Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 12,218
Due after one year through five years - amortized cost	3,869
Due after five years through ten years - amortized cost	600
Due after ten years - amortized cost	135
Amortized cost of mortgage-backed securities	147
Amortized cost of fixed maturity securities	16,969	$ 23,525
Due in one year or less - fair value	12,177
Due after one year through five years - fair value	3,889
Due after five years through ten years - fair value	798
Due after ten years - fair value	144
Fair value of mortgage-backed securities	159
Fair value of fixed maturity securities	$ 17,167	$ 23,758