Summary of Identifiable Assets and Equity (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Identifiable Assets	$ 25,752,941	$ 26,931,604
Invested Assets and Cash	18,363,500
Equity Attributable to Alleghany	7,857,062	$ 8,776,734	$ 8,236,487	$ 7,692,710
Operating Segments
Segment Reporting Information [Line Items]
Identifiable Assets	25,365,600
Invested Assets and Cash	18,046,300
Equity Attributable to Alleghany	8,153,100
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	16,819,800
Invested Assets and Cash	13,077,200
Equity Attributable to Alleghany	4,774,900
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	6,414,100
Invested Assets and Cash	4,697,600
Equity Attributable to Alleghany	2,377,500
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	23,233,900
Invested Assets and Cash	17,774,800
Equity Attributable to Alleghany	7,152,400
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Identifiable Assets	2,131,700
Invested Assets and Cash	271,500
Equity Attributable to Alleghany	1,000,700
Corporate activities
Segment Reporting Information [Line Items]
Identifiable Assets	387,400
Invested Assets and Cash	317,200
Equity Attributable to Alleghany	$ (296,000)