Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	$ 481	$ 634	$ 442	$ 537
Provision charged (credited) to income	(37)	29	15	141
Charge-offs	(38)	(54)	(54)	(71)
Recoveries	17	5	20	7
Net (charge-offs) recoveries	(21)	(49)	(34)	(64)
Allowance for credit losses - end of period	423	614	423	614
Commercial loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs			(48)	(67)
Commercial loans \| Commercial mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	108	199	108	135
Provision charged (credited) to income	(37)	0	(37)	64
Charge-offs	0	(27)	0	(27)
Recoveries	0	0	0	0
Net (charge-offs) recoveries	0	(27)	0	(27)
Allowance for credit losses - end of period	71	172	71	172
Commercial loans \| Business and corporate banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	363	423	327	390
Provision charged (credited) to income	9	39	58	87
Charge-offs	(35)	(25)	(48)	(40)
Recoveries	13	1	13	1
Net (charge-offs) recoveries	(22)	(24)	(35)	(39)
Allowance for credit losses - end of period	350	438	350	438
Commercial loans \| Other commercial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	1	1	1	1
Provision charged (credited) to income	(1)	0	(1)	0
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net (charge-offs) recoveries	0	0	0	0
Allowance for credit losses - end of period	0	1	0	1
Consumer loans \| Residential Mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	(10)	(11)	(12)	(12)
Provision charged (credited) to income	(6)	(8)	(4)	(8)
Charge-offs	(1)	0	(2)	0
Recoveries	3	2	4	3
Net (charge-offs) recoveries	2	2	2	3
Allowance for credit losses - end of period	(14)	(17)	(14)	(17)
Consumer loans \| Other Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	19	22	18	23
Provision charged (credited) to income	(2)	(2)	(1)	(2)
Charge-offs	(2)	(2)	(4)	(4)
Recoveries	1	2	3	3
Net (charge-offs) recoveries	(1)	0	(1)	(1)
Allowance for credit losses - end of period	$ 16	$ 20	$ 16	$ 20

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477657/942-210-S45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
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-Name Accounting Standards Codification
-Section S99
-Paragraph 1
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

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- Definition

Amount, excluding accrued interest, of increase in allowance for credit loss on financing receivable from recovery. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
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-Section 50
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- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
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-Publisher FASB
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- Definition

Amount, excluding accrued interest, of decrease (increase) in allowance for credit loss on financing receivable from writeoff (recovery). Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

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- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
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