Summary of Alleghany Capital Product and Service Revenues (Parenthetical) (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Operating Segments | Alleghany Capital Corporation Segment | Non-Industrial Segment | Transferred over Time [Member]
Other Revenues [Line Items]
Product and service revenue percentage	55.00%	55.00%	57.00%	59.00%