Operating Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) branch	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) branch segment	Sep. 30, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of bank operating segments | segment			7
Number of branches | branch	419		419
Net interest income	$ 555	$ 555	$ 1,655	$ 1,665
Provision for credit losses	(46)	55	(301)	481
Net interest income after provision for credit losses	601	500	1,956	1,184
Total noninterest income	139	157	513	408
Total noninterest expense	429	442	1,292	1,279
Income before income taxes	311	215	1,177	313
Total average loans	50,851	54,993	52,418	52,694
Total average deposits	77,411	66,503	74,521	62,153
Other
Segment Reporting Information [Line Items]
Net interest income	8	5	20	13
Provision for credit losses	(1)	0	(1)	3
Net interest income after provision for credit losses	9	5	21	10
Total noninterest income	(1)	35	106	38
Total noninterest expense	49	71	143	169
Income before income taxes	(41)	(31)	(16)	(121)
Total average loans	867	619	845	581
Total average deposits	1,144	2,276	1,500	2,634
Zions Bank | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	161	162	477	491
Provision for credit losses	(10)	(10)	(29)	84
Net interest income after provision for credit losses	171	172	506	407
Total noninterest income	43	40	128	116
Total noninterest expense	115	111	346	332
Income before income taxes	99	101	288	191
Total average loans	12,985	14,237	13,318	13,821
Total average deposits	24,399	19,283	23,001	17,735
Amegy | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	117	121	350	368
Provision for credit losses	(10)	6	(108)	133
Net interest income after provision for credit losses	127	115	458	235
Total noninterest income	37	32	104	99
Total noninterest expense	83	82	251	243
Income before income taxes	81	65	311	91
Total average loans	11,865	13,476	12,337	13,149
Total average deposits	15,925	13,628	15,179	12,775
CB&T | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	135	129	399	381
Provision for credit losses	(12)	36	(81)	120
Net interest income after provision for credit losses	147	93	480	261
Total noninterest income	25	22	74	66
Total noninterest expense	77	74	233	225
Income before income taxes	95	41	321	102
Total average loans	12,672	12,954	12,924	12,170
Total average deposits	15,900	14,462	15,564	13,375
NBAZ | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	50	54	155	162
Provision for credit losses	(5)	6	(29)	41
Net interest income after provision for credit losses	55	48	184	121
Total noninterest income	13	9	35	30
Total noninterest expense	38	37	112	109
Income before income taxes	30	20	107	42
Total average loans	4,689	5,305	4,914	5,097
Total average deposits	7,259	5,996	6,949	5,608
NSB | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	38	37	111	110
Provision for credit losses	(5)	(2)	(35)	49
Net interest income after provision for credit losses	43	39	146	61
Total noninterest income	13	10	38	31
Total noninterest expense	34	35	106	105
Income before income taxes	22	14	78	(13)
Total average loans	2,892	3,310	3,085	3,066
Total average deposits	6,870	5,704	6,500	5,313
Vectra | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	34	34	103	101
Provision for credit losses	(4)	18	(15)	41
Net interest income after provision for credit losses	38	16	118	60
Total noninterest income	8	8	24	24
Total noninterest expense	28	27	85	80
Income before income taxes	18	(3)	57	4
Total average loans	3,339	3,542	3,422	3,379
Total average deposits	4,362	3,838	4,344	3,497
TCBW | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	12	13	40	39
Provision for credit losses	1	1	(3)	10
Net interest income after provision for credit losses	11	12	43	29
Total noninterest income	1	1	4	4
Total noninterest expense	5	5	16	16
Income before income taxes	7	8	31	17
Total average loans	1,542	1,550	1,573	1,431
Total average deposits	$ 1,552	$ 1,316	$ 1,484	$ 1,216
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