Summary of Alleghany Capital Non-Insurance Revenue (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Other Revenues [Line Items]
Noninsurance revenue		$ 638,478	$ 438,338	$ 1,756,507	$ 1,032,690
Operating Segments
Other Revenues [Line Items]
Noninsurance revenue		635,500	413,700	1,746,300	995,900
Operating Segments | Alleghany Capital Corporation Segment
Other Revenues [Line Items]
Noninsurance revenue	[1]	628,000	407,500	1,726,800	979,200
Operating Segments | Alleghany Capital Corporation Segment | Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[2]	286,800	224,200	865,200	591,600
Operating Segments | Alleghany Capital Corporation Segment | Non-industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[3]	$ 341,200	183,700	861,200	387,900
Operating Segments | Alleghany Capital Corporation Segment | Corporate & Other
Other Revenues [Line Items]
Noninsurance revenue			$ (400)	$ 400	$ (300)

[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 the three and nine months ended September 30, 2019 and 2018, the vast majority of noninsurance revenue was recognized as goods and services transferred to customers over time. See Note 1(c) of this Form 10-Q for additional information on Alleghany’s adoption of new revenue recognition accounting guidance effective in the first quarter of 2018.

[3] For the three and nine months ended September 30, 2019, approximately 71 percent 71 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, 2018, approximately 60 percent and 65 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. See Note 1(c) of this Form 10-Q for additional information on Alleghany’s adoption of new revenue recognition accounting guidance effective in the first quarter of 2018.