Summary of Alleghany Capital Non-Insurance Revenue (Detail) - Noninsurance Revenue - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Revenues [Line Items]
Noninsurance revenue		$ 438,338	$ 296,309	$ 1,032,690	$ 650,413
Operating Segments
Other Revenues [Line Items]
Noninsurance revenue		413,700	294,000	995,900	637,300
Operating Segments | Alleghany Capital
Other Revenues [Line Items]
Noninsurance revenue		407,500	289,300	979,200	626,800
Operating Segments | Alleghany Capital | Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[1]	224,200	137,900	591,600	254,200
Operating Segments | Alleghany Capital | Non-Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[2]	183,700	150,900	387,900	372,100
Operating Segments | Alleghany Capital | Corporate & Other
Other Revenues [Line Items]
Noninsurance revenue		$ (400)	$ 500	$ (300)	$ 500

[1] For the three and nine months ended September 30, 2018, the vast majority of noninsurance revenues were recognized as goods and services transferred to customers over time. For the three and nine months ended September 30, 2017, approximately 77 percent and 67 percent, respectively, of noninsurance revenues were recognized as services were 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 regarding Alleghany's adoption of new revenue recognition accounting guidance effective in the first quarter of 2018.

[2] For the three and nine months ended September 30, 2018, approximately 60 percent and 65 percent, respectively, of 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 nine months ended September 30, 2017, approximately 56 percent and 69 percent, respectively, of noninsurance revenues were recognized as services were transferred to customers over time, with the remainder recognized as goods were transferred at a point in time. See Note 1(c) of this Form 10-Q for additional information regarding Alleghany's adoption of new revenue recognition accounting guidance effective in the first quarter of 2018.