Operating Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) branch	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) branch segment	Jun. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			7
Number of branches | branch	421		421
Net interest income	$ 563	$ 569	$ 1,111	$ 1,145
Provision for credit losses	168	21	426	25
Net interest income after provision for credit losses	395	548	685	1,120
Noninterest income	117	132	250	264
Total noninterest expense	430	424	837	854
Income before income taxes	82	256	98	530
Total average loans	54,268	48,324	51,532	47,750
Total average deposits	63,000	54,347	59,954	54,133
Other [Member]
Segment Reporting Information [Line Items]
Net interest income	30	(13)	50	(14)
Provision for credit losses	(3)	(1)	1	0
Net interest income after provision for credit losses	33	(12)	49	(14)
Noninterest income	0	9	3	28
Total noninterest expense	65	29	98	63
Income before income taxes	(32)	(32)	(46)	(49)
Total average loans	561	440	563	412
Total average deposits	2,410	3,051	2,814	2,946
Zions Bank [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	156	175	314	351
Provision for credit losses	40	16	94	26
Net interest income after provision for credit losses	116	159	220	325
Noninterest income	38	38	76	71
Total noninterest expense	111	120	221	237
Income before income taxes	43	77	75	159
Total average loans	14,222	13,067	13,611	13,004
Total average deposits	17,977	15,205	16,953	15,238
Amegy Corporation [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	120	127	237	252
Provision for credit losses	25	(8)	127	(23)
Net interest income after provision for credit losses	95	135	110	275
Noninterest income	31	34	67	68
Total noninterest expense	79	86	161	174
Income before income taxes	47	83	16	169
Total average loans	13,570	12,254	12,984	12,058
Total average deposits	13,023	11,361	12,344	11,401
California Bank And Trust [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	123	134	245	267
Provision for credit losses	46	9	84	12
Net interest income after provision for credit losses	77	125	161	255
Noninterest income	19	22	44	41
Total noninterest expense	74	82	151	164
Income before income taxes	22	65	54	132
Total average loans	12,524	10,838	11,773	10,707
Total average deposits	13,522	11,412	12,825	11,328
National Bank Of Arizona [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	53	59	105	117
Provision for credit losses	16	2	35	4
Net interest income after provision for credit losses	37	57	70	113
Noninterest income	10	11	21	21
Total noninterest expense	35	38	72	78
Income before income taxes	12	30	19	56
Total average loans	5,247	4,847	4,991	4,798
Total average deposits	5,722	5,048	5,413	4,972
Nevada State Bank [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	35	38	70	76
Provision for credit losses	40	0	52	(1)
Net interest income after provision for credit losses	(5)	38	18	77
Noninterest income	10	11	21	21
Total noninterest expense	34	37	70	73
Income before income taxes	(29)	12	(31)	25
Total average loans	3,169	2,570	2,942	2,535
Total average deposits	5,402	4,406	5,115	4,368
Vectra Bank Colorado [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	33	35	65	69
Provision for credit losses	(1)	1	23	5
Net interest income after provision for credit losses	34	34	42	64
Noninterest income	8	6	16	12
Total noninterest expense	27	27	53	54
Income before income taxes	15	13	5	22
Total average loans	3,490	3,105	3,296	3,080
Total average deposits	3,662	2,811	3,325	2,816
Commerce Bank Of Washington [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	13	14	25	27
Provision for credit losses	5	2	10	2
Net interest income after provision for credit losses	8	12	15	25
Noninterest income	1	1	2	2
Total noninterest expense	5	5	11	11
Income before income taxes	4	8	6	16
Total average loans	1,485	1,203	1,372	1,156
Total average deposits	$ 1,282	$ 1,053	$ 1,165	$ 1,064
Utah [Member] | Zions Bank [Member]
Segment Reporting Information [Line Items]
Number of branches | branch	95		95
Idaho [Member] | Zions Bank [Member]
Segment Reporting Information [Line Items]
Number of branches | branch	25		25
Wyoming [Member] | Zions Bank [Member]
Segment Reporting Information [Line Items]
Number of branches | branch	1		1
Texas [Member] | Amegy Corporation [Member]
Segment Reporting Information [Line Items]
Number of branches | branch	75		75
California [Member] | California Bank And Trust [Member]
Segment Reporting Information [Line Items]
Number of branches | branch	85		85
Arizona [Member] | National Bank Of Arizona [Member]
Segment Reporting Information [Line Items]
Number of branches | branch	56		56
Nevada [Member] | Nevada State Bank [Member]
Segment Reporting Information [Line Items]
Number of branches | branch	46		46
Colorado [Member] | Vectra Bank Colorado [Member]
Segment Reporting Information [Line Items]
Number of branches | branch	34		34
New Mexico [Member] | Vectra Bank Colorado [Member]
Segment Reporting Information [Line Items]
Number of branches | branch	1		1
Washington [Member] | Commerce Bank Of Washington [Member]
Segment Reporting Information [Line Items]
Number of branches | branch	2		2
Oregon [Member] | Commerce Bank Of Washington [Member]
Segment Reporting Information [Line Items]
Number of branches | branch	1		1