Loans, Leases, and Allowance for Credit Losses (Summary of Outstanding Loan Balances (Accruing and Nonaccruing) Categorized by Credit Quality Indicators) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	$ 7,206	$ 15,931
2020	10,492	9,700
2019	7,239	7,093
2018	5,016	4,439
2017	3,397	3,104
Prior	7,071	5,596
Revolving loans amortized cost basis	10,322	7,321
Revolving loans converted to term loans amortized cost basis	655	292
Total loans	51,398	53,476
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	5,057	11,233
2020	6,181	5,348
2019	3,605	3,934
2018	2,654	2,437
2017	1,829	1,087
Prior	2,995	2,526
Revolving loans amortized cost basis	6,564	3,766
Revolving loans converted to term loans amortized cost basis	276	141
Total loans	29,161	30,472
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,440	3,260
2020	2,889	2,992
2019	2,592	2,190
2018	1,659	876
2017	695	821
Prior	1,838	1,436
Revolving loans amortized cost basis	732	501
Revolving loans converted to term loans amortized cost basis	263	28
Total loans	12,108	12,104
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	709	1,438
2020	1,422	1,360
2019	1,042	969
2018	703	1,126
2017	873	1,196
Prior	2,238	1,634
Revolving loans amortized cost basis	3,026	3,054
Revolving loans converted to term loans amortized cost basis	116	123
Total loans	10,129	10,900
Commercial and industrial \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,075	2,844
2020	1,580	3,054
2019	1,683	2,225
2018	1,082	979
2017	492	328
Prior	435	303
Revolving loans amortized cost basis	6,408	3,585
Revolving loans converted to term loans amortized cost basis	192	126
Total loans	12,947	13,444
PPP \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	2,556	5,572
2020	1,905	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	4,461	5,572
Leasing \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	11	90
2020	54	122
2019	85	47
2018	76	36
2017	49	14
Prior	32	11
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	307	320
Owner-occupied \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	867	1,696
2020	1,641	1,345
2019	1,190	1,303
2018	1,183	1,003
2017	893	677
Prior	2,217	1,968
Revolving loans amortized cost basis	156	178
Revolving loans converted to term loans amortized cost basis	84	15
Total loans	8,231	8,185
Municipal \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	548	1,031
2020	1,001	827
2019	647	359
2018	313	419
2017	395	68
Prior	311	244
Revolving loans amortized cost basis	0	3
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	3,215	2,951
Construction and land development \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	310	582
2020	745	1,006
2019	735	278
2018	160	41
2017	43	1
Prior	3	6
Revolving loans amortized cost basis	524	428
Revolving loans converted to term loans amortized cost basis	56	3
Total loans	2,576	2,345
Term \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,130	2,678
2020	2,144	1,986
2019	1,857	1,912
2018	1,499	835
2017	652	820
Prior	1,835	1,430
Revolving loans amortized cost basis	208	73
Revolving loans converted to term loans amortized cost basis	207	25
Total loans	9,532	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	2,614	2,625
Revolving loans converted to term loans amortized cost basis	113	120
Total loans	2,727	2,745
1-4 family residential \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	591	1,187
2020	1,109	1,029
2019	842	841
2018	640	1,100
2017	860	1,191
Prior	2,227	1,621
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	6,269	6,969
Construction and other consumer real estate \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	71	200
2020	283	296
2019	176	106
2018	49	16
2017	6	1
Prior	8	11
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	593	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	412	429
Revolving loans converted to term loans amortized cost basis	3	3
Total loans	415	432
Other \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	47	51
2020	30	35
2019	24	22
2018	14	10
2017	7	4
Prior	3	2
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	125	124
Pass \| Commercial and industrial \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,052	2,585
2020	1,476	2,743
2019	1,478	1,903
2018	912	829
2017	418	296
Prior	315	228
Revolving loans amortized cost basis	5,966	3,298
Revolving loans converted to term loans amortized cost basis	169	109
Total loans	11,786	11,991
Pass \| PPP \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	2,556	5,572
2020	1,904	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	4,460	5,572
Pass \| Leasing \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	11	87
