Summary of Dividends Paid to Alleghany by Operating Subsidiaries (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
Dividends Payable [Line Items]
Dividends paid		$ 215,013	$ 0		$ 153,967
TransRe
Dividends Payable [Line Items]
Dividends paid	[1]	210,000	301,000	[2]	300,000
RSUI
Dividends Payable [Line Items]
Dividends paid		225,000	100,000	[2]	100,000
Transatlantic Holdings Incorporated And Rsui Group Incorporated
Dividends Payable [Line Items]
Dividends paid		$ 435,000	$ 401,000	[2]	$ 400,000

[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