Loans, Leases, and Allowance for Credit Losses (Summary of Outstanding Loan Balances (Accruing and Nonaccruing) Categorized by Credit Quality Indicators) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	$ 9,691	$ 15,931
2020	9,342	9,700
2019	6,675	7,093
2018	4,313	4,439
2017	3,080	3,104
Prior	6,375	5,596
Revolving loans amortized cost basis	10,553	7,321
Revolving loans converted to term loans amortized cost basis	649	292
Total loans	50,678	53,476
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	6,334	11,233
2020	5,203	5,348
2019	3,347	3,934
2018	2,337	2,437
2017	1,634	1,087
Prior	2,731	2,526
Revolving loans amortized cost basis	6,584	3,766
Revolving loans converted to term loans amortized cost basis	279	141
Total loans	28,449	30,472
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	2,170	3,260
2020	2,760	2,992
2019	2,408	2,190
2018	1,377	876
2017	661	821
Prior	1,667	1,436
Revolving loans amortized cost basis	850	501
Revolving loans converted to term loans amortized cost basis	260	28
Total loans	12,153	12,104
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,187	1,438
2020	1,379	1,360
2019	920	969
2018	599	1,126
2017	785	1,196
Prior	1,977	1,634
Revolving loans amortized cost basis	3,119	3,054
Revolving loans converted to term loans amortized cost basis	110	123
Total loans	10,076	10,900
Commercial and industrial \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,833	2,844
2020	1,481	3,054
2019	1,527	2,225
2018	955	979
2017	419	328
Prior	384	303
Revolving loans amortized cost basis	6,427	3,585
Revolving loans converted to term loans amortized cost basis	204	126
Total loans	13,230	13,444
PPP \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,950	5,572
2020	1,130	0
2019	0	0
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	3,080	5,572
Leasing \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	16	90
2020	53	122
2019	79	47
2018	70	36
2017	47	14
Prior	28	11
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	293	320
Owner-occupied \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,667	1,696
2020	1,551	1,345
2019	1,135	1,303
2018	1,022	1,003
2017	803	677
Prior	2,039	1,968
Revolving loans amortized cost basis	154	178
Revolving loans converted to term loans amortized cost basis	75	15
Total loans	8,446	8,185
Municipal \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	868	1,031
2020	988	827
2019	606	359
2018	290	419
2017	365	68
Prior	280	244
Revolving loans amortized cost basis	3	3
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	3,400	2,951
Construction and land development \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	461	582
2020	765	1,006
2019	751	278
2018	124	41
2017	43	1
Prior	2	6
Revolving loans amortized cost basis	637	428
Revolving loans converted to term loans amortized cost basis	60	3
Total loans	2,843	2,345
Term \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,709	2,678
2020	1,995	1,986
2019	1,657	1,912
2018	1,253	835
2017	618	820
Prior	1,665	1,430
Revolving loans amortized cost basis	213	73
Revolving loans converted to term loans amortized cost basis	200	25
Total loans	9,310	9,759
Home equity credit line \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	2,727	2,625
Revolving loans converted to term loans amortized cost basis	107	120
Total loans	2,834	2,745
1-4 family residential \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	969	1,187
2020	1,079	1,029
2019	796	841
2018	556	1,100
2017	774	1,191
Prior	1,966	1,621
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	6,140	6,969
Construction and other consumer real estate \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	162	200
2020	274	296
2019	104	106
2018	31	16
2017	5	1
Prior	8	11
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	584	630
Bankcard and other revolving plans \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	392	429
Revolving loans converted to term loans amortized cost basis	3	3
Total loans	395	432
Other \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	56	51
2020	26	35
2019	20	22
2018	12	10
2017	6	4
Prior	3	2
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	123	124
Pass \| Commercial and industrial \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,804	2,585
2020	1,419	2,743
2019	1,377	1,903
2018	855	829
2017	354	296
Prior	277	228
Revolving loans amortized cost basis	6,018	3,298
Revolving loans converted to term loans amortized cost basis	182	109
Total loans	12,286	11,991
Pass \| PPP \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,950	5,572
2020	1,130	0
2019	0	0
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	3,080	5,572
Pass \| Leasing \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	16	87
2020	52	121
2019	73	44
2018	69	34
2017	46	14
Prior	22	5
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	278	305
Pass \| Owner-occupied \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,591	1,588
2020	1,504	1,205
2019	1,052	1,167
2018	914	895
2017	743	585
Prior	1,858	1,806
Revolving loans amortized cost basis	135	161
Revolving loans converted to term loans amortized cost basis	68	11
Total loans	7,865	7,418
Pass \| Municipal \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	858	1,031
2020	979	827
2019	606	359
2018	290	419
2017	365	68
Prior	249	227
Revolving loans amortized cost basis	3	3
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	3,350	2,934
