Loans, Leases, and Allowance for Credit Losses (Summary of Major Portfolio Segment and Specific Loan Class) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans held for sale	$ 67	$ 81
Total loans and leases	50,678	53,476
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	28,449	30,472
Commercial | Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	13,230	13,444
Commercial | PPP
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	3,080	5,572
Commercial | Leasing
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	293	320
Commercial | Owner-occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	8,446	8,185
Commercial | Municipal
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	3,400	2,951
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	12,153	12,104
Commercial real estate | Construction and land development
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	2,843	2,345
Commercial real estate | Term
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	9,310	9,759
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	10,076	10,900
Consumer | Home equity credit line
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	2,834	2,745
Consumer | 1-4 family residential
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	6,140	6,969
Consumer | Construction and other consumer real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	584	630
Consumer | Bankcard and other revolving plans
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	395	432
Consumer | Other
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans and leases	$ 123	$ 124