Loans, Leases, and Allowance for Credit Losses (Summary of Past Due Loans (Accruing and Nonaccruing)) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 50,678	$ 53,476
Accruing loans 90+ days past due	4	12
Nonaccrual loans that are current	323	367
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	28,449	30,472
Accruing loans 90+ days past due	4	2
Nonaccrual loans that are current	224	216
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	12,153	12,104
Accruing loans 90+ days past due	0	8
Nonaccrual loans that are current	25	31
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	10,076	10,900
Accruing loans 90+ days past due	0	2
Nonaccrual loans that are current	74	120
Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	13,230	13,444
Accruing loans 90+ days past due	2	2
Nonaccrual loans that are current	157	140
PPP | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	3,080	5,572
Accruing loans 90+ days past due	0	0
Leasing | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	293	320
Accruing loans 90+ days past due	0	0
Owner-occupied | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	8,446	8,185
Accruing loans 90+ days past due	2	0
Nonaccrual loans that are current	67	76
Municipal | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	3,400	2,951
Accruing loans 90+ days past due	0	0
Construction and land development | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2,843	2,345
Accruing loans 90+ days past due	0	4
Term | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	9,310	9,759
Accruing loans 90+ days past due	0	4
Nonaccrual loans that are current	25	31
Home equity credit line | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2,834	2,745
Accruing loans 90+ days past due	0	0
Nonaccrual loans that are current	15	16
1-4 family residential | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	6,140	6,969
Accruing loans 90+ days past due	0	0
Nonaccrual loans that are current	58	103
Construction and other consumer real estate | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	584	630
Accruing loans 90+ days past due	0
Bankcard and other revolving plans | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	395	432
Accruing loans 90+ days past due	0	2
Nonaccrual loans that are current	1	1
Other | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	123	124
Accruing loans 90+ days past due	0	0
Current
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	50,464	53,198
Current | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	28,324	30,360
Current | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	12,112	12,033
Current | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	10,028	10,805
Current | Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	13,191	13,388
Current | PPP | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	3,078	5,572
Current | Leasing | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	293	320
Current | Owner-occupied | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	8,362	8,129
Current | Municipal | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	3,400	2,951
Current | Construction and land development | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2,824	2,341
Current | Term | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	9,288	9,692
Current | Home equity credit line | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2,828	2,733
Current | 1-4 family residential | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	6,103	6,891
Current | Construction and other consumer real estate | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	584	630
Current | Bankcard and other revolving plans | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	391	428
Current | Other | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	122	123
Total past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	214	278
Total past due | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	125	112
Total past due | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	41	71
Total past due | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	48	95
Total past due | Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	39	56
Total past due | PPP | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2	0
Total past due | Leasing | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	0
Total past due | Owner-occupied | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	84	56
Total past due | Municipal | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	0
Total past due | Construction and land development | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	19	4
Total past due | Term | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	22	67
Total past due | Home equity credit line | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	6	12
Total past due | 1-4 family residential | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	37	78
Total past due | Construction and other consumer real estate | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	0
Total past due | Bankcard and other revolving plans | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	4	4
Total past due | Other | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1	1
30-89 days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	124	140
30-89 days past due | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	84	60
30-89 days past due | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	26	57
30-89 days past due | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	14	23
30-89 days past due | Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	16	26
30-89 days past due | PPP | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2	0
30-89 days past due | Leasing | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	0
30-89 days past due | Owner-occupied | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	66	34
30-89 days past due | Municipal | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	0
30-89 days past due | Construction and land development | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	19	0
30-89 days past due | Term | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	7	57
30-89 days past due | Home equity credit line | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2	8
30-89 days past due | 1-4 family residential | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	8	12
30-89 days past due | Construction and other consumer real estate | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	0
30-89 days past due | Bankcard and other revolving plans | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	3	2
30-89 days past due | Other | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1	1
90+ days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	90	138
90+ days past due | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	41	52
90+ days past due | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	15	14
90+ days past due | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	34	72
90+ days past due | Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	23	30
90+ days past due | PPP | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	0
90+ days past due | Leasing | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	0
90+ days past due | Owner-occupied | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	18	22
90+ days past due | Municipal | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	0
90+ days past due | Construction and land development | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	4
90+ days past due | Term | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	15	10
90+ days past due | Home equity credit line | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	4	4
90+ days past due | 1-4 family residential | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	29	66
90+ days past due | Construction and other consumer real estate | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	0
90+ days past due | Bankcard and other revolving plans | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1	2
90+ days past due | Other | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0	0
Nonaccrual loans that are current
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	227	214
Nonaccrual loans that are current | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	182	158
Nonaccrual loans that are current | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	7	13
Nonaccrual loans that are current | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	38	43
Nonaccrual loans that are current | Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	133	109
Nonaccrual loans that are current | PPP | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	0	0
Nonaccrual loans that are current | Leasing | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	0	1
Nonaccrual loans that are current | Owner-occupied | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	49	48
Nonaccrual loans that are current | Municipal | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	0	0
Nonaccrual loans that are current | Construction and land development | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	0	0
Nonaccrual loans that are current | Term | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	7	13
Nonaccrual loans that are current | Home equity credit line | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	10	9
Nonaccrual loans that are current | 1-4 family residential | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	27	33
Nonaccrual loans that are current | Construction and other consumer real estate | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	0	0
Nonaccrual loans that are current | Bankcard and other revolving plans | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	1	1
Nonaccrual loans that are current | Other | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	$ 0	$ 0