Segments of Business - Additional Information (Detail) $ in Thousands		12 Months Ended
		Dec. 31, 2015 USD ($) Segment	Dec. 31, 2014 USD ($) Segment	Dec. 31, 2013 USD ($) Segment	Dec. 31, 2012 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment		2	2	2
Gross premiums written		$ 5,122,200	$ 5,096,600	$ 4,886,300
Net premiums earned		4,230,286	4,410,647	4,239,216
Identifiable assets		22,846,333	23,489,436
Stockholders' equity attributable to Alleghany stockholders		$ 7,554,707	$ 7,473,428	$ 6,923,757	$ 6,403,787
Reinsurance Segment | Gross Premiums Written | Broker One Concentration Risk
Segment Reporting Information [Line Items]
Percentage of gross premiums written		26.00%	29.00%	27.00%
Reinsurance Segment | Gross Premiums Written | Broker Two Concentration Risk
Segment Reporting Information [Line Items]
Percentage of gross premiums written		20.00%	22.00%	22.00%
Reinsurance Segment | Gross Premiums Written | Broker Three Concentration Risk
Segment Reporting Information [Line Items]
Percentage of gross premiums written		10.00%	11.00%	11.00%
Reinsurance Segment | Non-US
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,600,000	$ 1,700,000	$ 1,600,000
Net premiums earned		1,400,000	1,500,000	1,600,000
Reinsurance Segment | U.K.
Segment Reporting Information [Line Items]
Net premiums earned		640,400	654,800	661,200
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		5,150,200	5,125,400	4,909,400
Net premiums earned		4,230,300	4,410,600	4,239,200
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		3,662,100	3,600,100	3,423,000
Net premiums earned		3,115,500	3,330,700	3,278,700
Identifiable assets		15,600,000
Cash and invested assets		13,200,000
Stockholders' equity attributable to Alleghany stockholders		5,200,000
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,488,100	1,525,300	1,486,400
Net premiums earned		1,114,800	1,079,900	960,500
Identifiable assets		6,300,000
Cash and invested assets		4,800,000
Stockholders' equity attributable to Alleghany stockholders		2,600,000
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written	[1]	(28,000)	$ (28,800)	$ (23,100)
Identifiable assets		1,000,000
Cash and invested assets		400,000
Stockholders' equity attributable to Alleghany stockholders		(200,000)
Corporate activities | Kentucky Trailer
Segment Reporting Information [Line Items]
Mortgage loan, borrowings to finance small acquisitions and borrowings under available credit facility		$ 36,300

[1] Includes elimination of minor reinsurance activity between segments.