Summary of Dividends Paid to Alleghany by Operating Subsidiaries (Parenthetical) (Detail) - USD ($) $ in Thousands			12 Months Ended
		Jul. 01, 2019	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
Dividends Payable [Line Items]
Dividends paid			$ 215,013	$ 0		$ 153,967
Transatlantic Reinsurance Company
Dividends Payable [Line Items]
Dividends paid			245,000	220,000		280,000
Dividend of Ownership Interest in CapSpecialty, Inc. | Transatlantic Reinsurance Company
Dividends Payable [Line Items]
Dividends paid				101,000
TransRe
Dividends Payable [Line Items]
Dividends paid	[1]		$ 210,000	$ 301,000	[2]	$ 300,000
TransRe | Dividend of Ownership Interest in CapSpecialty, Inc.
Dividends Payable [Line Items]
Dividends paid		$ 101,000

[1] In 2020, 2019 and 2018, TRC paid cash dividends of $245.0 million, $220.0 million and $280.0 million, respectively, to the TransRe holding company. In addition, in 2019, TRC transferred its ownership interest in CapSpecialty to the TransRe holding company, and consequently, the TransRe holding company recorded a dividend from TRC in the amount of $101.0 million

[2] Includes $101.0 million representing the July 1, 2019 carrying value of TransRe’s ownership interest in CapSpecialty, which was transferred to Alleghany