Long-duration insurance contracts - Summary of weighted average discount rates, interest accretion rates established at contract inception and the average contract duration (Detail)	Sep. 30, 2023	Sep. 30, 2022
Periodic Payment Annuities [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Weighted average discount rate	5.90%	5.50%
Weighted average accretion rate	4.80%	4.70%
Weighted average duration (in years)	16 years	17 years
Life and Health [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Weighted average discount rate	5.40%	5.00%
Weighted average accretion rate	3.40%	3.40%
Weighted average duration (in years)	14 years	14 years