Loans, Leases, and Allowance for Credit Losses (Summary of Outstanding Loan Balances (Accruing and Nonaccruing) Categorized by Credit Quality Indicators - Prior Year) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Total loans
2023	$ 1,686		$ 8,155
2022	8,166		12,553
2021	12,354		8,774
2020	8,418		4,982
2018	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
2023	0		1
2022	1		12
2021	3		6
2020	1		0
2018	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
2023	794		4,492
2022	4,508		5,770
2021	5,406		4,436
2020	4,259		2,336
2018	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
2023	0		1
2022	1		10
2021	3		6
2020	1		0
2018	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
2023	662		2,571
2022	2,514		3,477
2021	3,556		2,294
2020	2,138		1,634
2018	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
2023	0		0
2022	0		2
2021	0		0
2020	0		0
2018	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
2023	230		1,092
2022	1,144		3,306
2021	3,392		2,044
2020	2,021		1,012
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	380		2,678
2022	2,658		2,572
2021	2,331		1,246
2020	1,167		645
2018	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
2023	0	1
2022	1	10
2021	3	6
2020	1	0
2018	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
2023	39		106
2022	96		136
2021	126		48
2020	43		30
2018	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
2023	0	0
2022	0	0
2021	0	0
2020	0	0
2018	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
2023	254		1,092
2022	1,141		1,982
2021	1,893		2,067
2020	2,029		1,035
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	121		616
2022	613		1,080
2021	1,056		1,075
2020	1,020		626
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	95		576
2022	632		940
2021	942		384
2020	343		91
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	567		1,995
2022	1,882		2,537
2021	2,614		1,910
2020	1,795		1,543
2018	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
2023	0		0
2022	0		2
2021	0		0
2020	0		0
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	196		814
2022	820		2,267
2021	2,452		1,826
2020	1,895		991
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	12		212
2022	272		1,002
2021	907		200
2020	110		15
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	22		66
2022	52		37
2021	33		18
2020	16		6
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	361		2,654
2022	2,617		2,420
2021	2,200		1,204
2020	1,133		639
2018	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
2023	39		104
2022	94		125
2021	116		47
2020	42		29
2018	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
2023	248		1,080
2022	1,126		1,945
2021	1,852		2,020
2020	1,979		1,002
2018	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
2023	121		601
2022	598		1,080
2021	1,056		1,069
2020	1,014		623
2018	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
2023	95		553
2022	613		938
2021	933		355
2020	273		56
2018	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
2023	431		1,861
2022	1,735		2,385
2021	2,375		1,833
2020	1,680		1,449
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	196		814
2022	819		2,264
2021	2,448		1,823
2020	1,890		988
2018	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
2023	12		212
2022	272		1,002
2021	907		200
2020	110		15
2018	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
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	22		66
2022	51		37
2021	33		18
2020	16		6
2018	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
2023	0		8
2022	7		98
2021	90		34
2020	20		2
2018	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
2023	0		2
2022	2		9
2021	8		1
2020	1		1
2018	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
2023	0		2
2022	5		5
2021	9		17
2020	21		5
2018	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
2023	0		7
2022	0		0
2021	0		0
2020	0		0
2018	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
2023	0		0
2022	0		0
2021	8		29
2020	70		30
2018	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
2023	95		55
2022	74		108
2021	183		65
2020	103		78
2018	109		44
Prior	4		6
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	10		0
Total loans	578		356
Special Mention | Home equity credit line | Consumer
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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 | 1-4 family residential | Consumer
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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 | Construction and other consumer real estate | Consumer
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
Total loans
2023	19		11
2022	30		18
2021	9		7
2020	13		2
2018	5		19
Prior	40		8
Revolving loans amortized cost basis	100		99
Revolving loans converted to term loans amortized cost basis	4		3
Total loans	220		167
Accruing Substandard | Leasing | Commercial
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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 | Owner-occupied | Commercial
Total loans
2023	6		10
2022	10		31
2021	31		29
2020	28		21
2018	20		16
Prior	115		90
Revolving loans amortized cost basis	8		2
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	218		199
Accruing Substandard | Municipal | Commercial
Total loans
2023	0		8
2022	15		0
2021	0		6
2020	6		3
2018	3		1
Prior	5		1
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	29		19
Accruing Substandard | Construction and land development | Commercial real estate
Total loans
2023	0		23
2022	19		2
2021	1		0
2020	0		5
2018	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	21		30
Accruing Substandard | Term | Commercial real estate
Total loans
2023	41		79
2022	69		18
2021	30		12
2020	12		16
2018	17		5
Prior	30		24
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	35		35
Total loans	234		189
Accruing Substandard | Home equity credit line | Consumer
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	0		0
Prior	0		0
Revolving loans amortized cost basis	4		4
Revolving loans converted to term loans amortized cost basis	0		1
Total loans	4		5
Accruing Substandard | 1-4 family residential | Consumer
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	0		0
Prior	0		1
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	0		1
Accruing Substandard | Construction and other consumer real estate | Consumer
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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 | Bankcard and other revolving plans | Consumer
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	0		0
Prior	0		0
Revolving loans amortized cost basis	2		2
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	2		2
Accruing Substandard | Other | Consumer
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
Nonaccrual | Commercial and industrial | Commercial
Total loans
2023	0		5
2022	4		36
2021	32		1
2020	1		2
2018	2		11
Prior	47		1
Revolving loans amortized cost basis	18		21
Revolving loans converted to term loans amortized cost basis	6		5
Total loans	110		82
Nonaccrual | Leasing | Commercial
Total loans
2023	0		0
2022	0		2
2021	2		0
2020	0		0
2018	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	2		2
Nonaccrual | Owner-occupied | Commercial
Total loans
2023	0		0
2022	0		1
2021	1		1
2020	1		7
2018	7		3
Prior	11		8
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	20		20
Nonaccrual | Municipal | Commercial
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
Nonaccrual | Construction and land development | Commercial real estate
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	0		21
Prior	0		0
Revolving loans amortized cost basis	1		1
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	1		22
Nonaccrual | Term | Commercial real estate
Total loans
2023	0		0
2022	4		26
2021	26		0
2020	0		0
2018	0		3
Prior	12		10
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	42		39
Nonaccrual | Home equity credit line | Consumer
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	0		0
Prior	0		0
Revolving loans amortized cost basis	21		15
Revolving loans converted to term loans amortized cost basis	6		2
Total loans	27		17
Nonaccrual | 1-4 family residential | Consumer
Total loans
2023	0		0
2022	1		3
2021	4		3
2020	5		3
2018	2		4
Prior	32		27
Revolving loans amortized cost basis	0		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	44		40
Nonaccrual | Construction and other consumer real estate | Consumer
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	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
Nonaccrual | Bankcard and other revolving plans | Consumer
Total loans
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2018	0		0
Prior	0		0
Revolving loans amortized cost basis	1		0
Revolving loans converted to term loans amortized cost basis	0		0
Total loans	1		0
Nonaccrual | Other | Consumer
Total loans
2023	0		0
2022	1		0
2021	0		0
2020	0		0
2018	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	$ 1		$ 0