Allowance for Credit Losses on Reinsurance Recoverables (Detail) - USD ($) $ in Millions		3 Months Ended	9 Months Ended
		Sep. 30, 2020	Sep. 30, 2020
Allowance for Credit Losses
Beginning balance		$ 6.9	$ 0.0
Provision for credit losses	[1]	0.0	3.6
Charge-offs		0.0	0.0
Recoveries		0.0	0.0
Balance as of September 30, 2020		6.9	6.9
Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for Credit Losses
Beginning balance	[2]	$ 0.0	$ 3.3

[1] For the nine months ended September 30, 2020, primarily reflects the change in estimates of expected credit losses based on current conditions and reasonable and supportable forecasts, including estimates 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.