Summary of Alleghany Capital Non-Insurance Revenue (Parenthetical) (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Operating Segments | Alleghany Capital Corporation Segment | Non-Industrial Segment | Transferred over Time
Other Revenues [Line Items]
Noninsurance revenue percentage	55.00%	74.00%	59.00%	78.00%