Segments of Business - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) Segment	Mar. 31, 2016 Segment	Dec. 31, 2016 USD ($)	Apr. 15, 2016
Segment Reporting Information [Line Items]
Number of reportable segments | Segment	2	2
Identifiable assets	$ 23,939,141		$ 23,756,591
Stockholders' equity attributable to Alleghany stockholders	8,184,849		7,939,945
Senior Notes and other debt	1,471,183		$ 1,476,489
Jazwares, LLC
Segment Reporting Information [Line Items]
Ownership interest acquired				50.00%
Ownership Percentage by Parent				80.00%
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	15,800,000
Cash and invested assets	13,200,000
Stockholders' equity attributable to Alleghany stockholders	5,300,000
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	6,800,000
Cash and invested assets	5,200,000
Stockholders' equity attributable to Alleghany stockholders	2,900,000
Other activities
Segment Reporting Information [Line Items]
Identifiable assets	1,300,000
Cash and invested assets	500,000
Stockholders' equity attributable to Alleghany stockholders	0
Other activities | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Senior Notes and other debt	87,400
Other activities | Kentucky Trailer | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Senior Notes and other debt	35,400
Other activities | Jazwares, LLC | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Senior Notes and other debt	31,200
Other activities | Bourn & Koch, Inc. | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Senior Notes and other debt	18,400
Other activities | Integrated Project Services LLC | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Senior Notes and other debt	$ 2,400