Long-duration insurance contracts - Summary of undiscounted and discounted expected future gross premiums to be collected and undiscounted expected future benefits for periodic payment annuities and life and health insurance (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Periodic Payment Annuities [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Undiscounted expected future benefits	$ 31,049	$ 31,065
Life and Health [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Undiscounted expected future gross premiums	106,518	101,577
Discounted expected future gross premiums	64,394	60,734
Undiscounted expected future benefits	$ 101,882	$ 97,031