Investments in fixed maturity securities - Narrative (Detail)	Mar. 31, 2024	Dec. 31, 2023
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]