Summary of Identifiable Assets and Equity (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Identifiable Assets	$ 31,894,427	$ 32,268,675
Invested Assets and Cash	22,294,800
Equity Attributable to Alleghany	8,595,373	$ 9,186,882	$ 8,695,881	$ 8,755,720
Operating Segments
Segment Reporting Information [Line Items]
Identifiable Assets	30,511,000
Invested Assets and Cash	20,871,800
Equity Attributable to Alleghany	8,927,800
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	18,771,400
Invested Assets and Cash	14,734,900
Equity Attributable to Alleghany	4,892,400
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	8,320,400
Invested Assets and Cash	5,967,300
Equity Attributable to Alleghany	2,677,800
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	27,091,800
Invested Assets and Cash	20,702,200
Equity Attributable to Alleghany	7,570,200
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Identifiable Assets	3,419,200
Invested Assets and Cash	169,600
Equity Attributable to Alleghany	1,357,600
Corporate activities
Segment Reporting Information [Line Items]
Identifiable Assets	1,383,400
Invested Assets and Cash	1,423,000
Equity Attributable to Alleghany	$ (332,400)