Goodwill and Intangible Asset, Net of Accumulated Amortization Expense on Consolidated Balance Sheets (Detail) - USD ($)		Dec. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill and other intangible assets, Gross carrying value		$ 990,600,000	$ 661,700,000
Intangible assets excluding goodwill, gross		705,600,000	520,700,000
Finite lived intangible assets, accumulated amortization		326,900,000	307,900,000
Goodwill and other intangible assets, Net carrying value	[1]	663,700,000	353,800,000
Intangible assets excluding goodwill, net of amortization	[1]	378,680,000	212,790,000
Goodwill, gross		284,974,000	141,015,000
Accumulated Amortization		0	0
Goodwill	[1]	284,974,000	141,015,000
Insurance Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill and other intangible assets, Gross carrying value	[2]	190,300,000	190,300,000
Intangible assets excluding goodwill, gross		141,300,000	141,300,000
Finite lived intangible assets, accumulated amortization	[2]	68,400,000	65,400,000
Goodwill and other intangible assets, Net carrying value	[1],[2]	121,900,000	124,900,000
Intangible assets excluding goodwill, net of amortization	[1]	72,900,000	75,900,000
Goodwill, gross	[2]	49,000,000	49,000,000
Accumulated Amortization	[2]	0	0
Goodwill	[1],[2]	49,000,000	49,000,000
Insurance Segment | Agency Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		17,900,000	17,900,000
Finite lived intangible assets, accumulated amortization		9,800,000	9,000,000
Finite lived intangible assets, net	[1]	8,100,000	8,900,000
Insurance Segment | Brokerage and Reinsurance Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		33,800,000	33,800,000
Finite lived intangible assets, accumulated amortization		30,400,000	28,200,000
Finite lived intangible assets, net	[1]	3,400,000	5,600,000
Insurance Segment | Renewal rights
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		24,200,000	24,200,000
Finite lived intangible assets, accumulated amortization		24,100,000	24,100,000
Finite lived intangible assets, net	[1]	100,000	100,000
Insurance Segment | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		4,100,000	4,100,000
Finite lived intangible assets, accumulated amortization		4,100,000	4,100,000
Insurance Segment | State insurance licenses
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross		25,800,000	25,800,000
Indefinite Lived Intangible Assets Accumulated Amortization		0	0
Indefinite lived intangible assets, net	[1]	25,800,000	25,800,000
Insurance Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross		35,500,000	35,500,000
Indefinite Lived Intangible Assets Accumulated Amortization		0	0
Indefinite lived intangible assets, net	[1]	35,500,000	35,500,000
Reinsurance Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Intangible assets excluding goodwill, gross	[2]	287,500,000	287,500,000
Finite lived intangible assets, accumulated amortization	[2]	232,700,000	238,900,000
Intangible assets excluding goodwill, net of amortization	[1],[2]	54,800,000	48,600,000
Reinsurance Segment | Renewal rights
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[2]	44,000,000	44,000,000
Finite lived intangible assets, accumulated amortization	[2]	17,800,000	13,500,000
Finite lived intangible assets, net	[1],[2]	26,200,000	30,500,000
Reinsurance Segment | Value of business in-force
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[2]	291,400,000	291,400,000
Finite lived intangible assets, accumulated amortization	[2]	291,400,000	291,400,000
Reinsurance Segment | Loss and LAE reserves
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[2]	(98,800,000)	(98,800,000)
Finite lived intangible assets, accumulated amortization	[2]	(72,700,000)	(65,100,000)
Finite lived intangible assets, net	[1],[2]	(26,100,000)	(33,700,000)
Reinsurance Segment | Leases
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[2]	(28,100,000)	(28,100,000)
Finite lived intangible assets, accumulated amortization	[2]	(13,800,000)	(10,900,000)
Finite lived intangible assets, net	[1],[2]	(14,300,000)	(17,200,000)
Reinsurance Segment | Internally-developed software
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[2]	10,000,000	10,000,000
Finite lived intangible assets, accumulated amortization	[2]	10,000,000	10,000,000
Reinsurance Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross	[2]	50,000,000	50,000,000
Indefinite Lived Intangible Assets Accumulated Amortization	[2]	0	0
Indefinite lived intangible assets, net	[1],[2]	50,000,000	50,000,000
Reinsurance Segment | State and foreign insurance licenses
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross	[2]	19,000,000	19,000,000
Indefinite Lived Intangible Assets Accumulated Amortization	[2]	0	0
Indefinite lived intangible assets, net	[1],[2]	19,000,000	19,000,000
Other activities
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill and other intangible assets, Gross carrying value	[2],[3]	512,800,000	183,900,000
Intangible assets excluding goodwill, gross	[3]	276,800,000	91,900,000
Finite lived intangible assets, accumulated amortization	[2],[3]	25,800,000	3,600,000
Goodwill and other intangible assets, Net carrying value	[1],[2],[3]	487,000,000	180,300,000
Intangible assets excluding goodwill, net of amortization	[1],[3]	251,000,000	88,300,000
Goodwill, gross	[2],[3]	236,000,000	92,000,000
Accumulated Amortization	[2],[3]	0	0
Goodwill	[1],[2],[3]	236,000,000	92,000,000
Other activities | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	84,000,000	53,000,000
Finite lived intangible assets, accumulated amortization	[3]	19,900,000	3,600,000
Finite lived intangible assets, net	[1],[3]	64,100,000	49,400,000
Other activities | License agreements
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	66,200,000
Finite lived intangible assets, accumulated amortization	[3]	5,900,000
Finite lived intangible assets, net	[1],[3]	60,300,000
Other activities | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross	[3]	126,600,000	38,900,000
Indefinite Lived Intangible Assets Accumulated Amortization	[3]	0	0
Indefinite lived intangible assets, net	[1],[3]	$ 126,600,000	$ 38,900,000

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

[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) Jazwares on April 15, 2016; (ii) IPS on October 31, 2015; (iii) Bourn & Koch on April 26, 2012; and (iv) a controlling equity interest in Kentucky Trailer on August 30, 2013. Also reflects minor acquisitions made by Jazwares and Bourn & Koch in 2016 and Kentucky Trailer in 2015 and 2014.