Summary of Identifiable Assets and Equity (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Identifiable Assets	$ 28,927,010	$ 26,931,604
Invested Assets and Cash	20,978,400
Equity Attributable to Alleghany	8,755,720	$ 8,776,734	$ 7,692,710	$ 8,514,063
Operating Segments
Segment Reporting Information [Line Items]
Identifiable Assets	28,057,100
Invested Assets and Cash	20,161,700
Equity Attributable to Alleghany	9,096,500
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	17,971,000
Invested Assets and Cash	14,552,900
Equity Attributable to Alleghany	5,377,000
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	7,493,300
Invested Assets and Cash	5,501,800
Equity Attributable to Alleghany	2,610,400
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	25,464,300
Invested Assets and Cash	20,054,700
Equity Attributable to Alleghany	7,987,400
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Identifiable Assets	2,592,800
Invested Assets and Cash	107,000
Equity Attributable to Alleghany	1,109,100
Corporate activities
Segment Reporting Information [Line Items]
Identifiable Assets	869,900
Invested Assets and Cash	816,700
Equity Attributable to Alleghany	$ (340,800)