Operating Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Branches	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Branches	Sep. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 522	$ 469	$ 1,539	$ 1,386
Provision for Loan and Lease Losses	5	19	35	95
Interest Income (Expense), after Provision for Loan Loss	517	450	1,504	1,291
Noninterest Income, Other Operating Income	3	2	3	1
Noninterest Expense	413	403	1,232	1,181
Income (Loss) from Continuing Operations before Income Taxes, Domestic	243	192	676	498
Zions Bank [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	164	157	483	462
Provision for Loan and Lease Losses	(15)	(1)	18	(31)
Interest Income (Expense), after Provision for Loan Loss	179	158	465	493
Noninterest Income, Other Operating Income	39	39	112	112
Noninterest Expense	107	108	327	320
Income (Loss) from Continuing Operations before Income Taxes, Domestic	111	89	250	285
Average Loans And Leases	12,543	12,629	12,505	12,512
Average Deposits	15,773	15,948	16,001	15,866
Amegy [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	122	114	359	344
Provision for Loan and Lease Losses	33	24	41	159
Interest Income (Expense), after Provision for Loan Loss	89	90	318	185
Noninterest Income, Other Operating Income	28	32	87	90
Noninterest Expense	86	75	259	244
Income (Loss) from Continuing Operations before Income Taxes, Domestic	31	47	146	31
Average Loans And Leases	11,170	10,666	10,890	10,599
Average Deposits	$ 10,862	11,068	$ 11,131	11,100
Number of Branches | Branches	73		73
California Bank Trust [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 118	110	$ 351	322
Provision for Loan and Lease Losses	(4)	(2)	(10)	(1)
Interest Income (Expense), after Provision for Loan Loss	122	112	361	323
Noninterest Income, Other Operating Income	20	18	56	51
Noninterest Expense	74	75	225	221
Income (Loss) from Continuing Operations before Income Taxes, Domestic	68	55	192	153
Average Loans And Leases	9,575	9,341	9,453	9,170
Average Deposits	$ 11,021	10,929	$ 10,953	10,764
Number of Branches | Branches	92		92
NBA [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 53	48	$ 153	140
Provision for Loan and Lease Losses	(8)	0	(7)	2
Interest Income (Expense), after Provision for Loan Loss	61	48	160	138
Noninterest Income, Other Operating Income	10	11	29	30
Noninterest Expense	38	36	111	109
Income (Loss) from Continuing Operations before Income Taxes, Domestic	33	23	78	59
Average Loans And Leases	4,267	4,156	4,258	4,010
Average Deposits	$ 4,816	4,632	$ 4,747	4,553
Number of Branches | Branches	58		58
Nevada State Bank [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 34	31	$ 98	91
Provision for Loan and Lease Losses	(3)	(4)	(8)	(29)
Interest Income (Expense), after Provision for Loan Loss	37	35	106	120
Noninterest Income, Other Operating Income	11	10	30	29
Noninterest Expense	35	34	105	104
Income (Loss) from Continuing Operations before Income Taxes, Domestic	13	11	31	45
Average Loans And Leases	2,347	2,288	2,352	2,275
Average Deposits	$ 4,276	4,223	$ 4,240	4,113
Number of Branches | Branches	50		50
Vectra Bank Colorado [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 32	31	$ 93	90
Provision for Loan and Lease Losses	0	2	0	(4)
Interest Income (Expense), after Provision for Loan Loss	32	29	93	94
Noninterest Income, Other Operating Income	7	6	19	17
Noninterest Expense	25	25	75	73
Income (Loss) from Continuing Operations before Income Taxes, Domestic	14	10	37	38
Average Loans And Leases	2,683	2,489	2,607	2,453
Average Deposits	2,757	2,663	2,758	2,704
TCBW [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	12	10	34	28
Provision for Loan and Lease Losses	3	0	2	0
Interest Income (Expense), after Provision for Loan Loss	9	10	32	28
Noninterest Income, Other Operating Income	1	1	3	3
Noninterest Expense	5	5	16	15
Income (Loss) from Continuing Operations before Income Taxes, Domestic	5	6	19	16
Average Loans And Leases	939	796	909	769
Average Deposits	1,098	1,010	1,098	970
Other Segment [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	(13)	(32)	(32)	(91)
Provision for Loan and Lease Losses	(1)	0	(1)	(1)
Interest Income (Expense), after Provision for Loan Loss	(12)	(32)	(31)	(90)
Noninterest Income, Other Operating Income	23	28	68	56
Noninterest Expense	43	45	114	95
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(32)	(49)	(77)	(129)
Average Loans And Leases	308	122	244	80
Average Deposits	$ 1,318	202	$ 1,227	(8)
Utah [Member] | Zions Bank [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	98		98
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	$ 522	469	$ 1,539	1,386
Provision for Loan and Lease Losses	5	19	35	95
Interest Income (Expense), after Provision for Loan Loss	517	450	1,504	1,291
Noninterest Income, Other Operating Income	139	145	404	388
Noninterest Expense	413	403	1,232	1,181
Income (Loss) from Continuing Operations before Income Taxes, Domestic	243	192	676	498
Average Loans And Leases	43,832	42,487	43,218	41,868
Average Deposits	$ 51,921	$ 50,675	$ 52,155	$ 50,062