Summary of Alleghany Capital Non-Insurance Revenue (Parenthetical) (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating Segments | Alleghany Capital Corporation Segment | Non-Industrial Segment | Transferred over Time
Other Revenues [Line Items]
Noninsurance revenue percentage	54.00%	71.00%	66.00%	71.00%