Credit Agreements - Additional Information (Detail) - Alleghany Corporation - Revolving Credit Facility - USD ($)		12 Months Ended
	Jul. 31, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Revolving credit facility, initiation date	Jul. 31, 2017
Line of credit facility, term	5 years
Revolving credit facility, maximum borrowing capacity	$ 300,000,000.0
Revolving credit facility expiration date	Jul. 31, 2022
Borrowings		$ 0	$ 0	$ 0
Credit facility borrowings outstanding		$ 0