Allowance for Credit Losses on Reinsurance Recoverables (Detail) - USD ($) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2020	Jun. 30, 2020
Allowance for Credit Losses
Beginning balance		$ 6.8
Provision for credit losses	[1]	0.1	$ 3.6
Balance as of June 30, 2020		$ 6.9	6.9
Accounting Standards Update 2016-13
Allowance for Credit Losses
Beginning balance	[2]		$ 3.3

[1] For the six months ended June 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.