Operating Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) Branches	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Branches	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Current Fiscal Year End Date									--12-31
Interest Income (Expense), Net	$ 526	$ 522	$ 528	$ 489	$ 480	$ 469	$ 465	$ 453	$ 2,065	$ 1,867	$ 1,715
Provision for Loan and Lease Losses	(11)	5	7	23	(3)	19	35	42	24	93	40
Interest Income (Expense), after Provision for Loan Loss									2,041	1,774	1,675
Noninterest Income	140	140	132	132	128	145	126	117	544	516	357
Noninterest Expense	$ 417	413	405	414	404	403	382	396	$ 1,649	1,585	1,581
Number of community regional banks operating in distinct geographical areas	7								7
Income (Loss) from Continuing Operations before Income Taxes, Domestic	$ 260	$ 244	$ 248	$ 184	$ 207	$ 192	$ 174	$ 132	$ 936	705	451
Income Tax Expense (Benefit)									344	236	142
Commerce Bank Of Washington [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									46	38	28
Provision for Loan and Lease Losses									2	0	(3)
Interest Income (Expense), after Provision for Loan Loss									44	38	31
Noninterest Income									5	5	4
Noninterest Expense									20	19	17
Income (Loss) from Continuing Operations before Income Taxes, Domestic									29	24	18
Average Loans And Leases									926	791	707
Average Deposits									1,107	1,007	879
California Bank And Trust [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									476	434	377
Provision for Loan and Lease Losses									(5)	(9)	(4)
Interest Income (Expense), after Provision for Loan Loss									481	443	381
Noninterest Income									75	67	63
Noninterest Expense									299	290	294
Income (Loss) from Continuing Operations before Income Taxes, Domestic									257	220	150
Average Loans And Leases									9,539	9,211	8,556
Average Deposits									11,030	10,827	10,063
Amegy Corporation [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									483	460	387
Provision for Loan and Lease Losses									25	163	91
Interest Income (Expense), after Provision for Loan Loss									458	297	296
Noninterest Income									118	123	121
Noninterest Expense									336	326	373
Income (Loss) from Continuing Operations before Income Taxes, Domestic									240	94	44
Average Loans And Leases									11,021	10,595	10,148
Average Deposits									11,096	11,130	11,495
Zions Bank [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									650	624	544
Provision for Loan and Lease Losses									19	(22)	(28)
Interest Income (Expense), after Provision for Loan Loss									631	646	572
Noninterest Income									151	149	133
Noninterest Expense									436	424	430
Income (Loss) from Continuing Operations before Income Taxes, Domestic									346	371	275
Average Loans And Leases									12,481	12,538	12,118
Average Deposits									15,986	15,991	15,688
Vectra Bank Colorado [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									126	120	101
Provision for Loan and Lease Losses									1	(8)	5
Interest Income (Expense), after Provision for Loan Loss									125	128	96
Noninterest Income									25	23	21
Noninterest Expense									101	97	98
Income (Loss) from Continuing Operations before Income Taxes, Domestic									49	54	19
Average Loans And Leases									2,644	2,469	2,400
Average Deposits									2,756	2,720	2,792
Nevada State Bank [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									134	122	94
Provision for Loan and Lease Losses									(11)	(28)	(28)
Interest Income (Expense), after Provision for Loan Loss									145	150	122
Noninterest Income									40	39	36
Noninterest Expense									139	137	131
Income (Loss) from Continuing Operations before Income Taxes, Domestic									46	52	27
Average Loans And Leases									2,357	2,284	2,344
Average Deposits									4,254	4,137	3,891
National Bank Of Arizona [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									206	190	152
Provision for Loan and Lease Losses									(8)	(3)	8
Interest Income (Expense), after Provision for Loan Loss									214	193	144
Noninterest Income									40	40	36
Noninterest Expense									148	144	133
Income (Loss) from Continuing Operations before Income Taxes, Domestic									106	89	47
Average Loans And Leases									4,267	4,086	3,811
Average Deposits									4,762	4,576	4,311
Consolidated Entities [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									2,065	1,867	1,715
Provision for Loan and Lease Losses									24	93	40
Interest Income (Expense), after Provision for Loan Loss									2,041	1,774	1,675
Noninterest Income									544	516	357
Noninterest Expense									1,649	1,585	1,581
Income (Loss) from Continuing Operations before Income Taxes, Domestic									936	705	451
Average Loans And Leases									43,501	42,062	40,171
Average Deposits									52,200	50,595	48,638
Other Segment [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									(56)	(121)	32
Provision for Loan and Lease Losses									1	0	(1)
Interest Income (Expense), after Provision for Loan Loss									(57)	(121)	33
Noninterest Income									90	70	(57)
Noninterest Expense									170	148	105
Income (Loss) from Continuing Operations before Income Taxes, Domestic									(137)	(199)	(129)
Average Loans And Leases									266	88	87
Average Deposits									$ 1,209	$ 207	$ (481)
Vectra Bank Colorado [Member] | COLORADO
Segment Reporting Information [Line Items]
Number of Branches | Branches	36								36
Vectra Bank Colorado [Member] | New Mexico [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	1								1
National Bank Of Arizona [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	58								58
Commerce Bank Of Washington [Member] | WASHINGTON
Segment Reporting Information [Line Items]
Number of Branches | Branches	1								1
Commerce Bank Of Washington [Member] | OREGON
Segment Reporting Information [Line Items]
Number of Branches | Branches	1								1
California Bank And Trust [Member] | CALIFORNIA
Segment Reporting Information [Line Items]
Number of Branches | Branches	92								92
Zions Bank [Member] | UTAH
Segment Reporting Information [Line Items]
Number of Branches | Branches	97								97
Zions Bank [Member] | IDAHO
Segment Reporting Information [Line Items]
Number of Branches | Branches	23								23
Zions Bank [Member] | WYOMING
Segment Reporting Information [Line Items]
Number of Branches | Branches	1								1
Amegy Corporation [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	73								73
Nevada State Bank [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	50								50