Operating Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) branch	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) branch segment	Sep. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			7
Number of branches | branch	422		422
Net interest income	$ 555	$ 567	$ 1,665	$ 1,713
Provision for credit losses	55	10	481	35
Net interest income after provision for credit losses	500	557	1,184	1,678
Total noninterest income	157	146	408	410
Total noninterest expense	442	415	1,279	1,270
Income before income taxes	215	288	313	818
Total average loans	54,993	48,828	52,694	48,114
Total average deposits	66,502	55,284	62,153	54,521
Other
Segment Reporting Information [Line Items]
Net interest income	5	5	12	19
Provision for credit losses	0	1	3	(1)
Net interest income after provision for credit losses	5	4	9	20
Total noninterest income	35	18	38	46
Total noninterest expense	71	36	169	98
Income before income taxes	(31)	(14)	(122)	(32)
Total average loans	619	469	581	432
Total average deposits	2,275	2,603	2,634	2,832
Zions Bank | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	162	172	492	517
Provision for credit losses	(10)	1	84	27
Net interest income after provision for credit losses	172	171	408	490
Total noninterest income	40	39	116	110
Total noninterest expense	111	114	332	351
Income before income taxes	101	96	192	249
Total average loans	14,237	13,229	13,821	13,079
Total average deposits	19,283	15,736	17,735	15,406
Amegy | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	121	123	368	368
Provision for credit losses	6	18	133	(5)
Net interest income after provision for credit losses	115	105	235	373
Total noninterest income	32	37	99	105
Total noninterest expense	82	84	243	258
Income before income taxes	65	58	91	220
Total average loans	13,476	12,438	13,149	12,186
Total average deposits	13,628	11,842	12,775	11,549
CB&T | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	129	127	381	387
Provision for credit losses	36	(6)	120	6
Net interest income after provision for credit losses	93	133	261	381
Total noninterest income	22	23	66	63
Total noninterest expense	74	75	225	239
Income before income taxes	41	81	102	205
Total average loans	12,954	10,819	12,170	10,745
Total average deposits	14,462	11,565	13,375	11,408
NBAZ | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	54	56	162	169
Provision for credit losses	6	0	41	5
Net interest income after provision for credit losses	48	56	121	164
Total noninterest income	9	10	30	31
Total noninterest expense	37	39	109	116
Income before income taxes	20	27	42	79
Total average loans	5,305	4,769	5,097	4,788
Total average deposits	5,996	5,028	5,608	4,990
NSB | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	37	37	110	111
Provision for credit losses	(2)	1	49	0
Net interest income after provision for credit losses	39	36	61	111
Total noninterest income	10	11	31	32
Total noninterest expense	35	35	105	109
Income before income taxes	14	12	(13)	34
Total average loans	3,310	2,708	3,066	2,594
Total average deposits	5,704	4,547	5,313	4,428
Vectra | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	34	34	101	102
Provision for credit losses	18	(3)	41	3
Net interest income after provision for credit losses	16	37	60	99
Total noninterest income	8	7	24	19
Total noninterest expense	27	27	80	82
Income before income taxes	(3)	17	4	36
Total average loans	3,542	3,161	3,379	3,107
Total average deposits	3,838	2,866	3,497	2,833
TCBW | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	13	13	39	40
Provision for credit losses	1	(2)	10	0
Net interest income after provision for credit losses	12	15	29	40
Total noninterest income	1	1	4	4
Total noninterest expense	5	5	16	17
Income before income taxes	8	11	17	27
Total average loans	1,550	1,235	1,431	1,183
Total average deposits	$ 1,316	$ 1,097	$ 1,216	$ 1,075
Utah | Zions Bank
Segment Reporting Information [Line Items]
Number of branches | branch	95		95
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	76		76
California | CB&T
Segment Reporting Information [Line Items]
Number of branches | branch	85		85
Arizona | NBAZ
Segment Reporting Information [Line Items]
Number of branches | branch	56		56
Nevada | NSB
Segment Reporting Information [Line Items]
Number of branches | branch	46		46
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