Investments in fixed maturity securities - Narrative (Detail)	Mar. 31, 2026	Dec. 31, 2025
Summary of Investment Holdings [Line Items]
Investment, Type [Extensible Enumeration]	us-gaap:USTreasuryBillSecuritiesMember	us-gaap:USTreasuryBillSecuritiesMember
AA or Higher Credit Rating [Member]
Summary of Investment Holdings [Line Items]
Percentage of fixed maturity investments by credit rating	95.00%
Investment, Type [Extensible Enumeration]	Debt Security, Government, Non-US [Member]

Indicates type of investment. Includes, but is not limited to, common stock, preferred stock, convertible security, fixed income security, government security, option purchased, warrant, loan participation and assignment, commercial paper, bankers' acceptance, certificates of deposit, short-term security, repurchase agreement, and other investment company.

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+ Details

- Definition

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.

+ References

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+ Details