Operating Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($) Branches	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Branches	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 576	$ 565	$ 548	$ 541	$ 526	$ 522	$ 528	$ 489	$ 2,230	$ 2,065	$ 1,867
Provision for Loan and Lease Losses	7	(11)	5	(40)	(11)	5	7	23	(39)	24	93
Interest Income (Expense), after Provision for Loan Loss									2,269	2,041	1,774
Noninterest Income	140	136	138	138	140	140	132	132	552	544	516
Noninterest Expense	$ 419	420	428	411	417	413	405	414	$ 1,678	1,649	1,585
Number of community regional banks operating in distinct geographical areas	7								7
Income (Loss) from Continuing Operations before Income Taxes, Domestic	$ 290	$ 292	$ 253	$ 308	$ 260	$ 244	$ 248	$ 184	$ 1,143	936	705
Consolidated Entities [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									2,230	2,065	1,867
Noninterest Income									552	544	516
California Bank Trust [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									538	481	434
Provision for Loan and Lease Losses									14	(5)	(9)
Interest Income (Expense), after Provision for Loan Loss									524	486	443
Noninterest Income									82	75	67
Noninterest Expense									309	299	290
Income (Loss) from Continuing Operations before Income Taxes, Domestic									297	262	220
Average Loans And Leases									10,033	9,539	9,211
Average Deposits									11,268	11,030	10,827
Other Segment [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									(100)	(54)	(119)
Provision for Loan and Lease Losses									1	1	0
Interest Income (Expense), after Provision for Loan Loss									(101)	(55)	(119)
Noninterest Income									85	90	70
Noninterest Expense									144	170	148
Income (Loss) from Continuing Operations before Income Taxes, Domestic									(160)	(135)	(197)
Average Loans And Leases									347	266	88
Average Deposits									1,811	1,209	207
Vectra Bank Colorado [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									137	127	120
Provision for Loan and Lease Losses									5	1	(8)
Interest Income (Expense), after Provision for Loan Loss									132	126	128
Noninterest Income									24	25	23
Noninterest Expense									105	101	97
Income (Loss) from Continuing Operations before Income Taxes, Domestic									51	50	54
Average Loans And Leases									2,924	2,644	2,469
Average Deposits									$ 2,761	2,756	2,720
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]
Interest Income (Expense), Net									$ 231	204	190
Provision for Loan and Lease Losses									7	(8)	(3)
Interest Income (Expense), after Provision for Loan Loss									224	212	193
Noninterest Income									39	40	40
Noninterest Expense									153	148	144
Income (Loss) from Continuing Operations before Income Taxes, Domestic									110	104	89
Average Loans And Leases									4,608	4,267	4,086
Average Deposits									$ 4,931	4,762	4,576
National Bank Of Arizona [Member] | ARIZONA
Segment Reporting Information [Line Items]
Number of Branches | Branches	58								58
Commerce Bank Of Washington [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									$ 55	46	38
Provision for Loan and Lease Losses									2	2	0
Interest Income (Expense), after Provision for Loan Loss									53	44	38
Noninterest Income									5	5	5
Noninterest Expense									22	20	19
Income (Loss) from Continuing Operations before Income Taxes, Domestic									36	29	24
Average Loans And Leases									1,118	926	791
Average Deposits									$ 1,092	1,107	1,007
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	90								90
Zions Bank [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 699								$ 699	648	622
Provision for Loan and Lease Losses									7	19	(22)
Interest Income (Expense), after Provision for Loan Loss									692	629	644
Noninterest Income	$ 150								150	151	149
Noninterest Expense									463	436	424
Income (Loss) from Continuing Operations before Income Taxes, Domestic									379	344	369
Average Loans And Leases									12,643	12,481	12,538
Average Deposits									$ 15,874	15,986	15,991
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]
Interest Income (Expense), Net									$ 519	482	460
Provision for Loan and Lease Losses									(75)	25	163
Interest Income (Expense), after Provision for Loan Loss									594	457	297
Noninterest Income									126	118	123
Noninterest Expense									338	336	326
Income (Loss) from Continuing Operations before Income Taxes, Domestic									382	239	94
Average Loans And Leases									11,358	11,021	10,595
Average Deposits									$ 11,160	11,096	11,130
Amegy Corporation [Member] | TEXAS
Segment Reporting Information [Line Items]
Number of Branches | Branches	75								75
Nevada State Bank [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									$ 151	131	122
Provision for Loan and Lease Losses									0	(11)	(28)
Interest Income (Expense), after Provision for Loan Loss									151	142	150
Noninterest Income									41	40	39
Noninterest Expense									144	139	137
Income (Loss) from Continuing Operations before Income Taxes, Domestic									48	43	52
Average Loans And Leases									2,394	2,357	2,284
Average Deposits									$ 4,286	4,254	4,137
Nevada State Bank [Member] | NEVADA
Segment Reporting Information [Line Items]
Number of Branches | Branches	50								50
Consolidated Entities [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net									$ 2,230	2,065	1,867
Provision for Loan and Lease Losses									(39)	24	93
Interest Income (Expense), after Provision for Loan Loss									2,269	2,041	1,774
Noninterest Income									552	544	516
Noninterest Expense									1,678	1,649	1,585
Income (Loss) from Continuing Operations before Income Taxes, Domestic									1,143	936	705
Average Loans And Leases									45,425	43,501	42,062
Average Deposits									$ 53,183	$ 52,200	$ 50,595