Operating Segment Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) branch segment	Mar. 31, 2023 USD ($)
Segment Reporting Information [Line Items]
Number of bank operating segments | segment	7
Number of branches | branch	408
Net interest income	$ 586	$ 679
Provision for credit losses	13	45
Net interest income after provision for credit losses	573	634
Total noninterest income	156	160
Total noninterest expense	526	512
Income before income taxes	203	282
Total average loans	57,907	56,153
Total average deposits	73,358	70,156
Other
Segment Reporting Information [Line Items]
Net interest income	8	38
Provision for credit losses	0	2
Net interest income after provision for credit losses	8	36
Total noninterest income	15	15
Total noninterest expense	35	60
Income before income taxes	(12)	(9)
Total average loans	977	1,144
Total average deposits	4,714	2,031
Zions Bank | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	166	185
Provision for credit losses	(11)	24
Net interest income after provision for credit losses	177	161
Total noninterest income	43	50
Total noninterest expense	144	135
Income before income taxes	76	76
Total average loans	14,726	13,978
Total average deposits	20,735	20,953
CB&T | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	140	160
Provision for credit losses	22	0
Net interest income after provision for credit losses	118	160
Total noninterest income	26	24
Total noninterest expense	101	92
Income before income taxes	43	92
Total average loans	14,165	14,016
Total average deposits	14,406	14,644
Amegy | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	113	124
Provision for credit losses	4	11
Net interest income after provision for credit losses	109	113
Total noninterest income	42	41
Total noninterest expense	113	98
Income before income taxes	38	56
Total average loans	13,115	12,844
Total average deposits	14,871	13,287
NBAZ | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	60	64
Provision for credit losses	5	(1)
Net interest income after provision for credit losses	55	65
Total noninterest income	10	10
Total noninterest expense	48	47
Income before income taxes	17	28
Total average loans	5,651	5,150
Total average deposits	6,859	7,179
NSB | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	47	51
Provision for credit losses	(5)	4
Net interest income after provision for credit losses	52	47
Total noninterest income	12	11
Total noninterest expense	43	41
Income before income taxes	21	17
Total average loans	3,506	3,327
Total average deposits	7,200	6,972
Vectra | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	37	41
Provision for credit losses	(6)	3
Net interest income after provision for credit losses	43	38
Total noninterest income	6	7
Total noninterest expense	34	33
Income before income taxes	15	12
Total average loans	4,039	3,983
Total average deposits	3,450	3,707
TCBW | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	15	16
Provision for credit losses	4	2
Net interest income after provision for credit losses	11	14
Total noninterest income	2	2
Total noninterest expense	8	6
Income before income taxes	5	10
Total average loans	1,728	1,711
Total average deposits	$ 1,123	$ 1,383
Utah | Zions Bank
Segment Reporting Information [Line Items]
Number of branches | branch	95
Idaho | Zions Bank
Segment Reporting Information [Line Items]
Number of branches | branch	25
Wyoming | Zions Bank
Segment Reporting Information [Line Items]
Number of branches | branch	1
Texas | Amegy
Segment Reporting Information [Line Items]
Number of branches | branch	75
California | CB&T
Segment Reporting Information [Line Items]
Number of branches | branch	75
Arizona | NBAZ
Segment Reporting Information [Line Items]
Number of branches | branch	56
Nevada | NSB
Segment Reporting Information [Line Items]
Number of branches | branch	43
Colorado | Vectra
Segment Reporting Information [Line Items]
Number of branches | branch	34
New Mexico | Vectra
Segment Reporting Information [Line Items]
Number of branches | branch	1
Washington | TCBW
Segment Reporting Information [Line Items]
Number of branches | branch	2
Oregon | TCBW
Segment Reporting Information [Line Items]
Number of branches | branch	1