Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 17,453
Due after one year through five years - amortized cost	4,018
Due after five years through ten years - amortized cost	696
Due after ten years - amortized cost	135
Amortized cost of mortgage-backed securities	160
Amortized cost of fixed maturity securities	22,462	$ 25,173
Due in one year or less - fair value	17,261
Due after one year through five years - fair value	3,999
Due after five years through ten years - fair value	867
Due after ten years - fair value	138
Fair value of mortgage-backed securities	170
Fair value of fixed maturity securities	$ 22,435	$ 25,128