Credit Agreements - Additional Information (Detail) - Alleghany Corporation - USD ($)			12 Months Ended
	Jul. 31, 2017	Jul. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 30, 2017
Revolving Credit Facility
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	$ 300,000,000
Revolving credit facility expiration date	Jul. 31, 2022
Borrowings			$ 0	$ 0
Credit facility borrowings outstanding			$ 0
Prior Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity						$ 200,000,000
Revolving credit facility expiration date		Jul. 31, 2017
Borrowings				$ 0	$ 0