Summary of Dividends Paid to Alleghany by Operating Subsidiaries (Detail) - USD ($) $ in Thousands			12 Months Ended
		Mar. 15, 2018	Dec. 31, 2019	[2]	Dec. 31, 2018	Dec. 31, 2017	[3]
Dividends Payable [Line Items]
Dividends paid		$ 154,000			$ 153,967
TransRe
Dividends Payable [Line Items]
Dividends paid	[1]		$ 301,000		300,000	$ 225,000
RSUI
Dividends Payable [Line Items]
Dividends paid	[4]		100,000		100,000	75,000
Transatlantic Holdings Incorporated And Rsui Group Incorporated
Dividends Payable [Line Items]
Dividends paid			$ 401,000		$ 400,000	$ 300,000

[1] In 2019, 2018 and 2017, TRC paid cash dividends of $220.0 million, $280.0 million and $270.0 million, respectively, to the TransRe holding company. In addition, 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.

[3] Dividends to Alleghany were reduced as a consequence of significant TransRe and RSUI catastrophe losses in the third quarter of 2017.

[4] Alleghany received an extraordinary dividend from RSUI in January 2020.