Loans and allowance for loan losses - Summary of Loan grades applied various classes of Commercial and Real Estate Loans (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		$ 18,344		$ 18,344		$ 19,234
Year two		15,460		15,460		15,083
Year three		12,360		12,360		17,786
Year four		15,149		15,149		11,704
Year five		9,759		9,759		8,328
Prior		30,693		30,693		28,783
Total	[1]	136,974		136,974		135,581
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one		13		25
Year two		36		70
Year three		24		67
Year four		30		72
Year five		8		28
Prior		20		96
Total		190	$ 154	506	$ 497
Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		7,099		7,099		9,221
Year two		7,551		7,551		6,895
Year three		5,208		5,208		6,345
Year four		5,042		5,042		3,509
Year five		2,598		2,598		1,668
Prior		6,015		6,015		5,895
Revolving Loans		28,222		28,222		27,815
Revolving Loans Converted to Term Loans		152		152		133
Total		61,887		61,887		61,481
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one		3		7
Year two		21		29
Year three		12		31
Year four		6		19
Year five		2		6
Prior		3		12
Revolving Loans		42		92
Revolving Loans Converted to Term Loans		0		0
Total		89		196
Commercial and industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		6,980		6,980		9,021
Year two		7,252		7,252		6,454
Year three		4,626		4,626		5,845
Year four		4,526		4,526		3,258
Year five		2,445		2,445		1,534
Prior		5,346		5,346		5,147
Revolving Loans		26,722		26,722		26,262
Revolving Loans Converted to Term Loans		97		97		79
Total		57,994		57,994		57,600
Commercial and industrial | Criticized accrual
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		115		115		189
Year two		272		272		385
Year three		492		492		402
Year four		440		440		210
Year five		120		120		75
Prior		427		427		528
Revolving Loans		1,232		1,232		1,359
Revolving Loans Converted to Term Loans		35		35		37
Total		3,133		3,133		3,185
Commercial and industrial | Criticized nonaccrual
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		4		4		11
Year two		27		27		56
Year three		90		90		98
Year four		76		76		41
Year five		33		33		59
Prior		242		242		220
Revolving Loans		268		268		194
Revolving Loans Converted to Term Loans		20		20		17
Total		760		760		696
Commercial Real Estate
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Total		22	$ 24	69	$ 92
Commercial Real Estate | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		2,675		2,675		714
Year two		437		437		1,867
Year three		1,852		1,852		2,076
Year four		1,983		1,983		1,501
Year five		1,338		1,338		2,382
Prior		11,378		11,378		11,819
Revolving Loans		399		399		421
Revolving Loans Converted to Term Loans		0		0		0
Total		20,062		20,062		20,780
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one		0		0
Year two		0		0
Year three		0		0
Year four		11		15
Year five		0		0
Prior		7		47
Revolving Loans		0		0
Revolving Loans Converted to Term Loans		0		0
Total		18		62
Commercial Real Estate | Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		2,675		2,675		674
Year two		408		408		1,477
Year three		1,548		1,548		1,358
Year four		1,610		1,610		1,222
Year five		1,205		1,205		1,774
Prior		9,538		9,538		9,611
Revolving Loans		393		393		413
Revolving Loans Converted to Term Loans		0		0		0
Total		17,377		17,377		16,529
Commercial Real Estate | Commercial | Criticized accrual
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		0		0		39
Year two		29		29		389
Year three		297		297		665
Year four		337		337		253
Year five		83		83		591
Prior		1,572		1,572		1,839
Revolving Loans		6		6		7
Revolving Loans Converted to Term Loans		0		0		0
Total		2,324		2,324		3,783
Commercial Real Estate | Commercial | Criticized nonaccrual
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		0		0		1
Year two		0		0		1
Year three		7		7		53
Year four		36		36		26
Year five		50		50		17
Prior		268		268		369
Revolving Loans		0		0		1
Revolving Loans Converted to Term Loans		0		0		0
Total		361		361		468
Commercial Real Estate | Residential builder and developer
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		3		3		396
Year two		0		0		278
Year three		3		3		74
Year four		30		30		12
Year five		0		0		4
Prior		5		5		11
Revolving Loans		51		51		60
Revolving Loans Converted to Term Loans		0		0		0
Total		92		92		835
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one		0		0
Year two		0		0
Year three		0		0
Year four		0		0
Year five		0		0
Prior		0		0
Revolving Loans		0		0
Revolving Loans Converted to Term Loans		0		0
Total		0		0
Commercial Real Estate | Residential builder and developer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		3		3		380
Year two		0		0		236
Year three		3		3		40
Year four		9		9		12
Year five		0		0		4
Prior		5		5		10
Revolving Loans		51		51		60
Revolving Loans Converted to Term Loans		0		0		0
Total		71		71		742
Commercial Real Estate | Residential builder and developer | Criticized accrual
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		0		0		15
Year two		0		0		42
Year three		0		0		34
Year four		21		21		0
Year five		0		0		0
Prior		0		0		0
Revolving Loans		0		0		0
Revolving Loans Converted to Term Loans		0		0		0
Total		21		21		91
Commercial Real Estate | Residential builder and developer | Criticized nonaccrual
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		0		0		1
Year two		0		0		0
Year three		0		0		0
Year four		0		0		0
Year five		0		0		0
Prior		0		0		1
Revolving Loans		0		0		0
Revolving Loans Converted to Term Loans		0		0		0
Total		0		0		2
Commercial Real Estate | Other commercial construction
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		138		138		150
Year two		211		211		1,499
Year three		1,418		1,418		1,795
Year four		1,291		1,291		648
Year five		225		225		472
Prior		565		565		540
Revolving Loans		44		44		45
Revolving Loans Converted to Term Loans		0		0		0
Total		3,892		3,892		5,149
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff, by Origination Year [Abstract]
Year one		0		0
Year two		0		0
Year three		0		0
Year four		4		7
Year five		0		0
Prior		0		0
Revolving Loans		0		0
Revolving Loans Converted to Term Loans		0		0
Total		4		7
Commercial Real Estate | Other commercial construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		138		138		108
Year two		205		205		1,395
Year three		1,265		1,265		1,091
Year four		652		652		269
Year five		78		78		175
Prior		339		339		379
Revolving Loans		38		38		42
Revolving Loans Converted to Term Loans		0		0		0
Total		2,715		2,715		3,459
Commercial Real Estate | Other commercial construction | Criticized accrual
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		0		0		42
Year two		6		6		104
Year three		153		153		687
Year four		629		629		346
Year five		143		143		297
Prior		219		219		145
Revolving Loans		6		6		3
Revolving Loans Converted to Term Loans		0		0		0
Total		1,156		1,156		1,624
Commercial Real Estate | Other commercial construction | Criticized nonaccrual
Financing Receivable, Credit Quality Indicator [Line Items]
Year one		0		0		0
Year two		0		0		0
Year three		0		0		17
Year four		10		10		33
Year five		4		4		0
Prior		7		7		16
Revolving Loans		0		0		0
Revolving Loans Converted to Term Loans		0		0		0
Total		$ 21		$ 21		$ 66

[1] Loans of $2.3 billion and $1.5 billion at September 30, 2025 and December 31, 2024, respectively, were held in special purpose trusts to settle the respective obligations of asset-backed notes issued by those trusts. The outstanding balances of those asset-backed notes issued to third party investors were included in Long-term borrowings and were $1.9 billion at September 30, 2025 and $1.2 billion at December 31, 2024.