Summary of Alleghany Capital Non-Insurance Revenue (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Other Revenues [Line Items]
Noninsurance revenue		$ 467,849	$ 514,123
Operating Segments
Other Revenues [Line Items]
Noninsurance revenue		464,700	510,400
Operating Segments | Alleghany Capital Corporation Segment
Other Revenues [Line Items]
Noninsurance revenue		457,300	505,000
Operating Segments | Alleghany Capital Corporation Segment | Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[1]	242,500	260,100
Operating Segments | Alleghany Capital Corporation Segment | Non-industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[2]	214,700	$ 244,900
Operating Segments | Alleghany Capital Corporation Segment | Corporate & Other
Other Revenues [Line Items]
Noninsurance revenue		$ 100

[1] For the three months ended March 31, 2020 and 2019, the vast majority of noninsurance revenue was recognized as goods and services transferred to customers over time.

[2] For the three months ended March 31, 2020 and 2019, approximately 82 percent 66 percent, respectively, of noninsurance revenue was recognized as services transferred to customers over time, with the remainder recognized as goods transferred at a point in time.