Loans, Leases, and Allowance for Credit Losses (Summary of Past Due Loans (Accruing and Nonaccruing)) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	$ 53,198	$ 48,510
Past Due	278	199
Total loans	53,476	48,709
Accruing loans 90+ days past due	12	10
Nonaccrual loans that are current	367
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	30,360	25,254
Past Due	112	134
Total loans	30,472	25,388
Accruing loans 90+ days past due	2	9
Nonaccrual loans that are current	216
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	12,033	11,539
Past Due	71	16
Total loans	12,104	11,555
Accruing loans 90+ days past due	8	0
Nonaccrual loans that are current	31
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	10,805	11,717
Past Due	95	49
Total loans	10,900	11,766
Accruing loans 90+ days past due	2	1
Nonaccrual loans that are current	120
Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	13,388	14,665
Past Due	56	95
Total loans	13,444	14,760
Accruing loans 90+ days past due	2	8
Nonaccrual loans that are current	140
PPP | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	5,572
Past Due	0
Total loans	5,572	0
Accruing loans 90+ days past due	0
Leasing | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	320	334
Past Due	0	0
Total loans	320	334
Accruing loans 90+ days past due	0	1
Nonaccrual loans that are current	0
Owner-occupied | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	8,129	7,862
Past Due	56	39
Total loans	8,185	7,901
Accruing loans 90+ days past due	0	0
Nonaccrual loans that are current	76
Municipal | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	2,951	2,393
Past Due	0	0
Total loans	2,951	2,393
Accruing loans 90+ days past due	0	0
Nonaccrual loans that are current	0
Construction and land development | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	2,341	2,206
Past Due	4	5
Total loans	2,345	2,211
Accruing loans 90+ days past due	4	0
Nonaccrual loans that are current	0
Term | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	9,692	9,333
Past Due	67	11
Total loans	9,759	9,344
Accruing loans 90+ days past due	4	0
Nonaccrual loans that are current	31
Home equity credit line | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	2,733	2,908
Past Due	12	9
Total loans	2,745	2,917
Accruing loans 90+ days past due	0	0
Nonaccrual loans that are current	16
1-4 family residential | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	6,891	7,532
Past Due	78	36
Total loans	6,969	7,568
Accruing loans 90+ days past due	0	0
Nonaccrual loans that are current	103
Construction and other consumer real estate | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	630	624
Past Due	0	0
Total loans	630	624
Accruing loans 90+ days past due		0
Nonaccrual loans that are current	0
Bankcard and other revolving plans | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	428	499
Past Due	4	3
Total loans	432	502
Accruing loans 90+ days past due	2	1
Nonaccrual loans that are current	1
Other | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	123	154
Past Due	1	1
Total loans	124	155
Accruing loans 90+ days past due	0	0
Nonaccrual loans that are current	0
30-89 days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	140	111
30-89 days past due | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	60	77
30-89 days past due | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	57	13
30-89 days past due | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	23	21
30-89 days past due | Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	26	57
30-89 days past due | PPP | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	0
30-89 days past due | Leasing | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	0	0
30-89 days past due | Owner-occupied | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	34	20
30-89 days past due | Municipal | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	0	0
30-89 days past due | Construction and land development | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	0	5
30-89 days past due | Term | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	57	8
30-89 days past due | Home equity credit line | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	8	6
30-89 days past due | 1-4 family residential | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	12	12
30-89 days past due | Construction and other consumer real estate | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	0	0
30-89 days past due | Bankcard and other revolving plans | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	2	2
30-89 days past due | Other | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	1	1
90+ days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	138	88
90+ days past due | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	52	57
90+ days past due | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	14	3
90+ days past due | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	72	28
90+ days past due | Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	30	38
90+ days past due | PPP | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	0
90+ days past due | Leasing | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	0	0
90+ days past due | Owner-occupied | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	22	19
90+ days past due | Municipal | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	0	0
90+ days past due | Construction and land development | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	4	0
90+ days past due | Term | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	10	3
90+ days past due | Home equity credit line | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	4	3
90+ days past due | 1-4 family residential | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	66	24
90+ days past due | Construction and other consumer real estate | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	0	0
90+ days past due | Bankcard and other revolving plans | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	2	1
90+ days past due | Other | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Past Due	0	0
Nonaccrual loans that are current
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	214	129
Nonaccrual loans that are current | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	158	98
Nonaccrual loans that are current | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	13	11
Nonaccrual loans that are current | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	43	20
Nonaccrual loans that are current | Commercial and industrial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	109	54
Nonaccrual loans that are current | PPP | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	0
Nonaccrual loans that are current | Leasing | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	1	0
Nonaccrual loans that are current | Owner-occupied | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	48	44
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	1
Nonaccrual loans that are current | Term | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	13	10
Nonaccrual loans that are current | Home equity credit line | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	9	7
Nonaccrual loans that are current | 1-4 family residential | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	33	13
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	0
Nonaccrual loans that are current | Other | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Nonaccrual loans that are current	$ 0	$ 0