Long-duration insurance contracts - Summary of Other Information Relating to Long-Duration Insurance Liabilities (Detail) - USD ($) $ in Millions	Mar. 31, 2025	Mar. 31, 2024
Periodic Payment Annuity [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Undiscounted expected future benefits	$ 30,603	$ 30,953
Weighted average discount rate	5.70%	5.40%
Weighted average accretion rate	4.80%	4.80%
Weighted average duration (in years)	16 years	17 years
Life and Health [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Undiscounted expected future gross premiums	$ 100,956	$ 95,514
Discounted expected future gross premiums	59,104	56,585
Undiscounted expected future benefits	$ 92,018	$ 86,800
Weighted average discount rate	5.20%	4.90%
Weighted average accretion rate	2.70%	2.70%
Weighted average duration (in years)	14 years	13 years