Loans and leases and allowance for credit losses - Summary of Payment Status of Loans Modified (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	$ 1,130	$ 1,586
Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	953	1,414
30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	117	123
Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	60	49
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	303	198
Commercial and industrial | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	276	182
Commercial and industrial | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	14	7
Commercial and industrial | Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	13	9
Commercial Real Estate | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	515	651
Commercial Real Estate | Residential builder and developer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	2	71
Commercial Real Estate | Other commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	130	488
Commercial Real Estate | Current | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	478	618
Commercial Real Estate | Current | Residential builder and developer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	1	71
Commercial Real Estate | Current | Other commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	101	440
Commercial Real Estate | 30-89 Days Past Due | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	37	21
Commercial Real Estate | 30-89 Days Past Due | Residential builder and developer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Commercial Real Estate | 30-89 Days Past Due | Other commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	25	48
Commercial Real Estate | Accruing Loans Past Due 90 Days or More | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	12
Commercial Real Estate | Accruing Loans Past Due 90 Days or More | Residential builder and developer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	1	0
Commercial Real Estate | Accruing Loans Past Due 90 Days or More | Other commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	4	0
Residential Real Estate | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	177	177
Residential Real Estate | Current | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	94	102
Residential Real Estate | 30-89 Days Past Due | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	41	47
Residential Real Estate | Accruing Loans Past Due 90 Days or More | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	42	28
Consumer: | Home equity lines and loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	2	1
Consumer: | Recreational finance
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	1	0
Consumer: | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer: | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer: | Current | Home equity lines and loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	2	1
Consumer: | Current | Recreational finance
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	1	0
Consumer: | Current | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer: | Current | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer: | 30-89 Days Past Due | Home equity lines and loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer: | 30-89 Days Past Due | Recreational finance
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer: | 30-89 Days Past Due | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer: | 30-89 Days Past Due | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer: | Accruing Loans Past Due 90 Days or More | Home equity lines and loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer: | Accruing Loans Past Due 90 Days or More | Recreational finance
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer: | Accruing Loans Past Due 90 Days or More | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer: | Accruing Loans Past Due 90 Days or More | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	$ 0	$ 0