Operating Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($) branch	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) branch segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment									7
Number of branches | branch	422								422
Net interest income	$ 550	$ 555	$ 563	$ 548	$ 559	$ 567	$ 569	$ 576	$ 2,216	$ 2,272	$ 2,230
Provision for credit losses	(67)	55	168	258	4	10	21	4	414	39	(40)
Net interest income after provision for credit losses									1,802	2,233	2,270
Noninterest income	166	157	117	134	152	146	132	132	574	562	552
Noninterest expense	424	442	430	408	472	415	424	430	1,704	1,742	1,679
Income before income taxes	$ 359	$ 215	$ 82	$ 16	$ 235	$ 288	$ 256	$ 274	672	1,053	1,143
Total average loans									53,016	48,265	45,425
Total average deposits									63,689	55,081	53,183
Other
Segment Reporting Information [Line Items]
Net interest income									20	24	35
Provision for credit losses									3	2	0
Net interest income after provision for credit losses									17	22	35
Noninterest income									67	73	88
Noninterest expense									205	180	145
Income before income taxes									(121)	(85)	(22)
Total average loans									629	451	347
Total average deposits									2,495	2,910	2,536
Zions Bank | Operating Segments
Segment Reporting Information [Line Items]
Net interest income									650	688	662
Provision for credit losses									67	18	8
Net interest income after provision for credit losses									583	670	654
Noninterest income									158	147	147
Noninterest expense									446	471	460
Income before income taxes									295	346	341
Total average loans									13,845	13,109	12,643
Total average deposits									18,370	15,561	15,149
Amegy | Operating Segments
Segment Reporting Information [Line Items]
Net interest income									485	489	485
Provision for credit losses									111	9	(80)
Net interest income after provision for credit losses									374	480	565
Noninterest income									133	138	126
Noninterest expense									329	344	344
Income before income taxes									178	274	347
Total average loans									13,114	12,235	11,358
Total average deposits									12,970	11,627	11,160
CB&T | Operating Segments
Segment Reporting Information [Line Items]
Net interest income									512	512	508
Provision for credit losses									120	7	15
Net interest income after provision for credit losses									392	505	493
Noninterest income									95	88	82
Noninterest expense									305	316	308
Income before income taxes									182	277	267
Total average loans									12,366	10,763	10,033
Total average deposits									13,763	11,522	11,268
NBAZ | Operating Segments
Segment Reporting Information [Line Items]
Net interest income									216	223	216
Provision for credit losses									35	2	8
Net interest income after provision for credit losses									181	221	208
Noninterest income									41	42	39
Noninterest expense									147	156	152
Income before income taxes									75	107	95
Total average loans									5,099	4,774	4,608
Total average deposits									5,771	5,002	4,931
NSB | Operating Segments
Segment Reporting Information [Line Items]
Net interest income									146	148	142
Provision for credit losses									37	(1)	1
Net interest income after provision for credit losses									109	149	141
Noninterest income									43	43	41
Noninterest expense									141	145	143
Income before income taxes									11	47	39
Total average loans									3,102	2,630	2,394
Total average deposits									5,427	4,512	4,286
Vectra | Operating Segments
Segment Reporting Information [Line Items]
Net interest income									135	135	130
Provision for credit losses									34	3	6
Net interest income after provision for credit losses									101	132	124
Noninterest income									32	26	24
Noninterest expense									109	108	105
Income before income taxes									24	50	43
Total average loans									3,401	3,109	2,924
Total average deposits									3,637	2,853	2,761
TCBW | Operating Segments
Segment Reporting Information [Line Items]
Net interest income									52	53	52
Provision for credit losses									7	(1)	2
Net interest income after provision for credit losses									45	54	50
Noninterest income									5	5	5
Noninterest expense									22	22	22
Income before income taxes									28	37	33
Total average loans									1,460	1,194	1,118
Total average deposits									$ 1,256	$ 1,094	$ 1,092
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
California | CB&T
Segment Reporting Information [Line Items]
Number of branches | branch	85								85
Texas | Amegy
Segment Reporting Information [Line Items]
Number of branches | branch	76								76
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