Summary of Alleghany Capital Non-Insurance Revenue (Parenthetical) (Detail) - Operating Segments - Alleghany Capital - Transferred over Time [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue percentage		77.00%		67.00%
Non-Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue percentage	60.00%	56.00%	65.00%	69.00%