$ 15,800	$ (64,700)	$ (34,700)

[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, 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.