Loans and Leases - Summary of Loans and Leases, Net of Unearned Income (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	$ 23,289	$ 22,153
Total commercial loans and leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	14,721	13,761
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	8,960	8,786
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	5,308	4,556
Commercial leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	432	373
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	21	46
Total consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	8,568	8,392
Direct installment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	1,821	1,764
Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	3,374	3,113
Indirect installment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	1,922	1,933
Consumer lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	1,451	1,582
Originated Loans and Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	20,397	18,062
Originated Loans and Leases | Total commercial loans and leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	12,630	10,730
Originated Loans and Leases | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	7,114	6,171
Originated Loans and Leases | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	5,063	4,140
Originated Loans and Leases | Commercial leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	432	373
Originated Loans and Leases | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	21	46
Originated Loans and Leases | Total consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	7,767	7,332
Originated Loans and Leases | Direct installment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	1,758	1,668
Originated Loans and Leases | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	2,995	2,612
Originated Loans and Leases | Indirect installment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	1,922	1,933
Originated Loans and Leases | Consumer lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	1,092	1,119
Loans Acquired in a Business Combination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	2,892	4,091
Loans Acquired in a Business Combination | Total commercial loans and leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	2,091	3,031
Loans Acquired in a Business Combination | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	1,846	2,615
Loans Acquired in a Business Combination | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	245	416
Loans Acquired in a Business Combination | Commercial leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	0	0
Loans Acquired in a Business Combination | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	0	0
Loans Acquired in a Business Combination | Total consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	801	1,060
Loans Acquired in a Business Combination | Direct installment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	63	96
Loans Acquired in a Business Combination | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	379	501
Loans Acquired in a Business Combination | Indirect installment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	0	0
Loans Acquired in a Business Combination | Consumer lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	$ 359	$ 463