OPERATING SEGMENT INFORMATION - Schedule of Segment Reporting Information, by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Net interest income	$ 620	$ 585	$ 1,803	$ 1,855
Provision for credit losses	13	41	31	132
Net interest income after provision for credit losses	607	544	1,772	1,723
Noninterest income	172	180	507	529
Noninterest expense	502	496	1,537	1,516
Income before income taxes	277	228	742	736
Total average loans	58,665	56,968	58,289	56,603
Total average deposits	75,029	75,643	74,208	71,836
Other
Segment Reporting Information [Line Items]
Net interest income	(5)	5	8	25
Provision for credit losses	1	(2)	(3)	(3)
Net interest income after provision for credit losses	(6)	7	11	28
Noninterest income	19	25	59	60
Noninterest expense	16	60	53	169
Income before income taxes	(3)	(28)	17	(81)
Total average loans	951	1,007	957	1,063
Total average deposits	5,628	8,608	5,267	6,030
Zions Bank | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	178	167	514	530
Provision for credit losses	11	1	(7)	32
Net interest income after provision for credit losses	167	166	521	498
Noninterest income	46	49	136	148
Noninterest expense	140	130	432	403
Income before income taxes	73	85	225	243
Total average loans	14,762	14,382	14,792	14,205
Total average deposits	21,386	20,041	21,010	20,058
CB&T | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	150	143	434	454
Provision for credit losses	0	24	17	39
Net interest income after provision for credit losses	150	119	417	415
Noninterest income	31	28	87	87
Noninterest expense	100	91	304	277
Income before income taxes	81	56	200	225
Total average loans	14,315	14,195	14,203	14,122
Total average deposits	14,643	14,335	14,530	14,103
Amegy | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	130	107	362	346
Provision for credit losses	4	7	22	30
Net interest income after provision for credit losses	126	100	340	316
Noninterest income	44	46	128	143
Noninterest expense	111	93	340	291
Income before income taxes	59	53	128	168
Total average loans	13,531	12,892	13,331	12,872
Total average deposits	14,633	13,997	14,705	13,055
NBAZ | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	62	66	181	194
Provision for credit losses	7	3	11	7
Net interest income after provision for credit losses	55	63	170	187
Noninterest income	11	10	31	31
Noninterest expense	49	44	147	136
Income before income taxes	17	29	54	82
Total average loans	5,647	5,364	5,631	5,253
Total average deposits	6,904	7,002	6,897	7,018
NSB | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	51	46	147	145
Provision for credit losses	(14)	7	(12)	18
Net interest income after provision for credit losses	65	39	159	127
Noninterest income	12	12	40	34
Noninterest expense	44	40	133	123
Income before income taxes	33	11	66	38
Total average loans	3,520	3,420	3,525	3,392
Total average deposits	7,156	7,059	7,187	6,887
Vectra | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	38	36	111	115
Provision for credit losses	2	(1)	(3)	5
Net interest income after provision for credit losses	36	37	114	110
Noninterest income	7	8	20	21
Noninterest expense	34	32	104	99
Income before income taxes	9	13	30	32
Total average loans	4,106	4,011	4,078	3,997
Total average deposits	3,531	3,463	3,486	3,479
TCBW | Operating Segments
Segment Reporting Information [Line Items]
Net interest income	16	15	46	46
Provision for credit losses	2	2	6	4
Net interest income after provision for credit losses	14	13	40	42
Noninterest income	2	2	6	5
Noninterest expense	8	6	24	18
Income before income taxes	8	9	22	29
Total average loans	1,833	1,697	1,772	1,699
Total average deposits	$ 1,148	$ 1,138	$ 1,126	$ 1,206