Long-duration insurance contracts - Summary of Other Information Relating to Long-Duration Insurance Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Periodic Payment Annuity [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Undiscounted expected future benefits	$ 30,915	$ 31,049
Weighted average discount rate	5.20%	5.90%
Weighted average accretion rate	4.80%	4.80%
Weighted average duration (in years)	17 years	16 years
Life and Health [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Undiscounted expected future gross premiums	$ 101,134	$ 106,518
Discounted expected future gross premiums	58,530	64,394
Undiscounted expected future benefits	$ 91,751	$ 101,882
Weighted average discount rate	4.70%	5.40%
Weighted average accretion rate	2.60%	3.40%
Weighted average duration (in years)	13 years	14 years