Allowance for Credit Losses on Available for Sale Securities (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Financing Receivable Allowance For Credit Losses [Line Items]
Credit losses for available for sale securities	$ 31,375
Ending balance	$ 31,375	[1]

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