Summary of Alleghany Capital Non-Insurance Revenue (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Other Revenues [Line Items]
Noninsurance revenue		$ 800,425	$ 492,701	$ 1,568,260	$ 960,550
Operating Segments
Other Revenues [Line Items]
Noninsurance revenue		800,300	491,500	1,568,100	956,100
Operating Segments | Alleghany Capital Corporation Segment
Other Revenues [Line Items]
Noninsurance revenue		789,200	483,200	1,548,400	940,500
Operating Segments | Alleghany Capital Corporation Segment | Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[1]	407,200	280,600	811,700	523,100
Operating Segments | Alleghany Capital Corporation Segment | Non-industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[2]	382,000	$ 202,600	736,700	417,300
Operating Segments | Alleghany Capital Corporation Segment | Corporate & Other
Other Revenues [Line Items]
Noninsurance revenue		$ 0		$ 0	$ 100

[1] For the three and six months ended June 30, 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 and six months ended June 30, 2021, approximately 55 percent and 59 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. For the three and six months ended June 30, 2020, approximately 74 percent and 78 percent, respectively, of non-industrial noninsurance revenues were recognized as services transferred to customers over time, with the remainder recognized as goods transferred in a point in time.