Allowance for Credit Losses on Commercial Mortgage Loans (Detail) - USD ($) $ in Millions		3 Months Ended	9 Months Ended
		Sep. 30, 2020	Sep. 30, 2020
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance		$ 1.3	$ 0.0
Provision for credit losses		0.2	0.7
Charge-offs		0.0	0.0
Recoveries		0.0	0.0
Ending balance		1.5	1.5
Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	[1]	$ 0.0	$ 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.