Summary of Alleghany Capital Non-Insurance Revenue (Parenthetical) (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating Segments | Alleghany Capital Corporation Segment | Non-Industrial Segment | Transferred over Time [Member]
Other Revenues [Line Items]
Noninsurance revenue percentage	71.00%	60.00%	71.00%	65.00%