Retroactive reinsurance contracts - Narrative (Detail) - USD ($) $ in Billions	Jun. 30, 2026	Dec. 31, 2025
Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Deferred charge assets on retroactive reinsurance contracts	$ 7.7	$ 8.1

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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+ Details