Reclassifications of Accumulated Other Comprehensive Income (Parenthetical) (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on early extinguishment of debt		$ (7.1)
Pre tax loss from sale of privately held investment accounted under equity method	$ 13.6		$ 13.6
Put Option | Equity Contract
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross realized capital loss from derivative			38.4
Insurance Segment | Put Option | Equity Contract
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross realized capital loss from derivative			$ 38.4
Alleghany Corporation | 2020 Senior Notes
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on early extinguishment of debt		(7.1)
Stranded Oil Resources Corporation
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Write-down of oil field assets		76.0		$ 35.4
Precision Cutting Technologies, Inc
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on reduction of contingent consideration liabilities		$ 5.0