Allowance for Credit Losses on Available for Sale Securities (Detail) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended
		Jun. 30, 2020	Jun. 30, 2020
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance		$ 31,400
Change in allowance for credit losses on available for sale securities		(17,021)	$ 14,354
Charge-offs		(5,500)	(5,500)
Ending balance	[1]	$ 8,904	$ 8,904

[1] See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance.