Long-duration insurance contracts - Summary of Other Information Relating to Long-Duration Insurance Liabilities (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Mar. 31, 2023
Periodic Payment Annuity [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Undiscounted expected future benefits	$ 30,953	$ 31,244
Weighted average discount rate	5.40%	5.00%
Weighted average accretion rate	4.80%	4.80%
Weighted average duration (in years)	17 years	18 years
Life and Health [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Undiscounted expected future gross premiums	$ 95,514	$ 107,831
Discounted expected future gross premiums	56,585	64,421
Undiscounted expected future benefits	$ 86,800	$ 102,881
Weighted average discount rate	4.90%	4.90%
Weighted average accretion rate	2.70%	3.20%
Weighted average duration (in years)	13 years	14 years