Summary of Identifiable Assets and Equity (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Identifiable Assets	$ 26,490,831			$ 25,344,896
Invested Assets and Cash	19,762,700
Equity Attributable to Alleghany	8,828,658	$ 8,685,914	$ 8,236,487	$ 7,692,710	$ 8,595,085	$ 8,404,389	$ 8,373,400	$ 8,514,063
Operating Segments
Segment Reporting Information [Line Items]
Identifiable Assets	25,995,100
Invested Assets and Cash	19,304,900
Equity Attributable to Alleghany	9,391,100
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	16,914,100
Invested Assets and Cash	13,540,700
Equity Attributable to Alleghany	5,347,800
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	7,166,500
Invested Assets and Cash	5,615,800
Equity Attributable to Alleghany	3,141,800
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	24,080,600
Invested Assets and Cash	19,156,500
Equity Attributable to Alleghany	8,489,600
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Identifiable Assets	1,914,500
Invested Assets and Cash	148,400
Equity Attributable to Alleghany	901,500
Corporate activities
Segment Reporting Information [Line Items]
Identifiable Assets	495,700
Invested Assets and Cash	457,800
Equity Attributable to Alleghany	$ (562,400)