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Long-duration insurance contracts - Summary of Other Information Relating to Long-Duration Insurance Liabilities (Detail) - USD ($) $ in Millions | Mar. 31, 2025 | Mar. 31, 2024
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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

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