Operating Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) Branches	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Branches	Jun. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 528	$ 465	$ 1,017	$ 918
Provision for Loan and Lease Losses	7	35	30	77
Interest Income (Expense), after Provision for Loan Loss	521	430	987	841
Noninterest Income, Other Operating Income	(1)	(1)	(1)	(1)
Noninterest Expense	405	382	819	777
Income (Loss) from Continuing Operations before Income Taxes, Domestic	248	174	432	306
Zions Bank [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	164	154	319	304
Provision for Loan and Lease Losses	(2)	0	33	(30)
Interest Income (Expense), after Provision for Loan Loss	166	154	286	334
Noninterest Income, Other Operating Income	37	37	72	73
Noninterest Expense	111	115	224	221
Income (Loss) from Continuing Operations before Income Taxes, Domestic	92	76	134	186
Average Loans And Leases	12,483	12,600	12,486	12,453
Average Deposits	15,987	15,977	16,127	15,839
Amegy [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	124	117	238	230
Provision for Loan and Lease Losses	7	31	8	135
Interest Income (Expense), after Provision for Loan Loss	117	86	230	95
Noninterest Income, Other Operating Income	30	28	59	57
Noninterest Expense	89	83	173	168
Income (Loss) from Continuing Operations before Income Taxes, Domestic	58	31	116	(16)
Average Loans And Leases	10,856	10,761	10,747	10,566
Average Deposits	$ 11,218	10,959	$ 11,268	11,116
Number of Branches | Branches	73		73
California Bank Trust [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 123	110	$ 233	213
Provision for Loan and Lease Losses	(1)	4	(6)	1
Interest Income (Expense), after Provision for Loan Loss	124	106	239	212
Noninterest Income, Other Operating Income	19	16	36	32
Noninterest Expense	76	73	151	145
Income (Loss) from Continuing Operations before Income Taxes, Domestic	67	49	124	99
Average Loans And Leases	9,476	9,260	9,391	9,083
Average Deposits	$ 10,917	10,882	$ 10,919	10,681
Number of Branches | Branches	92		92
NBA [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 51	47	$ 100	92
Provision for Loan and Lease Losses	(1)	0	1	2
Interest Income (Expense), after Provision for Loan Loss	52	47	99	90
Noninterest Income, Other Operating Income	10	10	20	20
Noninterest Expense	36	38	73	72
Income (Loss) from Continuing Operations before Income Taxes, Domestic	26	19	46	38
Average Loans And Leases	4,246	4,008	4,254	3,936
Average Deposits	$ 4,762	4,582	$ 4,712	4,513
Number of Branches | Branches	58		58
Nevada State Bank [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 33	30	$ 65	60
Provision for Loan and Lease Losses	0	1	(5)	(25)
Interest Income (Expense), after Provision for Loan Loss	33	29	70	85
Noninterest Income, Other Operating Income	10	10	20	19
Noninterest Expense	35	36	70	69
Income (Loss) from Continuing Operations before Income Taxes, Domestic	8	3	20	35
Average Loans And Leases	2,372	2,274	2,355	2,269
Average Deposits	$ 4,233	4,103	$ 4,222	4,057
Number of Branches | Branches	50		50
Vectra Bank Colorado [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 32	29	$ 61	59
Provision for Loan and Lease Losses	3	(3)	0	(6)
Interest Income (Expense), after Provision for Loan Loss	29	32	61	65
Noninterest Income, Other Operating Income	7	6	12	11
Noninterest Expense	25	25	50	48
Income (Loss) from Continuing Operations before Income Taxes, Domestic	11	13	23	28
Average Loans And Leases	2,603	2,415	2,569	2,434
Average Deposits	2,728	2,667	2,759	2,725
TCBW [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	11	9	22	18
Provision for Loan and Lease Losses	0	1	(1)	0
Interest Income (Expense), after Provision for Loan Loss	11	8	23	18
Noninterest Income, Other Operating Income	1	1	2	2
Noninterest Expense	5	5	10	10
Income (Loss) from Continuing Operations before Income Taxes, Domestic	7	4	15	10
Average Loans And Leases	911	777	894	755
Average Deposits	1,094	947	1,097	950
Other Segment [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	(10)	(31)	(21)	(58)
Provision for Loan and Lease Losses	1	1	0	0
Interest Income (Expense), after Provision for Loan Loss	(11)	(32)	(21)	(58)
Noninterest Income, Other Operating Income	18	18	43	28
Noninterest Expense	28	7	68	44
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(21)	(21)	(46)	(74)
Average Loans And Leases	296	13	210	60
Average Deposits	$ 1,395	(167)	$ 1,170	(129)
Utah [Member] | Zions Bank [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	99		99
Idaho [Member] | Zions Bank [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	23		23
WYOMING | Zions Bank [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	1		1
Colorado [Member] | Vectra Bank Colorado [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	36		36
New Mexico [Member] | Vectra Bank Colorado [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	1		1
WASHINGTON | TCBW [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	1		1
OREGON | TCBW [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	1		1
Consolidated Entities [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 528	465	$ 1,017	918
Provision for Loan and Lease Losses	7	35	30	77
Interest Income (Expense), after Provision for Loan Loss	521	430	987	841
Noninterest Income, Other Operating Income	132	126	264	242
Noninterest Expense	405	382	819	777
Income (Loss) from Continuing Operations before Income Taxes, Domestic	248	174	432	306
Average Loans And Leases	43,243	42,108	42,906	41,556
Average Deposits	$ 52,334	$ 49,950	$ 52,274	$ 49,752