Reclassifications of Accumulated Other Comprehensive Income (Parenthetical) (Detail) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on early redemption of debt	$ (7.1)
Alleghany Corporation \| 2020 Senior Notes
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on early redemption 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
Precision Cutting Technologies, Inc
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on reduction of contingent consideration liabilities	$ 5.0

The expense recorded to reduce the value of oil and gas assets consisting of proved properties and unproved properties as the estimate of future successful production from these properties is reduced.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109226691&loc=d3e2921-110230

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 360
-Section 35
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=66906256&loc=d3e66150-109466

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration