Summary of Dividends Paid to Alleghany by Operating Subsidiaries (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
TransRe
Dividends Payable [Line Items]
Dividends paid	[1]	$ 250.0	$ 300.0	$ 150.0
RSUI
Dividends Payable [Line Items]
Dividends paid		150.0	225.0	100.0
CapSpecialty Incorporated
Dividends Payable [Line Items]
Dividends paid				15.0
Transatlantic Holding Inc Rsui Group Incorporated And Cap Specialty Incorporated
Dividends Payable [Line Items]
Dividends paid		$ 400.0	$ 525.0	$ 265.0

[1] In 2015, Transatlantic Reinsurance Company ("TRC") paid dividends of $400.0 million to the TransRe holding company, of which $250.0 million was in turn paid to Alleghany. In 2014, TRC paid dividends of $400.0 million to the TransRe holding company, of which $300.0 million was in turn paid to Alleghany. In 2013, TRC paid dividends of $200.0 million to the TransRe holding company, of which $150.0 million was in turn paid to Alleghany.