Summary of Alleghany Capital Non-Insurance Revenue (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Other Revenues [Line Items]
Noninsurance revenue		$ 767,835	$ 467,849
Operating Segments
Other Revenues [Line Items]
Noninsurance revenue		767,700	464,700
Operating Segments | Alleghany Capital Corporation Segment
Other Revenues [Line Items]
Noninsurance revenue		759,100	457,300
Operating Segments | Alleghany Capital Corporation Segment | Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[1]	404,400	242,500
Operating Segments | Alleghany Capital Corporation Segment | Non-industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[2]	354,700	214,700
Operating Segments | Alleghany Capital Corporation Segment | Corporate & Other
Other Revenues [Line Items]
Noninsurance revenue		$ 0	$ 100

[1] For the three months ended March 31, 2021 and 2020, the vast majority of industrial noninsurance revenues were recognized as goods and services transferred to customers over time.

[2] For the three months ended March 31, 2021 and 2020, approximately 63 percent and 82 percent, respectively, of non-industrial noninsurance revenues were recognized as services transferred to customers over time, with the remainder recognized as goods transferred at a point in time.