Summary of Alleghany Capital Non-Insurance Revenue (Parenthetical) (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating Segments | Alleghany Capital Corporation Segment | Non-Industrial Segment | Transferred over Time [Member]
Other Revenues [Line Items]
Noninsurance revenue percentage	77.00%	70.00%	72.00%	70.00%