Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	$ 117	$ 101	$ 330	$ 298
Other noninterest income (non-ASC 606 customer-related)	34	38	93	113
Total customer-related fees	151	139	423	411
Other noninterest income (noncustomer-related)	(12)	18	90	(3)
Total noninterest income	139	157	513	408
Other real estate owned gain from sale	0	1	0	1
Net interest income	555	555	1,655	1,665
Total income less interest expense	$ 694	713	2,168	2,074
Interest Income on loans and securities | Revenue from Contract with Customer Benchmark | Product Concentration Risk
Segment Reporting, Revenue Reconciling Item [Line Items]
Percentage of revenue	79.00%
Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	$ 34	32	100	93
Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	35	28	98	83
Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	20	17	54	50
Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	2	3	6	9
Wealth management fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	12	9	34	28
Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	14	12	38	35
Other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	8	9	26	20
Other noninterest income (non-ASC 606 customer-related)	5	9	(3)	21
Total customer-related fees	13	18	23	41
Other noninterest income (noncustomer-related)	(14)	17	83	(3)
Total noninterest income	(1)	35	106	38
Other real estate owned gain from sale	0	0	0	0
Net interest income	8	5	20	13
Total income less interest expense	7	40	126	51
Other | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	(3)	1	1	0
Other | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	1	0	0
Other | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	0	(1)	0
Other | Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	2	4	6
Other | Wealth management fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	(1)	0	(2)
Other | Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	9	6	22	16
Zions Bank | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	40	35	113	100
Other noninterest income (non-ASC 606 customer-related)	4	5	16	17
Total customer-related fees	44	40	129	117
Other noninterest income (noncustomer-related)	(1)	0	(1)	(1)
Total noninterest income	43	40	128	116
Other real estate owned gain from sale	0	0	0	0
Net interest income	161	162	477	491
Total income less interest expense	204	202	605	607
Zions Bank | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	12	11	34	31
Zions Bank | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	15	12	42	36
Zions Bank | Operating Segments | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	6	5	17	15
Zions Bank | Operating Segments | Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	0	(1)
Zions Bank | Operating Segments | Wealth management fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	5	5	15	13
Zions Bank | Operating Segments | Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	2	2	5	6
Amegy | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	27	24	75	74
Other noninterest income (non-ASC 606 customer-related)	10	8	27	25
Total customer-related fees	37	32	102	99
Other noninterest income (noncustomer-related)	0	0	2	0
Total noninterest income	37	32	104	99
Other real estate owned gain from sale	0	0	0	0
Net interest income	117	121	350	368
Total income less interest expense	154	153	454	467
Amegy | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	10	9	28	28
Amegy | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	8	6	21	18
Amegy | Operating Segments | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	4	4	11	11
Amegy | Operating Segments | Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	1	2	4
Amegy | Operating Segments | Wealth management fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	3	2	9	8
Amegy | Operating Segments | Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	2	4	5
CB&T | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	17	13	47	42
Other noninterest income (non-ASC 606 customer-related)	6	8	23	23
Total customer-related fees	23	21	70	65
Other noninterest income (noncustomer-related)	2	1	4	1
Total noninterest income	25	22	74	66
Other real estate owned gain from sale	0	1	0	1
Net interest income	135	129	399	381
Total income less interest expense	160	152	473	448
CB&T | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	7	5	19	17
CB&T | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	4	3	12	10
CB&T | Operating Segments | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	3	3	9	8
CB&T | Operating Segments | Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	0	0
CB&T | Operating Segments | Wealth management fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	2	1	4	3
CB&T | Operating Segments | Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	1	3	4
NBAZ | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	8	7	23	21
Other noninterest income (non-ASC 606 customer-related)	4	2	10	9
Total customer-related fees	12	9	33	30
Other noninterest income (noncustomer-related)	1	0	2	0
Total noninterest income	13	9	35	30
Other real estate owned gain from sale	0	0	0	0
Net interest income	50	54	155	162
Total income less interest expense	63	63	190	192
NBAZ | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	2	2	5	5
NBAZ | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	3	2	8	7
NBAZ | Operating Segments | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	2	2	7	6
NBAZ | Operating Segments | Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	0	0
NBAZ | Operating Segments | Wealth management fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	1	2	2
NBAZ | Operating Segments | Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	1	1
NSB | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	10	8	27	23
Other noninterest income (non-ASC 606 customer-related)	3	2	11	8
Total customer-related fees	13	10	38	31
Other noninterest income (noncustomer-related)	0	0	0	0
Total noninterest income	13	10	38	31
Other real estate owned gain from sale	0	0	0	0
Net interest income	38	37	111	110
Total income less interest expense	51	47	149	141
NSB | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	3	2	7	6
NSB | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	3	3	9	7
NSB | Operating Segments | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	3	2	8	7
NSB | Operating Segments | Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	0	0
NSB | Operating Segments | Wealth management fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	1	3	3
NSB | Operating Segments | Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	0	0
Vectra | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	6	5	16	15
Other noninterest income (non-ASC 606 customer-related)	2	3	8	9
Total customer-related fees	8	8	24	24
Other noninterest income (noncustomer-related)	0	0	0	0
Total noninterest income	8	8	24	24
Other real estate owned gain from sale	0	0	0	0
Net interest income	34	34	103	101
Total income less interest expense	42	42	127	125
Vectra | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	2	2	5	5
Vectra | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	2	1	5	4
Vectra | Operating Segments | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	1	3	3
Vectra | Operating Segments | Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	0	0
Vectra | Operating Segments | Wealth management fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	1	1
Vectra | Operating Segments | Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	1	2	2
TCBW | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	0	3	3
Other noninterest income (non-ASC 606 customer-related)	0	1	1	1
Total customer-related fees	1	1	4	4
Other noninterest income (noncustomer-related)	0	0	0	0
Total noninterest income	1	1	4	4
Other real estate owned gain from sale	0	0	0	0
Net interest income	12	13	40	39
Total income less interest expense	13	14	44	43
TCBW | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	0	1	1
TCBW | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	1	1
TCBW | Operating Segments | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	0	0
TCBW | Operating Segments | Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	0	0
TCBW | Operating Segments | Wealth management fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	0	0
TCBW | Operating Segments | Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	$ 0	$ 0	$ 1	$ 1