Allowance for Credit Losses on Commercial Mortgage Loans (Detail) - USD ($) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2020	Jun. 30, 2020
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance		$ 2.0
Provision for credit losses		(0.7)	$ 0.5
Ending balance		$ 1.3	1.3
Accounting Standards Update 2016-13
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	[1]		$ 0.8

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