Loans, Leases, and Allowance for Credit Losses (Summary of Outstanding Loan Balances (Accruing and Nonaccruing) Categorized by Credit Quality Indicators - Current Year) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Total loans
2024	$ 1,686		$ 8,155
2023	8,166		12,553
2022	12,354		8,774
2021	8,418		4,982
2020	4,664		3,222
Prior	9,296		6,610
Revolving loans amortized cost basis	12,909		12,896
Revolving loans converted to term loans amortized cost basis	616		587
Total loans	58,109		57,779
Gross charge-offs
2024	0		1
2023	1		12
2022	3		6
2021	1		0
2020	0		1
Prior	2		4
Revolving loans amortized cost basis	6		36
Revolving loans converted to term loans amortized cost basis	1		2
Total	14	$ 7	62
Commercial
Total loans
2024	794		4,492
2023	4,508		5,770
2022	5,406		4,436
2021	4,259		2,336
2020	2,223		1,729
Prior	4,651		3,201
Revolving loans amortized cost basis	8,402		8,410
Revolving loans converted to term loans amortized cost basis	236		214
Total loans	30,479		30,588
Gross charge-offs
2024	0		1
2023	1		10
2022	3		6
2021	1		0
2020	0		0
Prior	0		2
Revolving loans amortized cost basis	4		24
Revolving loans converted to term loans amortized cost basis	1		2
Total	10	3	45
Commercial real estate
Total loans
2024	662		2,571
2023	2,514		3,477
2022	3,556		2,294
2021	2,138		1,634
2020	1,448		884
Prior	2,194		1,482
Revolving loans amortized cost basis	788		757
Revolving loans converted to term loans amortized cost basis	278		272
Total loans	13,578		13,371
Gross charge-offs
2024	0		0
2023	0		2
2022	0		0
2021	0		0
2020	0		1
Prior	0		0
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total	0	0	3
Consumer
Total loans
2024	230		1,092
2023	1,144		3,306
2022	3,392		2,044
2021	2,021		1,012
2020	993		609
Prior	2,451		1,927
Revolving loans amortized cost basis	3,719		3,729
Revolving loans converted to term loans amortized cost basis	102		101
Total loans	14,052		13,820
Gross charge-offs
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	2		2
Revolving loans amortized cost basis	2		12
Revolving loans converted to term loans amortized cost basis	0		0
Total	4	4	14
Commercial and industrial \| Commercial
Total loans
2024	380		2,678
2023	2,658		2,572
2022	2,331		1,246
2021	1,167		645
2020	578		544
Prior	1,059		644
Revolving loans amortized cost basis	8,168		8,196
Revolving loans converted to term loans amortized cost basis	178		159
Total loans	16,519		16,684
Gross charge-offs
2024	0	1
2023	1	10
2022	3	6
2021	1	0
2020	0	0
Prior	0	2
Revolving loans amortized cost basis	4	24
Revolving loans converted to term loans amortized cost basis	1	2
Total	10	45
Leasing \| Commercial
Total loans
2024	39		106
2023	96		136
2022	126		48
2021	43		30
2020	27		45
Prior	57		18
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	388		383
Gross charge-offs
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Revolving loans converted to term loans amortized cost basis	0	0
Total	0	$ 0
Owner-occupied \| Commercial
Total loans
2024	254		1,092
2023	1,141		1,982
2022	1,893		2,067
2021	2,029		1,035
2020	1,016		757
Prior	2,675		2,020
Revolving loans amortized cost basis	232		214
Revolving loans converted to term loans amortized cost basis	55		52
Total loans	9,295		9,219
Gross charge-offs
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	0		0
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total	0		0
Municipal \| Commercial
Total loans
2024	121		616
2023	613		1,080
2022	1,056		1,075
2021	1,020		626
2020	602		383
Prior	860		519
Revolving loans amortized cost basis	2		0
Revolving loans converted to term loans amortized cost basis	3		3
Total loans	4,277		4,302
Gross charge-offs
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	0		0
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total	0		0
Construction and land development \| Commercial real estate
Total loans
2024	95		576
2023	632		940
2022	942		384
2021	343		91
2020	36		28
Prior	12		4
Revolving loans amortized cost basis	502		519
Revolving loans converted to term loans amortized cost basis	124		127
Total loans	2,686		2,669
Gross charge-offs
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		1
Prior	0		0
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total	0		1
Term \| Commercial real estate
Total loans
2024	567		1,995
2023	1,882		2,537
2022	2,614		1,910
2021	1,795		1,543
2020	1,412		856
Prior	2,182		1,478
Revolving loans amortized cost basis	286		238
Revolving loans converted to term loans amortized cost basis	154		145
Total loans	10,892		10,702
Gross charge-offs
2024	0		0
2023	0		2
2022	0		0
2021	0		0
2020	0		0
Prior	0		0
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total	0		2
Home equity credit line \| Consumer
Total loans
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	0		0
Revolving loans amortized cost basis	3,281		3,256
Revolving loans converted to term loans amortized cost basis	101		100
Total loans	3,382		3,356
Gross charge-offs
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	0		0
Revolving loans amortized cost basis	0		3
Revolving loans converted to term loans amortized cost basis	0		0
Total	0		3
1-4 family residential \| Consumer
Total loans
2024	196		814
2023	820		2,267
2022	2,452		1,826
2021	1,895		991
2020	978		598
Prior	2,437		1,919
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	8,778		8,415
Gross charge-offs
