Contingencies and Commitments - Narrative (Detail) - Alleghany Corporation [Member] $ in Billions	Mar. 21, 2022 USD ($)
Business Acquisition [Line Items]
Acquisition of outstanding shares, value	$ 11.6
Business acquisition, agreement date	Mar. 21, 2022

Fair value at acquisition-date of the equity interest in the acquiree held by the acquirer, immediately before the acquisition date for businesses combined in stages.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (g)(1)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration