Summary of Identifiable Assets and Equity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Identifiable Assets	$ 25,097,256	$ 25,384,317
Invested Assets and Cash	19,200,300
Equity attributable to Alleghany	8,373,400	$ 8,514,063
Operating Segments
Segment Reporting Information [Line Items]
Identifiable Assets	24,514,200
Invested Assets and Cash	18,715,400
Equity attributable to Alleghany	8,806,700
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	16,515,300
Invested Assets and Cash	13,422,500
Equity attributable to Alleghany	5,155,300
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	6,692,900
Invested Assets and Cash	5,143,100
Equity attributable to Alleghany	2,854,300
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	23,208,200
Invested Assets and Cash	18,565,600
Equity attributable to Alleghany	8,009,600
Operating Segments | Alleghany Capital
Segment Reporting Information [Line Items]
Identifiable Assets	1,306,000
Invested Assets and Cash	149,800
Equity attributable to Alleghany	797,100
Corporate Activities
Segment Reporting Information [Line Items]
Identifiable Assets	583,000
Invested Assets and Cash	484,900
Equity attributable to Alleghany	$ (433,300)