Pass \| Construction and land development \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	449	558
2020	765	933
2019	723	267
2018	124	41
2017	43	1
Prior	2	6
Revolving loans amortized cost basis	637	423
Revolving loans converted to term loans amortized cost basis	60	3
Total loans	2,803	2,232
Pass \| Term \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,636	2,524
2020	1,964	1,858
2019	1,595	1,639
2018	1,121	761
2017	590	778
Prior	1,561	1,291
Revolving loans amortized cost basis	194	73
Revolving loans converted to term loans amortized cost basis	195	20
Total loans	8,856	8,944
Pass \| Home equity credit line \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	2,715	2,606
Revolving loans converted to term loans amortized cost basis	100	115
Total loans	2,815	2,721
Pass \| 1-4 family residential \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	969	1,185
2020	1,076	1,017
2019	791	833
2018	553	1,081
2017	762	1,174
Prior	1,929	1,570
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	6,080	6,860
Pass \| Construction and other consumer real estate \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	162	200
2020	273	296
2019	104	106
2018	31	16
2017	5	1
Prior	8	11
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	583	630
Pass \| Bankcard and other revolving plans \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	390	426
Revolving loans converted to term loans amortized cost basis	3	2
Total loans	393	428
Pass \| Other \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	56	51
2020	26	35
2019	20	22
2018	12	10
2017	6	4
Prior	3	2
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	123	124
Special Mention \| Commercial and industrial \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	6	79
2020	15	152
2019	37	183
2018	15	98
2017	30	4
Prior	4	43
Revolving loans amortized cost basis	172	110
Revolving loans converted to term loans amortized cost basis	1	1
Total loans	280	670
Special Mention \| Leasing \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	1
2020	1	0
2019	6	2
2018	1	1
2017	1	0
Prior	0	6
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	9	10
Special Mention \| Owner-occupied \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	9	72
2020	18	65
2019	28	60
2018	45	60
2017	21	51
Prior	59	41
Revolving loans amortized cost basis	3	9
Revolving loans converted to term loans amortized cost basis	6	3
Total loans	189	361
Special Mention \| Municipal \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	10	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior	26	8
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	36	8
Special Mention \| Construction and land development \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	24
2020	0	43
2019	1	11
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	0	5
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	1	83
Special Mention \| Term \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	65	110
2020	21	89
2019	32	177
2018	73	42
2017	12	23
Prior	31	85
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	3	5
Total loans	237	531
Special Mention \| Home equity credit line \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	1	2
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	1	2
Special Mention \| 1-4 family residential \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior	0	2
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	0	2
Special Mention \| Construction and other consumer real estate \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	0	0
Special Mention \| Bankcard and other revolving plans \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	0	0
Special Mention \| Other \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	0	0
Accruing Substandard \| Commercial and industrial \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	20	123
2020	36	157
2019	109	129
2018	82	44
2017	31	26
Prior	77	17
Revolving loans amortized cost basis	150	141
Revolving loans converted to term loans amortized cost basis	2	6
Total loans	507	643
Accruing Substandard \| Leasing \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	2
2020	0	1
2019	0	1
2018	0	1
2017	0	0
Prior	6	0
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	6	5
Accruing Substandard \| Owner-occupied \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	67	28
2020	23	64
2019	42	61
2018	48	37
2017	29	35
Prior	101	98
Revolving loans amortized cost basis	14	6
Revolving loans converted to term loans amortized cost basis	1	1
Total loans	325	330
Accruing Substandard \| Municipal \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	9	0
2019	0	0
2018	0	0
2017	0	0
Prior	5	9
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	14	9
Accruing Substandard \| Construction and land development \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	12	0
2020	0	30
2019	27	0
2018	0	0
2017	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	39	30
Accruing Substandard \| Term \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	8	41
2020	9	34
2019	25	96
2018	59	30
2017	16	18
Prior	73	34
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	2	0
Total loans	192	253
Accruing Substandard \| Home equity credit line \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0

Loans, Leases, and Allowance for Credit Losses (Summary of Outstanding Loan Balances (Accruing and Nonaccruing) Categorized by Credit Quality Indicators) (Details) - USD ($)
$ in Millions