Derivative financial instruments - Information about Interest Rate Swap Agreements (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Mar. 31, 2025	Sep. 30, 2025	Dec. 31, 2024
Derivative [Line Items]
Notional Amount		$ 31,100	$ 36,184
Weighted-Average Maturity (In years)	2 years 2 months 12 days	2 years 2 months 12 days
Estimated fair value gain (loss)	$ (1)	$ 12
Fair value hedges:
Derivative [Line Items]
Impact of estimated fair value gain (losses) on hedging instruments	(153)	1
Fair value hedges: \| Forward-Starting Interest Rate Swap Agreements
Derivative [Line Items]
Notional Amount		2,800	3,400
Cash flow hedges:
Derivative [Line Items]
Impact of estimated fair value gain (losses) on hedging instruments	$ (136)	83
Cash flow hedges: \| Forward-Starting Interest Rate Swap Agreements
Derivative [Line Items]
Notional Amount		10,400	10,000
Interest Payments On Variable Rate Commercial Real Estate Loans \| Cash flow hedges:
Derivative [Line Items]
Notional Amount		$ 25,000	$ 30,819
Weighted-Average Maturity (In years)	1 year 7 months 6 days	1 year 6 months
Fixed		3.65%	3.41%
Variable		4.28%	4.47%
Estimated fair value gain (loss)	$ 1	$ 8
Fixed Rate Long-Term Borrowings \| Fair value hedges:
Derivative [Line Items]
Notional Amount		$ 6,100	$ 5,350
Weighted-Average Maturity (In years)	5 years 10 months 24 days	5 years 1 month 6 days
Fixed		3.56%	3.55%
Variable		4.26%	4.71%
Estimated fair value gain (loss)	$ (2)	$ 4
Fixed Rate Available For Sale Securities \| Fair value hedges:
Derivative [Line Items]
Notional Amount			$ 15
Weighted-Average Maturity (In years)	1 month 6 days
Fixed			4.84%
Variable			4.36%
Estimated fair value gain (loss)	$ 0

Impact in estimated fair value gain (losses ) on hedging instruments.

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- Definition

Average fixed interest rate related to the group of interest rate derivatives.

+ References

No definition available.

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- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

Average variable interest rate related to the group of interest rate derivatives.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 20
-Name Accounting Standards Codification
-Section 25
-Paragraph 6A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480682/815-20-25-6A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480237/815-40-50-5

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- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 181
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480401/815-10-55-181

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
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-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1A

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- Definition

Amount of unrealized gain (loss) related to the fair value of interest rate derivatives designated as fair value hedging instruments, as offset by the gain (loss) on the hedged item to the extent that the fair value hedge is determined to be effective.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4C

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