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Long-duration insurance contracts - Summary of Other Information Relating to Long-Duration Insurance Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Periodic Payment Annuity [Member]
Liability for Future Policy Benefit, Activity [Line Items]
Undiscounted expected future benefits	$ 30,522	$ 30,915
Weighted average discount rate	5.70%	5.20%
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	$ 103,457	$ 101,134
Discounted expected future gross premiums	61,069	58,530
Undiscounted expected future benefits	$ 94,059	$ 91,751
Weighted average discount rate	5.00%	4.70%
Weighted average accretion rate	2.60%	2.60%
Weighted average duration (in years)	13 years	13 years

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Current weighted-average discount rate used to measure liability for future policy benefit.

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Amount, before effect of reinsurance, of discounted balance for expected future gross premium component of liability for future policy benefit.

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Amount, before effect of reinsurance, of undiscounted balance for expected future gross premium component of liability for future policy benefit.

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Amount, before effect of reinsurance, of undiscounted balance for expected future policy benefit and expense component of liability for future policy benefit.

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Weighted-average duration of liability for future policy benefit, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Original weighted-average discount rate at contract issue date used to measure liability for future policy benefit.

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