Goodwill and Intangible Asset, Net of Accumulated Amortization Expense on Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill and other intangible assets, Gross carrying value		$ 1,378,100	$ 1,140,200
Finite lived intangible assets, accumulated amortization		369,900	346,300
Goodwill and other intangible assets, Net carrying value	[1]	1,008,200	793,900
Goodwill, gross		455,100	334,900
Goodwill		455,142	334,905
Intangible assets, gross		923,000	805,300
Goodwill and other intangible assets, accumulated amortization		369,900	346,300
Intangible assets, net		553,136	459,037
Insurance Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill and other intangible assets, Gross carrying value		191,000	189,600
Finite lived intangible assets, accumulated amortization		73,400	71,500
Goodwill and other intangible assets, Net carrying value	[1]	117,600	118,100
Goodwill, gross	[2]	49,000	49,000
Goodwill	[1],[2]	49,000	49,000
Intangible assets, gross	[2]	142,000	140,600
Goodwill and other intangible assets, accumulated amortization	[2]	73,400	71,500
Intangible assets, net	[1],[2]	68,600	69,100
Insurance Segment | Agency Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		18,400	17,900
Finite lived intangible assets, accumulated amortization		11,400	10,600
Finite lived intangible assets, net	[1]	7,000	7,300
Insurance Segment | Brokerage and Reinsurance Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		33,800	33,800
Finite lived intangible assets, accumulated amortization		33,800	32,700
Finite lived intangible assets, net	[1]		1,100
Insurance Segment | Renewal rights
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		25,100	24,200
Finite lived intangible assets, accumulated amortization		24,100	24,100
Finite lived intangible assets, net	[1]	1,000	100
Insurance Segment | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		4,100	4,100
Finite lived intangible assets, accumulated amortization		4,100	4,100
Insurance Segment | State insurance licenses
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross		25,100	25,100
Indefinite lived intangible assets, net	[1]	25,100	25,100
Insurance Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross		35,500	35,500
Indefinite lived intangible assets, net	[1]	35,500	35,500
Reinsurance Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill and other intangible assets, Gross carrying value		308,400	301,400
Finite lived intangible assets, accumulated amortization		225,300	228,100
Goodwill and other intangible assets, Net carrying value	[1]	83,100	73,300
Goodwill, gross	[2]	8,800	8,800
Goodwill	[1],[2]	8,800	8,800
Intangible assets, gross	[2]	299,600	292,600
Goodwill and other intangible assets, accumulated amortization	[2]	225,300	228,100
Intangible assets, net	[1],[2]	74,300	64,500
Reinsurance Segment | Renewal rights
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		51,500	44,000
Finite lived intangible assets, accumulated amortization		26,000	21,800
Finite lived intangible assets, net	[1]	25,500	22,200
Reinsurance Segment | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		14,600	15,100
Finite lived intangible assets, accumulated amortization		10,000	10,200
Finite lived intangible assets, net	[1]	4,600	4,900
Reinsurance Segment | Value of business in-force
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		291,400	291,400
Finite lived intangible assets, accumulated amortization		291,400	291,400
Reinsurance Segment | Loss and LAE reserves
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		(98,800)	(98,800)
Finite lived intangible assets, accumulated amortization		(82,600)	(78,600)
Finite lived intangible assets, net	[1]	(16,200)	(20,200)
Reinsurance Segment | Leases
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		(28,100)	(28,100)
Finite lived intangible assets, accumulated amortization		(19,500)	(16,700)
Finite lived intangible assets, net	[1]	(8,600)	(11,400)
Reinsurance Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross		50,000	50,000
Indefinite lived intangible assets, net	[1]	50,000	50,000
Reinsurance Segment | State and foreign insurance licenses
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross		19,000	19,000
Indefinite lived intangible assets, net	[1]	19,000	19,000
Alleghany Capital Corporation Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill and other intangible assets, Gross carrying value	[3]	878,700	649,200
Finite lived intangible assets, accumulated amortization	[3]	71,200	46,700
Goodwill and other intangible assets, Net carrying value	[1],[3]	807,500	602,500
Goodwill, gross	[2],[3]	397,300	277,100
Goodwill	[1],[2],[3]	397,300	277,100
Intangible assets, gross	[2],[3]	481,400	372,100
Goodwill and other intangible assets, accumulated amortization	[2],[3]	71,200	46,700
Intangible assets, net	[1],[2],[3]	410,200	325,400
Alleghany Capital Corporation Segment | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	21,200	20,800
Finite lived intangible assets, accumulated amortization	[3]	20,800	12,100
Finite lived intangible assets, net	[1],[3]	400	8,700
Alleghany Capital Corporation Segment | License agreements
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	68,200	66,200
Finite lived intangible assets, accumulated amortization	[3]	22,600	14,100
Finite lived intangible assets, net	[1],[3]	45,600	52,100
Alleghany Capital Corporation Segment | Customer relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	215,100	133,200
Finite lived intangible assets, accumulated amortization	[3]	27,700	20,500
Finite lived intangible assets, net	[1],[3]	187,400	112,700
Alleghany Capital Corporation Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, accumulated amortization	[3]	100
Intangible assets, gross	[3]	176,900	151,900
Intangible assets, net	[1],[3]	$ 176,800	$ 151,900

[1] Goodwill and intangible assets have been reduced by amounts written-down in prior periods, as applicable.

[2] See Note 13 for additional information on Alleghany's segments of business.

[3] Primarily represents goodwill and other intangible assets related to the acquisition of: (i) Concord on October 1, 2018; (ii) Hirschfeld by W&W|AFCO Steel on February 8, 2018; (iii) W&W|AFCO Steel on April 28, 2017; (iv) Jazwares on April 15, 2016; (v) IPS on October 31, 2015; (vi) Bourn & Koch on April 26, 2012; and (vii) a controlling equity interest in Kentucky Trailer on August 30, 2013. Also reflects minor acquisitions made by Jazwares in 2016 and 2018, Bourn & Koch in 2016, and Kentucky Trailer in 2015, 2014 and 2018.