Summary of Identifiable Assets and Equity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Identifiable Assets	$ 25,085,887	$ 25,384,317
Invested Assets and Cash	19,267,200
Equity attributable to Alleghany	8,404,389	$ 8,514,063
Operating Segments
Segment Reporting Information [Line Items]
Identifiable Assets	24,523,100
Invested Assets and Cash	18,781,600
Equity attributable to Alleghany	8,877,200
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	16,356,900
Invested Assets and Cash	13,387,800
Equity attributable to Alleghany	5,208,500
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	6,837,900
Invested Assets and Cash	5,263,300
Equity attributable to Alleghany	2,891,500
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	23,194,800
Invested Assets and Cash	18,651,100
Equity attributable to Alleghany	8,100,000
Operating Segments | Alleghany Capital
Segment Reporting Information [Line Items]
Identifiable Assets	1,328,300
Invested Assets and Cash	130,500
Equity attributable to Alleghany	777,200
Corporate Activities
Segment Reporting Information [Line Items]
Identifiable Assets	562,800
Invested Assets and Cash	485,600
Equity attributable to Alleghany	$ (472,800)