Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 10,228
Due after one year through five years - amortized cost	4,202
Due after five years through ten years - amortized cost	532
Due after ten years - amortized cost	123
Amortized cost of mortgage-backed securities	129
Amortized cost of fixed maturity securities	15,214	$ 23,525
Due in one year or less - fair value	10,107
Due after one year through five years - fair value	4,267
Due after five years through ten years - fair value	718
Due after ten years - fair value	133
Fair value of mortgage-backed securities	139
Fair value of fixed maturity securities	$ 15,364	$ 23,758