Revenue Recognition (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($) state	Sep. 30, 2020 USD ($)	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 ($) state	Dec. 31, 2019 USD ($)	Dec. 31, 2018 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)									$ 402	$ 414	$ 412
Other noninterest income (Non-ASC 606 customer related)									147	111	96
Total customer-related fees									549	525	508
Other noninterest income (noncustomer-related)									25	37	44
Total noninterest income	$ 166	$ 157	$ 117	$ 134	$ 152	$ 146	$ 132	$ 132	574	562	552
Other real estate owned income									1	4	1
Net interest income	$ 550	$ 555	$ 563	$ 548	$ 559	$ 567	$ 569	$ 576	2,216	2,272	2,230
Total income less interest expense									2,791	2,838	2,783
Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									125	121	122
Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									113	127	125
Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									68	78	78
Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									12	13	12
Wealth management and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									59	57	51
Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									25	18	24
Other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									31	37	37
Other noninterest income (Non-ASC 606 customer related)									15	3	10
Total customer-related fees									46	40	47
Other noninterest income (noncustomer-related)									21	33	41
Total noninterest income									67	73	88
Other real estate owned income									0	0	0
Net interest income									20	24	35
Total income less interest expense									87	97	123
Other | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									2	0	0
Other | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									1	2	0
Other | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									0	1	1
Other | Capital markets and foreign exchange fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									8	7	6
Other | Wealth management and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									1	16	14
Other | Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									19	11	16
Zions Bank | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									136	134	134
Other noninterest income (Non-ASC 606 customer related)									23	12	12
Total customer-related fees									159	146	146
Other noninterest income (noncustomer-related)									(1)	1	1
Total noninterest income									158	147	147
Other real estate owned income									0	3	1
Net interest income									650	688	662
Total income less interest expense									808	838	810
Zions Bank | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									42	41	41
Zions Bank | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									49	51	50
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)									21	23	24
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)									(1)	0	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)									23	17	16
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	3
Amegy | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									98	98	96
Other noninterest income (Non-ASC 606 customer related)									34	40	30
Total customer-related fees									132	138	126
Other noninterest income (noncustomer-related)									1	0	0
Total noninterest income									133	138	126
Other real estate owned income									0	0	0
Net interest income									485	489	485
Total income less interest expense									618	627	611
Amegy | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									37	34	34
Amegy | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									24	28	29
Amegy | Operating Segments | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									15	17	16
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)									5	6	6
Amegy | Operating Segments | Wealth management and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									16	12	10
Amegy | Operating Segments | Other customer-related fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									1	1	1
CB&T | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									56	58	58
Other noninterest income (Non-ASC 606 customer related)									36	28	22
Total customer-related fees									92	86	80
Other noninterest income (noncustomer-related)									3	2	2
Total noninterest income									95	88	82
Other real estate owned income									1	0	0
Net interest income									512	512	508
Total income less interest expense									608	600	590
CB&T | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									22	23	23
CB&T | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									14	16	15
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)									11	13	14
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
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)									8	4	4
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	2	2
NBAZ | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									28	29	30
Other noninterest income (Non-ASC 606 customer related)									12	12	9
Total customer-related fees									40	41	39
Other noninterest income (noncustomer-related)									1	1	0
Total noninterest income									41	42	39
Other real estate owned income									0	0	0
Net interest income									216	223	216
Total income less interest expense									257	265	255
NBAZ | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									7	7	7
NBAZ | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									9	11	11
NBAZ | Operating Segments | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									8	8	9
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
NBAZ | Operating Segments | Wealth management and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									3	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)									1	1	1
NSB | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									31	35	35
Other noninterest income (Non-ASC 606 customer related)									12	8	6
Total customer-related fees									43	43	41
Other noninterest income (noncustomer-related)									0	0	0
Total noninterest income									43	43	41
Other real estate owned income									0	1	0
Net interest income									146	148	142
Total income less interest expense									189	192	183
NSB | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									8	8	9
NSB | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									10	12	12
NSB | Operating Segments | Retail and business banking fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									9	11	10
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
NSB | Operating Segments | Wealth management and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									3	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)									1	1	1
Vectra | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									19	19	18
Other noninterest income (Non-ASC 606 customer related)									13	7	6
Total customer-related fees									32	26	24
Other noninterest income (noncustomer-related)									0	0	0
Total noninterest income									32	26	24
Other real estate owned income									0	0	0
Net interest income									135	135	130
Total income less interest expense									167	161	154
Vectra | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									6	6	6
Vectra | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									5	6	6
Vectra | 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	4
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
Vectra | Operating Segments | Wealth management and trust fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									4	2	2
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
TCBW | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									3	4	4
Other noninterest income (Non-ASC 606 customer related)									2	1	1
Total customer-related fees									5	5	5
Other noninterest income (noncustomer-related)									0	0	0
Total noninterest income									5	5	5
Other real estate owned income									0	0	0
Net interest income									52	53	52
Total income less interest expense									57	58	57
TCBW | Operating Segments | Commercial account fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									1	2	2
TCBW | Operating Segments | Card fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Total noninterest income from contracts with customers (ASC 606)									1	1	2
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
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
TCBW | 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	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	$ 0