Summary of Dividends Paid to Alleghany by Operating Subsidiaries (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
TransRe
Dividends Payable [Line Items]
Dividends paid	[1]	$ 375.0	$ 250.0	$ 300.0
RSUI
Dividends Payable [Line Items]
Dividends paid		100.0	150.0	225.0
Transatlantic Holdings Incorporated And Rsui Group Incorporated
Dividends Payable [Line Items]
Dividends paid		$ 475.0	$ 400.0	$ 525.0

[1] In 2016, 2015 and 2014, TRC paid dividends of $350.0 million, $400.0 million and $400.0 million, respectively, to the TransRe holding company.