Reclassifications of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains		$ (63,205)	$ (213,897)	$ (247,058)
Other than temporary impairment losses		45,165	133,868	36,294
Income taxes		187,141	195,173	$ 251,777
Total reclassifications		(67,400)	(68,700)
Unrealized Appreciation of Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications		(67,400)	(68,700)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Appreciation of Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains	[1]	(148,800)	(239,700)
Other than temporary impairment losses		45,200	133,900
Income taxes		$ 36,200	$ 37,100

[1] For 2016, excludes a ($98.8) million impairment charge from a write-down of certain SORC assets and the Jazwares Remeasurement Gain of $13.2 million. For 2015, excludes a ($25.8) million realized capital loss related to an impairment charge related to a write-off of Alleghany's investment in ORX.