Summary of Alleghany Capital Non-Insurance Revenue (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Segments | Alleghany Capital Corporation Segment | Non-Industrial Segment | Transferred over Time [Member]
Other Revenues [Line Items]
Noninsurance revenue percentage	59.00%	70.00%	63.00%