Summary of Identifiable Assets and Equity (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Identifiable Assets	$ 26,931,604	$ 25,344,896
Invested Assets and Cash	20,070,900
Equity Attributable to Alleghany	8,776,734	$ 7,692,710	$ 8,514,063	$ 7,939,945
Operating Segments
Segment Reporting Information [Line Items]
Identifiable Assets	26,437,100
Invested Assets and Cash	19,664,800
Equity Attributable to Alleghany	9,317,200
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	17,158,200
Invested Assets and Cash	13,803,900
Equity Attributable to Alleghany	5,243,300
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	7,230,600
Invested Assets and Cash	5,641,800
Equity Attributable to Alleghany	3,173,000
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	24,388,800
Invested Assets and Cash	19,445,700
Equity Attributable to Alleghany	8,416,300
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Identifiable Assets	2,048,300
Invested Assets and Cash	219,100
Equity Attributable to Alleghany	900,900
Corporate activities
Segment Reporting Information [Line Items]
Identifiable Assets	494,500
Invested Assets and Cash	406,100
Equity Attributable to Alleghany	$ (540,500)