Operating Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) branch	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) branch segment	Jun. 30, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of bank operating segments | segment			7
Number of branches | branch	419		419
Net interest income	$ 555	$ 563	$ 1,100	$ 1,111
Provision for credit losses	(123)	168	(255)	426
Net interest income after provision for credit losses	678	395	1,355	685
Total noninterest income	205	117	374	250
Total noninterest expense	428	430	863	837
Income before income taxes	455	82	866	98
Total average loans	52,770	54,268	53,215	51,532
Total average deposits	74,640	63,000	73,052	59,954
Other
Segment Reporting Information [Line Items]
Net interest income	7	6	13	10
Provision for credit losses	0	(3)	(1)	1
Net interest income after provision for credit losses	7	9	14	9
Total noninterest income	66	0	107	3
Total noninterest expense	48	65	92	98
Income before income taxes	25	(56)	29	(86)
Total average loans	865	561	833	563
Total average deposits	1,350	2,410	1,684	2,814
Zions Bank | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	159	165	316	329
Provision for credit losses	(8)	40	(19)	94
Net interest income after provision for credit losses	167	125	335	235
Total noninterest income	46	38	85	76
Total noninterest expense	113	111	231	221
Income before income taxes	100	52	189	90
Total average loans	13,248	14,222	13,488	13,611
Total average deposits	22,862	17,977	22,289	16,953
Amegy | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	117	126	233	246
Provision for credit losses	(45)	25	(98)	127
Net interest income after provision for credit losses	162	101	331	119
Total noninterest income	35	31	67	67
Total noninterest expense	84	79	169	161
Income before income taxes	113	53	229	25
Total average loans	12,452	13,570	12,577	12,984
Total average deposits	15,350	13,023	14,800	12,344
CB&T | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	133	127	264	252
Provision for credit losses	(32)	46	(69)	84
Net interest income after provision for credit losses	165	81	333	168
Total noninterest income	25	19	49	44
Total noninterest expense	76	74	156	151
Income before income taxes	114	26	226	61
Total average loans	13,053	12,524	13,052	11,773
Total average deposits	15,602	13,522	15,393	12,825
NBAZ | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	53	54	105	108
Provision for credit losses	(14)	16	(24)	35
Net interest income after provision for credit losses	67	38	129	73
Total noninterest income	11	10	22	21
Total noninterest expense	37	35	75	72
Income before income taxes	41	13	76	22
Total average loans	4,950	5,247	5,029	4,991
Total average deposits	7,036	5,722	6,791	5,413
NSB | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	37	37	74	73
Provision for credit losses	(12)	40	(30)	52
Net interest income after provision for credit losses	49	(3)	104	21
Total noninterest income	12	10	25	21
Total noninterest expense	36	34	72	70
Income before income taxes	25	(27)	57	(28)
Total average loans	3,120	3,169	3,183	2,942
Total average deposits	6,552	5,402	6,312	5,115
Vectra | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	35	34	68	67
Provision for credit losses	(11)	(1)	(11)	23
Net interest income after provision for credit losses	46	35	79	44
Total noninterest income	8	8	16	16
Total noninterest expense	29	27	57	53
Income before income taxes	25	16	38	7
Total average loans	3,476	3,490	3,463	3,296
Total average deposits	4,388	3,662	4,334	3,325
TCBW | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	14	14	27	26
Provision for credit losses	(1)	5	(3)	10
Net interest income after provision for credit losses	15	9	30	16
Total noninterest income	2	1	3	2
Total noninterest expense	5	5	11	11
Income before income taxes	12	5	22	7
Total average loans	1,606	1,485	1,590	1,372
Total average deposits	$ 1,500	$ 1,282	$ 1,449	$ 1,165
Utah | Zions Bank
Segment Reporting Information [Line Items]
Number of branches | branch	96		96
Idaho | Zions Bank
Segment Reporting Information [Line Items]
Number of branches | branch	25		25
Wyoming | Zions Bank
Segment Reporting Information [Line Items]
Number of branches | branch	1		1
Texas | Amegy
Segment Reporting Information [Line Items]
Number of branches | branch	75		75
California | CB&T
Segment Reporting Information [Line Items]
Number of branches | branch	83		83
Arizona | NBAZ
Segment Reporting Information [Line Items]
Number of branches | branch	56		56
Nevada | NSB
Segment Reporting Information [Line Items]
Number of branches | branch	45		45
Colorado | Vectra
Segment Reporting Information [Line Items]
Number of branches | branch	34		34
New Mexico | Vectra
Segment Reporting Information [Line Items]
Number of branches | branch	1		1
Washington | TCBW
Segment Reporting Information [Line Items]
Number of branches | branch	2		2
Oregon | TCBW
Segment Reporting Information [Line Items]
Number of branches | branch	1		1