Summary of Dividends Paid to Alleghany by Operating Subsidiaries (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	[1]	Dec. 31, 2016	Dec. 31, 2015
TransRe
Dividends Payable [Line Items]
Dividends paid	[2]	$ 225.0		$ 375.0	$ 250.0
RSUI
Dividends Payable [Line Items]
Dividends paid		75.0		100.0	150.0
Transatlantic Holdings Incorporated And Rsui Group Incorporated
Dividends Payable [Line Items]
Dividends paid		$ 300.0		$ 475.0	$ 400.0

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

[2] In 2017, 2016 and 2015, TRC paid dividends of $270.0 million, $350.0 million and $400.0 million, respectively, to the TransRe holding company.