2020	53	121
2019	79	44
2018	74	34
2017	48	14
Prior	26	5
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	291	305
Pass \| Owner-occupied \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	804	1,588
2020	1,568	1,205
2019	1,060	1,167
2018	1,059	895
2017	824	585
Prior	2,001	1,806
Revolving loans amortized cost basis	132	161
Revolving loans converted to term loans amortized cost basis	76	11
Total loans	7,524	7,418
Pass \| Municipal \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	548	1,031
2020	992	827
2019	647	359
2018	313	419
2017	395	68
Prior	272	227
Revolving loans amortized cost basis	0	3
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	3,167	2,934
Pass \| Construction and land development \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	298	558
2020	745	933
2019	708	267
2018	160	41
2017	43	1
Prior	3	6
Revolving loans amortized cost basis	524	423
Revolving loans converted to term loans amortized cost basis	56	3
Total loans	2,537	2,232
Pass \| Term \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	1,115	2,524
2020	2,047	1,858
2019	1,796	1,639
2018	1,318	761
2017	620	778
Prior	1,701	1,291
Revolving loans amortized cost basis	207	73
Revolving loans converted to term loans amortized cost basis	202	20
Total loans	9,006	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	2,601	2,606
Revolving loans converted to term loans amortized cost basis	105	115
Total loans	2,706	2,721
Pass \| 1-4 family residential \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	591	1,185
2020	1,103	1,017
2019	834	833
2018	636	1,081
2017	846	1,174
Prior	2,178	1,570
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	6,188	6,860
Pass \| Construction and other consumer real estate \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	71	200
2020	282	296
2019	176	106
2018	49	16
2017	6	1
Prior	8	11
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	592	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	410	426
Revolving loans converted to term loans amortized cost basis	2	2
Total loans	412	428
Pass \| Other \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	47	51
2020	30	35
2019	24	22
2018	14	10
2017	7	4
Prior	3	2
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	125	124
Special Mention \| Commercial and industrial \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	2	79
2020	56	152
2019	64	183
2018	74	98
2017	32	4
Prior	29	43
Revolving loans amortized cost basis	186	110
Revolving loans converted to term loans amortized cost basis	1	1
Total loans	444	670
Special Mention \| Leasing \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	1
2020	1	0
2019	6	2
2018	2	1
2017	1	0
Prior	6	6
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	16	10
Special Mention \| Owner-occupied \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	8	72
2020	56	65
2019	57	60
2018	55	60
2017	23	51
Prior	68	41
Revolving loans amortized cost basis	8	9
Revolving loans converted to term loans amortized cost basis	7	3
Total loans	282	361
Special Mention \| Municipal \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	0	0
2020	0	0
2019	0	0
2018	0	0
2017	0	0
Prior	34	8
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	34	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	7	110
2020	87	89
2019	26	177
2018	82	42
2017	11	23
Prior	36	85
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	3	5
Total loans	252	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	1	2
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	1	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	38	157
2019	139	129
2018	92	44
2017	34	26
Prior	64	17
Revolving loans amortized cost basis	217	141
Revolving loans converted to term loans amortized cost basis	2	6
Total loans	606	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	0	0
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total loans	0	5
Accruing Substandard \| Owner-occupied \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	55	28
2020	12	64
2019	59	61
2018	54	37
2017	37	35
Prior	125	98
Revolving loans amortized cost basis	13	6
Revolving loans converted to term loans amortized cost basis	1	1
Total loans	356	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	26	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	38	30
Accruing Substandard \| Term \| Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
2021	8	41
2020	10	34
2019	30	96
2018	99	30
2017	20	18
Prior	76	34
Revolving loans amortized cost basis	1	0
Revolving loans converted to term loans amortized cost basis	2	0
Total loans	246	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