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	1		2
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total	1		2
Construction and other consumer real estate \| Consumer
Total loans
2024	12		212
2023	272		1,002
2022	907		200
2021	110		15
2020	10		7
Prior	10		6
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	1,321		1,442
Gross charge-offs
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	0		0
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total	0		0
Bankcard and other revolving plans \| Consumer
Total loans
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	0		0
Revolving loans amortized cost basis	438		473
Revolving loans converted to term loans amortized cost basis	1		1
Total loans	439		474
Gross charge-offs
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	0		0
Revolving loans amortized cost basis	2		9
Revolving loans converted to term loans amortized cost basis	0		0
Total	2		9
Other \| Consumer
Total loans
2024	22		66
2023	52		37
2022	33		18
2021	16		6
2020	5		4
Prior	4		2
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	132		133
Gross charge-offs
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	1		0
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total	1		0
Pass \| Commercial and industrial \| Commercial
Total loans
2024	361		2,654
2023	2,617		2,420
2022	2,200		1,204
2021	1,133		639
2020	568		494
Prior	968		598
Revolving loans amortized cost basis	7,978		7,973
Revolving loans converted to term loans amortized cost basis	166		151
Total loans	15,991		16,133
Pass \| Leasing \| Commercial
Total loans
2024	39		104
2023	94		125
2022	116		47
2021	42		29
2020	26		45
Prior	57		18
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	374		368
Pass \| Owner-occupied \| Commercial
Total loans
2024	248		1,080
2023	1,126		1,945
2022	1,852		2,020
2021	1,979		1,002
2020	975		721
Prior	2,524		1,907
Revolving loans amortized cost basis	209		212
Revolving loans converted to term loans amortized cost basis	50		52
Total loans	8,963		8,939
Pass \| Municipal \| Commercial
Total loans
2024	121		601
2023	598		1,080
2022	1,056		1,069
2021	1,014		623
2020	599		382
Prior	855		512
Revolving loans amortized cost basis	2		0
Revolving loans converted to term loans amortized cost basis	3		3
Total loans	4,248		4,270
Pass \| Construction and land development \| Commercial real estate
Total loans
2024	95		553
2023	613		938
2022	933		355
2021	273		56
2020	35		7
Prior	12		4
Revolving loans amortized cost basis	501		518
Revolving loans converted to term loans amortized cost basis	124		127
Total loans	2,586		2,558
Pass \| Term \| Commercial real estate
Total loans
2024	431		1,861
2023	1,735		2,385
2022	2,375		1,833
2021	1,680		1,449
2020	1,286		804
Prior	2,136		1,438
Revolving loans amortized cost basis	286		238
Revolving loans converted to term loans amortized cost basis	109		110
Total loans	10,038		10,118
Pass \| Home equity credit line \| Consumer
Total loans
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	0		0
Revolving loans amortized cost basis	3,256		3,237
Revolving loans converted to term loans amortized cost basis	95		97
Total loans	3,351		3,334
Pass \| 1-4 family residential \| Consumer
Total loans
2024	196		814
2023	819		2,264
2022	2,448		1,823
2021	1,890		988
2020	976		594
Prior	2,405		1,891
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	8,734		8,374
Pass \| Construction and other consumer real estate \| Consumer
Total loans
2024	12		212
2023	272		1,002
2022	907		200
2021	110		15
2020	10		7
Prior	10		6
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	1,321		1,442
Pass \| Bankcard and other revolving plans \| Consumer
Total loans
2024	0		0
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	0		0
Revolving loans amortized cost basis	435		471
Revolving loans converted to term loans amortized cost basis	1		1
Total loans	436		472
Pass \| Other \| Consumer
Total loans
2024	22		66
2023	51		37
2022	33		18
2021	16		6
2020	5		4
Prior	4		2
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	131		133
Special Mention \| Commercial and industrial \| Commercial
Total loans
2024	0		8
2023	7		98
2022	90		34
2021	20		2
2020	3		20
Prior	4		37
Revolving loans amortized cost basis	72		103
Revolving loans converted to term loans amortized cost basis	2		0
Total loans	198		302
Special Mention \| Leasing \| Commercial
Total loans
2024	0		2
2023	2		9
2022	8		1
2021	1		1
2020	1		0
Prior	0		0
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	12		13
Special Mention \| Owner-occupied \| Commercial
Total loans
2024	0		2
2023	5		5
2022	9		17
2021	21		5
2020	14		17
Prior	25		15
Revolving loans amortized cost basis	15		0
Revolving loans converted to term loans amortized cost basis	5		0
Total loans	94		61
Special Mention \| Municipal \| Commercial
Total loans
2024	0		7
2023	0		0
2022	0		0
2021	0		0
2020	0		0
Prior	0		6
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	0		13
Special Mention \| Construction and land development \| Commercial real estate
Total loans
2024	0		0
2023	0		0
2022	8		29
2021	70		30
2020	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	78		59
Special Mention \| Term \| Commercial real estate
Total loans
2024	95		55
2023	74		108
2022	183		65
2021	103		78
2020	109		44
Prior	4		6
Revolving loans amortized cost basis	0		0

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