Summary of Alleghany Capital Non-Insurance Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other Revenues [Line Items]
Other revenues	$ 251,627	$ 151,292
Operating Segments
Other Revenues [Line Items]
Other revenues	248,400	147,200
Operating Segments | Alleghany Capital
Other Revenues [Line Items]
Other revenues	242,700	144,700
Operating Segments | Alleghany Capital | Industrial Segment
Other Revenues [Line Items]
Other revenues	145,500	41,900
Operating Segments | Alleghany Capital | Non-industrial Segment
Other Revenues [Line Items]
Other revenues	97,100	$ 102,800
Operating Segments | Alleghany Capital | Corporate & Other
Other Revenues [Line Items]
Other revenues	$ 100