Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 9,735
Due after one year through five years - amortized cost	5,128
Due after five years through ten years - amortized cost	573
Due after ten years - amortized cost	130
Amortized cost of mortgage-backed securities	134
Amortized cost of fixed maturity securities	15,700	$ 23,525
Due in one year or less - fair value	9,801
Due after one year through five years - fair value	5,163
Due after five years through ten years - fair value	784
Due after ten years - fair value	145
Fair value of mortgage-backed securities	149
Fair value of fixed maturity securities	$ 16,042	$ 23,758