Revenue Recognition and Deferred Revenue (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) state	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) state	Sep. 30, 2019 USD ($)
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of states in which entity operates | state	11		11
Total noninterest income from contracts with customers (ASC 606)	$ 101	$ 106	$ 298	$ 307
Other noninterest income (Non-ASC 606 customer related)	38	34	113	85
Total customer-related fees	139	140	411	392
Other noninterest income (non-customer related)	18	6	(3)	18
Total noninterest income	157	146	408	410
Other real estate owned gain from sale	1	2	1	4
Net interest income	555	567	1,665	1,713
Total income less interest expense	713	715	2,074	2,127
Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	32	31	93	90
Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	28	32	83	95
Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	17	20	50	58
Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	3	3	9	9
Wealth management and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	14	15	44	42
Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	7	5	19	13
Other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	9	9	20	26
Other noninterest income (Non-ASC 606 customer related)	9	4	21	5
Total customer-related fees	18	13	41	31
Other noninterest income (non-customer related)	17	5	(3)	15
Total noninterest income	35	18	38	46
Other real estate owned gain from sale	0	0	0	0
Net interest income	5	5	12	19
Total income less interest expense	40	23	50	65
Other | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	(1)	1
Other | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	0	0	1
Other | 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
Other | Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	2	1	6	5
Other | Wealth management and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	5	1	12
Other | Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	6	3	14	7
Zions Bank | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	35	34	100	100
Other noninterest income (Non-ASC 606 customer related)	5	5	17	9
Total customer-related fees	40	39	117	109
Other noninterest income (non-customer related)	0	0	(1)	1
Total noninterest income	40	39	116	110
Other real estate owned gain from sale	0	1	0	3
Net interest income	162	172	492	517
Total income less interest expense	202	212	608	630
Zions Bank | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	11	10	32	30
Zions Bank | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	12	13	36	38
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)	5	6	15	18
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	(1)	0
Zions Bank | Operating Segments | Wealth management and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	6	4	16	12
Zions Bank | 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
Amegy | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	24	25	74	73
Other noninterest income (Non-ASC 606 customer related)	8	12	25	32
Total customer-related fees	32	37	99	105
Other noninterest income (non-customer related)	0	0	0	0
Total noninterest income	32	37	99	105
Other real estate owned gain from sale	0	0	0	0
Net interest income	121	123	368	368
Total income less interest expense	153	160	467	473
Amegy | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	9	8	28	25
Amegy | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	6	7	18	21
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	5	11	13
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	2	4	4
Amegy | Operating Segments | Wealth management and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	4	3	12	9
Amegy | 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
CB&T | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	13	15	42	43
Other noninterest income (Non-ASC 606 customer related)	8	7	23	19
Total customer-related fees	21	22	65	62
Other noninterest income (non-customer related)	1	1	1	1
Total noninterest income	22	23	66	63
Other real estate owned gain from sale	1	0	1	0
Net interest income	129	127	381	387
Total income less interest expense	152	150	448	450
CB&T | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	6	6	17	17
CB&T | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	3	4	10	12
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	8	10
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 and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	1	6	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)	0	1	1	1
NBAZ | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	7	8	21	21
Other noninterest income (Non-ASC 606 customer related)	2	2	9	9
Total customer-related fees	9	10	30	30
Other noninterest income (non-customer related)	0	0	0	1
Total noninterest income	9	10	30	31
Other real estate owned gain from sale	0	0	0	0
Net interest income	54	56	162	169
Total income less interest expense	63	66	192	200
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)	2	3	7	8
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	6	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 and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	1	2	1
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)	8	9	23	27
Other noninterest income (Non-ASC 606 customer related)	2	2	8	5
Total customer-related fees	10	11	31	32
Other noninterest income (non-customer related)	0	0	0	0
Total noninterest income	10	11	31	32
Other real estate owned gain from sale	0	1	0	1
Net interest income	37	37	110	111
Total income less interest expense	47	49	141	144
NSB | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	2	2	6	6
NSB | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	3	3	7	9
NSB | Operating Segments | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	2	3	7	8
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 and trust 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	1
Vectra | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	5	5	15	14
Other noninterest income (Non-ASC 606 customer related)	3	2	9	5
Total customer-related fees	8	7	24	19
Other noninterest income (non-customer related)	0	0	0	0
Total noninterest income	8	7	24	19
Other real estate owned gain from sale	0	0	0	0
Net interest income	34	34	101	102
Total income less interest expense	42	41	125	121
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)	1	2	4	5
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 and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	1	0	3	1
Vectra | 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
TCBW | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	1	3	3
Other noninterest income (Non-ASC 606 customer related)	1	0	1	1
Total customer-related fees	1	1	4	4
Other noninterest income (non-customer 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	13	13	39	40
Total income less interest expense	14	14	43	44
TCBW | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	1	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 and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)	0	0	1	1
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	$ 0	$ 0