Summary of Alleghany Capital Non-Insurance Revenue (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Other Revenues [Line Items]
Noninsurance revenue		$ 603,906	$ 342,725	$ 1,118,029	$ 594,352
Operating Segments
Other Revenues [Line Items]
Noninsurance revenue		600,300	333,700	1,110,700	582,100
Operating Segments | Alleghany Capital Corporation Segment
Other Revenues [Line Items]
Noninsurance revenue	[1]	593,700	329,000	1,098,700	571,700
Operating Segments | Alleghany Capital Corporation Segment | Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[2]	318,300	221,900	578,400	367,400
Operating Segments | Alleghany Capital Corporation Segment | Non-industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[3]	275,100	$ 107,100	520,000	204,200
Operating Segments | Alleghany Capital Corporation Segment | Corporate & Other
Other Revenues [Line Items]
Noninsurance revenue		$ 300		$ 300	$ 100

[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 six months ended June 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 six months ended June 30, 2019, approximately 77 percent and 72 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 six months ended June 30, 2018, approximately 70 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. 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.