Revenue Recognition and Deferred Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	$ 31	$ 31	$ 90	$ 93
Card fees	24	24	70	70
Retail and business banking fees	20	19	58	58
Capital Market and Foreign Exchange Gain (Loss)	23	14	59	41
Wealth Management and Trust Income	16	14	45	41
Noninterest income, Customer-related Fees	140	126	392	377
Noninterest Income	146	136	410	412
Gains (Losses) on Sales of Other Real Estate	2		3	1
Net interest income	567	565	1,713	1,654
Corporate Segment
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	31	31	90	93
Card fees	32	32	95	93
Retail and business banking fees	20	19	58	57
Capital Market and Foreign Exchange Gain (Loss)	3	3	8	8
Wealth Management and Trust Income	15	13	42	38
Other Noninterest Income From Contracts With Customers	5	6	13	17
Total non-interest income from contracts with customers (ASC 606)	106	104	306	306
Other non-interest income (Non-ASC 606 customer related)	34	22	86	71
Noninterest income, Customer-related Fees	140	126	392	377
Other non-interest income (non-customer related)	6	10	18	35
Noninterest Income	146	136	410	412
Gains (Losses) on Sales of Other Real Estate	2	0	4	1
Net interest income	567	565	1,713	1,654
Revenues	715	701	2,127	2,067
Zions Bank [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	11	10	31	32
Card fees	15	15	45	44
Retail and business banking fees	6	7	17	18
Capital Market and Foreign Exchange Gain (Loss)	1	1	4	4
Wealth Management and Trust Income	5	4	13	12
Other Noninterest Income From Contracts With Customers	1	1	2	2
Total non-interest income from contracts with customers (ASC 606)	39	38	112	112
Other non-interest income (Non-ASC 606 customer related)	1	0	(1)	(2)
Noninterest income, Customer-related Fees	40	38	111	110
Other non-interest income (non-customer related)	0	0	1	1
Noninterest Income	40	38	112	111
Gains (Losses) on Sales of Other Real Estate	1	0	3	0
Net interest income	177	171	528	496
Revenues	218	209	643	607
Amegy Corporation [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	8	9	25	26
Card fees	7	7	21	22
Retail and business banking fees	5	4	13	12
Capital Market and Foreign Exchange Gain (Loss)	(1)	(1)	(4)	(4)
Wealth Management and Trust Income	3	3	8	8
Other Noninterest Income From Contracts With Customers	0	0	1	1
Total non-interest income from contracts with customers (ASC 606)	22	22	64	65
Other non-interest income (Non-ASC 606 customer related)	15	9	41	30
Noninterest income, Customer-related Fees	37	31	105	95
Other non-interest income (non-customer related)	0	0	0	0
Noninterest Income	37	31	105	95
Gains (Losses) on Sales of Other Real Estate	0	0	0	0
Net interest income	126	126	375	366
Revenues	163	157	480	461
California Bank Trust [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	6	6	17	17
Card fees	4	4	12	11
Retail and business banking fees	3	3	10	10
Capital Market and Foreign Exchange Gain (Loss)	2	1	4	3
Wealth Management and Trust Income	1	1	3	3
Other Noninterest Income From Contracts With Customers	0	0	1	1
Total non-interest income from contracts with customers (ASC 606)	16	15	47	45
Other non-interest income (Non-ASC 606 customer related)	6	4	15	12
Noninterest income, Customer-related Fees	22	19	62	57
Other non-interest income (non-customer related)	1	0	1	2
Noninterest Income	23	19	63	59
Gains (Losses) on Sales of Other Real Estate	0	0	0	0
Net interest income	130	132	395	381
Revenues	153	151	458	440
National Bank Of Arizona [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	2	2	5	6
Card fees	2	2	6	6
Retail and business banking fees	2	2	6	7
Capital Market and Foreign Exchange Gain (Loss)	0	0	0	0
Wealth Management and Trust Income	1	1	1	1
Other Noninterest Income From Contracts With Customers	0	0	1	0
Total non-interest income from contracts with customers (ASC 606)	7	7	19	20
Other non-interest income (Non-ASC 606 customer related)	3	3	11	9
Noninterest income, Customer-related Fees	10	10	30	29
Other non-interest income (non-customer related)	0	0	1	0
Noninterest Income	10	10	31	29
Gains (Losses) on Sales of Other Real Estate	0	0	0	0
Net interest income	57	56	173	161
Revenues	67	66	204	190
Nevada State Bank [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	2	2	6	7
Card fees	2	2	6	6
Retail and business banking fees	3	3	8	7
Capital Market and Foreign Exchange Gain (Loss)	0	0	1	1
Wealth Management and Trust Income	1	1	3	2
Other Noninterest Income From Contracts With Customers	0	0	1	0
Total non-interest income from contracts with customers (ASC 606)	8	8	25	23
Other non-interest income (Non-ASC 606 customer related)	3	2	7	7
Noninterest income, Customer-related Fees	11	10	32	30
Other non-interest income (non-customer related)	0	0	0	0
Noninterest Income	11	10	32	30
Gains (Losses) on Sales of Other Real Estate	1	0	1	0
Net interest income	39	37	115	106
Revenues	51	47	148	136
Vectra Bank Colorado [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	2	2	5	5
Card fees	1	1	3	3
Retail and business banking fees	1	1	3	3
Capital Market and Foreign Exchange Gain (Loss)	0	0	1	1
Wealth Management and Trust Income	0	0	1	1
Other Noninterest Income From Contracts With Customers	0	0	0	0
Total non-interest income from contracts with customers (ASC 606)	4	4	13	13
Other non-interest income (Non-ASC 606 customer related)	3	2	6	5
Noninterest income, Customer-related Fees	7	6	19	18
Other non-interest income (non-customer related)	0	0	0	0
Noninterest Income	7	6	19	18
Gains (Losses) on Sales of Other Real Estate	0	0	0	0
Net interest income	34	33	103	96
Revenues	41	39	122	114
Commerce Bank Of Washington [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	0	0	1	1
Card fees	1	1	1	1
Retail and business banking fees	0	0	0	0
Capital Market and Foreign Exchange Gain (Loss)	0	0	1	1
Wealth Management and Trust Income	0	0	1	0
Other Noninterest Income From Contracts With Customers	0	0	0	0
Total non-interest income from contracts with customers (ASC 606)	1	1	4	3
Other non-interest income (Non-ASC 606 customer related)	0	0	0	1
Noninterest income, Customer-related Fees	1	1	4	4
Other non-interest income (non-customer related)	0	0	0	0
Noninterest Income	1	1	4	4
Gains (Losses) on Sales of Other Real Estate	0	0	0	0
Net interest income	13	13	41	36
Revenues	14	14	45	40
Other Segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Commercial account fees	0	0	0	1
Card fees	0	0	1	0
Retail and business banking fees	0	1	1	0
Capital Market and Foreign Exchange Gain (Loss)	1	2	1	2
Wealth Management and Trust Income	4	3	12	11
Other Noninterest Income From Contracts With Customers	4	5	7	13
Total non-interest income from contracts with customers (ASC 606)	9	9	22	25
Other non-interest income (Non-ASC 606 customer related)	3	2	7	9
Noninterest income, Customer-related Fees	12	11	29	34
Other non-interest income (non-customer related)	5	10	15	32
Noninterest Income	17	21	44	66
Gains (Losses) on Sales of Other Real Estate	0	0	0	1
Net interest income	(9)	(3)	(17)	12
Revenues	$ 8	$ 18	$ 27	$ 79