Summary of Alleghany Capital Non-Insurance Revenue (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other Revenues [Line Items]
Noninsurance revenue		$ 2,521,092	$ 2,328,848	$ 1,643,999
Operating Segments
Other Revenues [Line Items]
Noninsurance revenue		2,512,400	2,316,300	1,596,600
Operating Segments | Alleghany Capital Corporation Segment
Other Revenues [Line Items]
Noninsurance revenue	[1]	2,477,500	2,289,300	1,574,600
Operating Segments | Alleghany Capital Corporation Segment | Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[2]	1,220,900	1,105,600	917,100
Operating Segments | Alleghany Capital Corporation Segment | Non-industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[3]	1,256,500	1,183,700	657,500
Operating Segments | Alleghany Capital Corporation Segment | Corporate & Other
Other Revenues [Line Items]
Noninsurance revenue		$ 100	$ 0	$ 0

[1] Excludes certain minor, legacy investments that were previously reflected in Alleghany Capital in 2018 and prior periods, to align with management’s view of reportable segments.

[2] For 2020, 2019 and 2018, the vast majority of industrial noninsurance revenues were recognized as goods and services transferred to customers over time.

[3] For 2020, 2019 and 2018, approximately 59 percent, 70 percent and 63 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.