Summary of Identifiable Assets and Equity (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Identifiable Assets	$ 26,356,378		$ 25,344,896
Invested Assets and Cash	19,572,400
Equity attributable to Alleghany	8,685,914	$ 8,236,487	$ 7,692,710	$ 8,404,389	$ 8,373,400	$ 8,514,063
Operating Segments
Segment Reporting Information [Line Items]
Identifiable Assets	25,876,800
Invested Assets and Cash	19,132,600
Equity attributable to Alleghany	9,298,100
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	16,874,000
Invested Assets and Cash	13,462,700
Equity attributable to Alleghany	5,321,700
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	7,122,300
Invested Assets and Cash	5,535,200
Equity attributable to Alleghany	3,096,500
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	23,996,300
Invested Assets and Cash	18,997,900
Equity attributable to Alleghany	8,418,200
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Identifiable Assets	1,880,500
Invested Assets and Cash	134,700
Equity attributable to Alleghany	879,900
Corporate activities
Segment Reporting Information [Line Items]
Identifiable Assets	479,600
Invested Assets and Cash	439,800
Equity attributable to Alleghany	$ (612,200)