Summary of Dividends Paid to Alleghany Capital Corporation by its Subsidiaries (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Dividends Payable [Line Items]
Payments of Dividends		$ 0	$ 215,013	$ 0
Alleghany Capital Corporation Subsidiaries
Dividends Payable [Line Items]
Payments of Dividends	[1]	$ 133,300	$ 60,900	$ 82,400

[1] Federal income taxes for most Alleghany Capital subsidiaries are incurred at the Alleghany Capital corporate level. As such, these dividends were arrived at after deducting a provision for estimated income taxes to be paid at the Alleghany Capital-level, as applicable. Income tax provisions are finalized when tax returns are filed in future periods.