Debt - Additional Information (Detail) - USD ($) $ in Thousands				9 Months Ended
	May 18, 2020	Jan. 15, 2020	Sep. 20, 2010	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Debt Instrument [Line Items]
Interest paid				$ 55,683	$ 69,029
Loss on early redemption of debt				(7,100)
Senior Notes and other debt				2,041,610		$ 1,751,113
Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt				462,500		367,100
Alleghany Corporation | 2020 Senior Notes
Debt Instrument [Line Items]
Senior notes, face value			$ 300,000
Senior notes, maturity date			Sep. 15, 2020
Senior notes, interest rate			5.625%
Senior notes, issuance rate			99.60%
Proceeds from issuance of Senior notes			$ 298,900
Senior notes, effective yield			5.67%
Cost of senior note redemption		$ 312,700
Aggregate principal amount redeemed		300,000
Redemption premium		7,100
Interest paid		$ 5,600
Loss on early redemption of debt				$ (7,100)
Alleghany Corporation | 2030 Senior Notes
Debt Instrument [Line Items]
Senior notes, face value	$ 500,000
Senior notes, maturity date	May 15, 2030
Senior notes, frequency of interest payment				Interest on the 2030 Senior Notes is payable semi-annually in arrears on May 15 and November 15 of each year.
Senior notes, interest rate	3.625%
Senior notes, issuance rate	99.90%
Proceeds from issuance of Senior notes	$ 494,800
Senior notes, effective yield	3.64%
Amortization of deferred charges	$ 4,600
Jazwares, LLC | Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt				$ 180,000
Wilbert Funeral Services, Inc | Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt				73,800
Kentucky Trailer | Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt				61,800
WWSC Holdings, LLC | Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt				75,900
Integrated Project Services LLC | Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt				37,000
Precision Cutting Technologies, Inc | Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt				$ 34,000
Concord | Operating Segments | Alleghany Capital Corporation Segment | Floating Rate Debt Funded by Alleghany Corporation
Debt Instrument [Line Items]
Senior Notes and other debt						$ 33,000