Summary of Dividends Paid to Alleghany by Operating Subsidiaries (Detail) - USD ($) $ in Thousands		12 Months Ended
	Mar. 15, 2018	Dec. 31, 2018	Dec. 31, 2017	[2]	Dec. 31, 2016
Dividends Payable [Line Items]
Cash dividends paid	$ 154,000	$ 153,967
TransRe
Dividends Payable [Line Items]
Cash dividends paid		300,000	$ 225,000	[1]	$ 375,000	[1]
RSUI
Dividends Payable [Line Items]
Cash dividends paid		100,000	75,000		100,000
Transatlantic Holdings Incorporated And Rsui Group Incorporated
Dividends Payable [Line Items]
Cash dividends paid		$ 400,000	$ 300,000		$ 475,000

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

[2] In 2018, 2017 and 2016, TRC paid dividends of $280.0 million, $270.0 million and $350.0 million, respectively, to the TransRe holding company.