Loans, Leases, and Allowance for Credit Losses (Summary of Major Portfolio Segment and Specific Loan Class) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans held for sale	$ 66	$ 81
Total loans and leases	51,398	53,476
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	29,161	30,472
Commercial | Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	12,947	13,444
Commercial | PPP
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	4,461	5,572
Commercial | Leasing
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	307	320
Commercial | Owner-occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	8,231	8,185
Commercial | Municipal
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	3,215	2,951
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	12,108	12,104
Commercial real estate | Construction and land development
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	2,576	2,345
Commercial real estate | Term
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	9,532	9,759
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	10,129	10,900
Consumer | Home equity credit line
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	2,727	2,745
Consumer | 1-4 family residential
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	6,269	6,969
Consumer | Construction and other consumer real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	593	630
Consumer | Bankcard and other revolving plans
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	415	432
Consumer | Other
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	$ 125	$ 124