Summary of Identifiable Assets and Equity (Detail) - USD ($) $ in Thousands	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Identifiable Assets	$ 31,007,865		$ 32,268,675
Invested Assets and Cash	21,076,300
Equity Attributable to Alleghany	7,906,670	$ 8,595,373	$ 9,186,882	$ 9,142,031	$ 8,695,881	$ 8,755,720
Operating Segments
Segment Reporting Information [Line Items]
Identifiable Assets	30,121,400
Invested Assets and Cash	20,162,100
Equity Attributable to Alleghany	8,275,300
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	18,104,600
Invested Assets and Cash	14,073,000
Equity Attributable to Alleghany	4,402,500
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	8,480,500
Invested Assets and Cash	5,860,200
Equity Attributable to Alleghany	2,504,600
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	26,585,100
Invested Assets and Cash	19,933,200
Equity Attributable to Alleghany	6,907,100
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Identifiable Assets	3,536,300
Invested Assets and Cash	228,900
Equity Attributable to Alleghany	1,368,200
Corporate activities
Segment Reporting Information [Line Items]
Identifiable Assets	886,500
Invested Assets and Cash	914,200
Equity Attributable to Alleghany	$ (368,600)