Operating Segment Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($) branch segment	Mar. 31, 2022 USD ($)
Segment Reporting Information [Line Items]
Number of bank operating segments | segment	7
Number of branches | branch	412
Net interest income	$ 679	$ 544
Provision for credit losses	45	(33)
Net interest income after provision for credit losses	634	577
Total noninterest income	160	142
Total noninterest expense	512	464
Income before income taxes	282	255
Total average loans	56,153	50,933
Total average deposits	70,156	81,605
Other
Segment Reporting Information [Line Items]
Net interest income	38	11
Provision for credit losses	2	1
Net interest income after provision for credit losses	36	10
Total noninterest income	15	3
Total noninterest expense	60	58
Income before income taxes	(9)	(45)
Total average loans	1,144	896
Total average deposits	2,031	1,335
Zions Bank | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	185	157
Provision for credit losses	24	(2)
Net interest income after provision for credit losses	161	159
Total noninterest income	50	46
Total noninterest expense	135	123
Income before income taxes	76	82
Total average loans	13,978	12,817
Total average deposits	20,953	26,120
CB&T | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	160	129
Provision for credit losses	0	6
Net interest income after provision for credit losses	160	123
Total noninterest income	24	24
Total noninterest expense	92	84
Income before income taxes	92	63
Total average loans	14,016	12,845
Total average deposits	14,644	16,468
Amegy | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	124	112
Provision for credit losses	11	(27)
Net interest income after provision for credit losses	113	139
Total noninterest income	41	37
Total noninterest expense	98	86
Income before income taxes	56	90
Total average loans	12,844	11,795
Total average deposits	13,287	16,413
NBAZ | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	64	51
Provision for credit losses	(1)	(4)
Net interest income after provision for credit losses	65	55
Total noninterest income	10	11
Total noninterest expense	47	40
Income before income taxes	28	26
Total average loans	5,150	4,774
Total average deposits	7,179	7,953
NSB | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	51	37
Provision for credit losses	4	(3)
Net interest income after provision for credit losses	47	40
Total noninterest income	11	12
Total noninterest expense	41	37
Income before income taxes	17	15
Total average loans	3,327	2,817
Total average deposits	6,972	7,437
Vectra | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	41	33
Provision for credit losses	3	(4)
Net interest income after provision for credit losses	38	37
Total noninterest income	7	8
Total noninterest expense	33	30
Income before income taxes	12	15
Total average loans	3,983	3,398
Total average deposits	3,707	4,298
TCBW | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	16	14
Provision for credit losses	2	0
Net interest income after provision for credit losses	14	14
Total noninterest income	2	1
Total noninterest expense	6	6
Income before income taxes	10	9
Total average loans	1,711	1,591
Total average deposits	$ 1,383	$ 1,581
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	77
Arizona | NBAZ
Segment Reporting Information [Line Items]
Number of branches | branch	56
Nevada | NSB
Segment Reporting Information [Line Items]
Number of branches | branch	46
Colorado | Vectra
Segment Reporting Information [Line Items]
Number of branches | branch	33
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