Summary of Identifiable Assets and Equity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Identifiable Assets	$ 25,344,896	$ 25,384,317
Invested Assets and Cash	18,561,900
Equity attributable to Alleghany	7,692,710	$ 8,514,063	$ 7,939,945	$ 7,554,707
Operating Segments
Segment Reporting Information [Line Items]
Identifiable Assets	24,936,200
Invested Assets and Cash	18,205,900
Equity attributable to Alleghany	8,299,200
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	16,502,900
Invested Assets and Cash	12,967,900
Equity attributable to Alleghany	4,723,500
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	6,701,600
Invested Assets and Cash	5,105,000
Equity attributable to Alleghany	2,696,400
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	23,204,500
Invested Assets and Cash	18,072,900
Equity attributable to Alleghany	7,419,900
Operating Segments | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Identifiable Assets	1,731,700
Invested Assets and Cash	133,000
Equity attributable to Alleghany	879,300
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Identifiable Assets	408,700
Invested Assets and Cash	356,000
Equity attributable to Alleghany	$ (606,500)