Revenue Recognition and Deferred Revenue (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) States	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) States	Jun. 30, 2019 USD ($)
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of states in which entity operates | States	11		11
Commercial account fees	$ 30	$ 30	$ 61	$ 60
Card fees	19	23	39	46
Retail and business banking fees	15	20	33	38
Capital markets and foreign exchange fees	18	20	42	35
Wealth management and trust fees	15	15	31	28
Total customer-related fees	130	130	271	252
Total noninterest income	117	132	250	264
Net interest income	563	569	1,111	1,145
Other [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	0	(1)	(2)	0
Card fees	0	0	2	(1)
Retail and business banking fees	0	2	0	0
Capital markets and foreign exchange fees	3	1	4	2
Wealth management and trust fees	1	5	2	8
Other customer-related fees	5	3	8	5
Total noninterest income from contracts with customers (ASC 606)	9	10	14	14
Other noninterest income (Non-ASC 606 customer related)	3	0	10	3
Total customer-related fees	12	10	24	17
Other non-interest income (non-customer related)	(12)	(1)	(21)	11
Total noninterest income	0	9	3	28
Other real estate owned gain from sale	0	0	0	1
Net interest income	30	(13)	50	(14)
Total income less interest expense	30	(4)	53	15
Zions Bank [Member] | Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	10	10	21	20
Card fees	11	13	23	25
Retail and business banking fees	4	5	10	11
Capital markets and foreign exchange fees	(1)	0	(1)	0
Wealth management and trust fees	6	4	11	8
Other customer-related fees	1	1	1	2
Total noninterest income from contracts with customers (ASC 606)	31	33	65	66
Other noninterest income (Non-ASC 606 customer related)	7	3	12	4
Total customer-related fees	38	36	77	70
Other non-interest income (non-customer related)	0	2	(1)	1
Total noninterest income	38	38	76	71
Other real estate owned gain from sale	0	0	0	1
Net interest income	156	175	314	351
Total income less interest expense	194	213	390	423
Amegy Corporation [Member] | Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	10	9	19	17
Card fees	6	7	12	14
Retail and business banking fees	3	4	7	8
Capital markets and foreign exchange fees	1	1	3	3
Wealth management and trust fees	4	3	8	5
Other customer-related fees	0	0	1	1
Total noninterest income from contracts with customers (ASC 606)	24	24	50	48
Other noninterest income (Non-ASC 606 customer related)	8	10	17	20
Total customer-related fees	32	34	67	68
Other non-interest income (non-customer related)	(1)	0	0	0
Total noninterest income	31	34	67	68
Other real estate owned gain from sale	0	0	0	0
Net interest income	120	127	237	252
Total income less interest expense	151	161	304	320
California Bank And Trust [Member] | Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	5	6	11	11
Card fees	3	4	6	8
Retail and business banking fees	3	3	6	7
Capital markets and foreign exchange fees	0	0	0	0
Wealth management and trust fees	2	1	4	2
Other customer-related fees	0	1	1	1
Total noninterest income from contracts with customers (ASC 606)	13	15	28	29
Other noninterest income (Non-ASC 606 customer related)	6	7	15	12
Total customer-related fees	19	22	43	41
Other non-interest income (non-customer related)	0	0	1	0
Total noninterest income	19	22	44	41
Other real estate owned gain from sale	0	0	0	0
Net interest income	123	134	245	267
Total income less interest expense	142	156	289	308
National Bank Of Arizona [Member] | Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	2	2	4	4
Card fees	2	3	5	6
Retail and business banking fees	2	2	4	4
Capital markets and foreign exchange fees	0	0	0	0
Wealth management and trust fees	0	0	1	1
Other customer-related fees	0	0	0	0
Total noninterest income from contracts with customers (ASC 606)	6	7	14	15
Other noninterest income (Non-ASC 606 customer related)	4	3	7	6
Total customer-related fees	10	10	21	21
Other non-interest income (non-customer related)	0	1	0	0
Total noninterest income	10	11	21	21
Other real estate owned gain from sale	0	0	0	0
Net interest income	53	59	105	117
Total income less interest expense	63	70	126	138
Nevada State Bank [Member] | Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	1	2	4	4
Card fees	2	3	5	6
Retail and business banking fees	2	3	4	6
Capital markets and foreign exchange fees	0	0	0	0
Wealth management and trust fees	1	1	2	2
Other customer-related fees	0	0	0	0
Total noninterest income from contracts with customers (ASC 606)	6	9	15	18
Other noninterest income (Non-ASC 606 customer related)	4	2	6	3
Total customer-related fees	10	11	21	21
Other non-interest income (non-customer related)	0	0	0	0
Total noninterest income	10	11	21	21
Other real estate owned gain from sale	0	0	0	0
Net interest income	35	38	70	76
Total income less interest expense	45	49	91	97
Vectra Bank Colorado [Member] | Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	1	1	3	3
Card fees	1	2	2	3
Retail and business banking fees	1	1	2	2
Capital markets and foreign exchange fees	0	0	0	0
Wealth management and trust fees	1	0	2	1
Other customer-related fees	0	0	0	0
Total noninterest income from contracts with customers (ASC 606)	4	4	9	9
Other noninterest income (Non-ASC 606 customer related)	4	2	7	3
Total customer-related fees	8	6	16	12
Other non-interest income (non-customer related)	0	0	0	0
Total noninterest income	8	6	16	12
Other real estate owned gain from sale	0	0	0	0
Net interest income	33	35	65	69
Total income less interest expense	41	41	81	81
Commerce Bank Of Washington [Member] | Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	1	1	1	1
Card fees	0	0	0	1
Retail and business banking fees	0	0	0	0
Capital markets and foreign exchange fees	0	0	0	0
Wealth management and trust fees	0	0	0	0
Other customer-related fees	0	0	0	0
Total noninterest income from contracts with customers (ASC 606)	1	1	1	2
Other noninterest income (Non-ASC 606 customer related)	0	0	1	0
Total customer-related fees	1	1	2	2
Other non-interest income (non-customer related)	0	0	0	0
Total noninterest income	1	1	2	2
Other real estate owned gain from sale	0	0	0	0
Net interest income	13	14	25	27
Total income less interest expense	14	15	27	29
Corporate and Other [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	30	30	61	60
Card fees	25	32	55	62
Retail and business banking fees	15	20	33	38
Capital markets and foreign exchange fees	3	2	6	5
Wealth management and trust fees	15	14	30	27
Other customer-related fees	6	5	11	9
Total noninterest income from contracts with customers (ASC 606)	94	103	196	201
Other noninterest income (Non-ASC 606 customer related)	36	27	75	51
Total customer-related fees	130	130	271	252
Other non-interest income (non-customer related)	(13)	2	(21)	12
Total noninterest income	117	132	250	264
Other real estate owned gain from sale	0	0	0	2
Net interest income	563	569	1,111	1,145
Total income less interest expense	$ 680	$ 701	$ 1,361	$ 1,411