Segments of Business - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) Segment	Mar. 31, 2015 Segment	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment	2	2
Identifiable assets	$ 23,055,343		$ 22,839,079
Stockholders' equity attributable to Alleghany stockholders	7,771,736		$ 7,554,707
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	15,800,000
Cash and invested assets	13,300,000
Stockholders' equity attributable to Alleghany stockholders	5,300,000
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	6,400,000
Cash and invested assets	4,800,000
Stockholders' equity attributable to Alleghany stockholders	2,700,000
Other activities
Segment Reporting Information [Line Items]
Identifiable assets	900,000
Cash and invested assets	400,000
Stockholders' equity attributable to Alleghany stockholders	(200,000)
Other activities | Alleghany Capital Corporation | Kentucky Trailer
Segment Reporting Information [Line Items]
Mortgage loan, borrowings to finance small acquisitions and borrowings under available credit facility	$ 40,400