Long duration insurance contracts - Summary of weighted average current discount rates and interest accretion rates established at contract inception (Detail)	Mar. 31, 2023	Mar. 31, 2022
Periodic Payment Annuities [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Weighted average discount rate	5.00%	3.60%
Weighted average accretion rate	4.80%	4.90%
Weighted average duration (in years)	18 years	20 years
Life and Health [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Weighted average discount rate	4.90%	3.40%
Weighted average accretion rate	3.20%	3.50%
Weighted average duration (in years)	14 years	14 years