Allowance for Credit Losses on Commercial Mortgage Loans (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($)
Financing Receivable Allowance For Credit Losses [Line Items]
Provision for credit losses	$ 1.2	[1]
Ending balance	2.0
Accounting Standards Update 2016-13
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	$ 0.8	[2]

[1] Primarily reflects an increase in the estimate of expected credit losses based on current conditions and reasonable and supportable forecasts as of March 31, 2020 arising from the Pandemic, as defined in Note 9(a) of this Form 10-Q.

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