Summary of Alleghany Capital Non-Insurance Revenue (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Other Revenues [Line Items]
Noninsurance revenue		$ 723,785	$ 638,478	$ 1,684,335	$ 1,756,507
Operating Segments
Other Revenues [Line Items]
Noninsurance revenue		722,700	635,500	1,678,700	1,746,300
Operating Segments | Alleghany Capital Corporation Segment
Other Revenues [Line Items]
Noninsurance revenue		713,800	628,000	1,654,300	1,726,800
Operating Segments | Alleghany Capital Corporation Segment | Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[1]	324,200	286,800	847,300	865,200
Operating Segments | Alleghany Capital Corporation Segment | Non-industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[2]	389,600	341,200	806,900	861,200
Operating Segments | Alleghany Capital Corporation Segment | Corporate & Other
Other Revenues [Line Items]
Noninsurance revenue		$ 0	$ 0	$ 100	$ 400

[1] For the three and nine months ended September 30, 2020 and 2019, the vast majority of noninsurance revenue was recognized as goods and services transferred to customers over time.

[2] For the three and nine months ended September 30, 2020, approximately 54 percent and 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. For the three and nine months ended September 30, 2019, 71 percent of noninsurance revenue was recognized as services transferred to customers over time, with the remainder recognized as goods transferred at a point in time.