Reclassifications of Accumulated Other Comprehensive Income (Parenthetical) (Detail) - USD ($) $ in Thousands			12 Months Ended
	Mar. 15, 2018	Dec. 29, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains/ losses			$ (3,241)	$ 107,222	$ 63,205
Alleghany Capital
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on settlement of contingent consideration liabilities				20,900
Reinsurance Segment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Write-down of assets				(7,900)
Insurance Segment | Ares
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains/ losses	$ 45,700		45,700
Pacific Comp
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pre-tax gain from sale of business				8,400
Stranded Oil Resources Corporation
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on sale of oil field		$ (4,800)		$ (4,800)
Write down of oil field assets			$ 35,400		$ 98,800