Reclassifications of Accumulated Other Comprehensive Income (Parenthetical) (Detail) - USD ($) $ in Millions			12 Months Ended
	Dec. 29, 2017	Apr. 15, 2016	Dec. 31, 2017	Dec. 31, 2016
Jazwares, LLC
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on acquisition		$ 13.2		$ 13.2
Reinsurance Segment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Write-down of assets			$ (7.9)
Pacific Comp
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pre-tax gain from sale of business			8.4
Stranded Oil Resources Corporation
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on sale of oil field	$ (4.8)		(4.8)
Write down of oil field assets				$ (98.8)
Other activities | Alleghany Capital Corporation
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on settlement of contingent consideration liabilities			$ 20.9