Long-duration insurance contracts - Summary of life, annuity and health insurance benefits liabilities (Detail) - USD ($) $ in Millions	Mar. 31, 2026	Mar. 31, 2025
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Life, annuity and health insurance benefits	$ 17,479	$ 17,734
Periodic Payment Annuity [Member]
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Life, annuity and health insurance benefits	10,121	10,376
Life and Health [Member]
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Life, annuity and health insurance benefits	4,492	4,485
Other [Member]
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Life, annuity and health insurance benefits	$ 2,866	$ 2,873

Amount, before effect of reinsurance, of present value of future benefit to be paid to or on behalf of policyholder and related expense less present value of future net premium receivable under insurance contract.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 605
-Name Accounting Standards Codification
-Section 55
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477548/944-605-55-14

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 65
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-Publisher FASB
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Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
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-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-6

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant