Loans, Leases, and Allowance for Credit Losses (Summary of Major Portfolio Segment and Specific Loan Class) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans held for sale	$ 81	$ 129
Total loans and leases	53,476	48,709
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	30,472	25,388
Commercial | Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	13,444	14,760
Commercial | Leasing
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	320	334
Commercial | Owner-occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	8,185	7,901
Commercial | Municipal
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	2,951	2,393
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	12,104	11,555
Commercial real estate | Construction and land development
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	2,345	2,211
Commercial real estate | Term
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	9,759	9,344
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	10,900	11,766
Consumer | Home equity credit line
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	2,745	2,917
Consumer | 1-4 family residential
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	6,969	7,568
Consumer | Construction and other consumer real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	630	624
Consumer | Bankcard and other revolving plans
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	432	502
Consumer | Other
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	$ 124	$ 155