Reclassifications of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other than temporary impairment losses		$ 16,871	$ 45,165	$ 133,868
Income taxes		(63,802)	187,141	$ 195,173
Total reclassifications		(47,900)	(67,400)
Unrealized Appreciation of Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications		(47,900)	(67,400)
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]	(90,600)	(148,800)
Other than temporary impairment losses		16,900	45,200
Income taxes		$ 25,800	$ 36,200

[1] For 2017, excludes an $8.4 million pre-tax gain from the sale of PacificComp, a $20.9 million realized gain on bulk settlement of certain contingent liabilities by Alleghany Capital, a ($4.8) million loss realized from the sale of a SORC legacy oil field on December 29, 2017 and a ($7.9) million write-down of certain Reinsurance Segment assets. 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.