Operating Segment Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) Branches	Mar. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 489	$ 453
Provision for Loan and Lease Losses	23	42
Interest Income (Expense), after Provision for Loan Loss	466	411
Noninterest Income, Other Operating Income	0	0
Noninterest Expense	414	396
Income (Loss) from Continuing Operations before Income Taxes, Domestic	184	132
Zions Bank [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	155	150
Provision for Loan and Lease Losses	35	(31)
Interest Income (Expense), after Provision for Loan Loss	120	181
Noninterest Income, Other Operating Income	35	36
Noninterest Expense	113	105
Income (Loss) from Continuing Operations before Income Taxes, Domestic	42	112
Average Loans And Leases	12,488	12,306
Average Deposits	16,268	15,700
Amegy [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	114	113
Provision for Loan and Lease Losses	1	104
Interest Income (Expense), after Provision for Loan Loss	113	9
Noninterest Income, Other Operating Income	29	29
Noninterest Expense	85	86
Income (Loss) from Continuing Operations before Income Taxes, Domestic	57	(48)
Average Loans And Leases	10,637	10,370
Average Deposits	$ 11,318	11,274
Number of Branches | Branches	74
California Bank Trust [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 110	103
Provision for Loan and Lease Losses	(5)	(3)
Interest Income (Expense), after Provision for Loan Loss	115	106
Noninterest Income, Other Operating Income	17	16
Noninterest Expense	75	72
Income (Loss) from Continuing Operations before Income Taxes, Domestic	57	50
Average Loans And Leases	9,306	8,905
Average Deposits	$ 10,921	10,479
Number of Branches | Branches	93
NBA [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 50	45
Provision for Loan and Lease Losses	1	2
Interest Income (Expense), after Provision for Loan Loss	49	43
Noninterest Income, Other Operating Income	9	10
Noninterest Expense	37	35
Income (Loss) from Continuing Operations before Income Taxes, Domestic	21	18
Average Loans And Leases	4,262	3,863
Average Deposits	$ 4,661	4,445
Number of Branches | Branches	58
Nevada State Bank [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 32	30
Provision for Loan and Lease Losses	(4)	(26)
Interest Income (Expense), after Provision for Loan Loss	36	56
Noninterest Income, Other Operating Income	10	9
Noninterest Expense	35	33
Income (Loss) from Continuing Operations before Income Taxes, Domestic	11	32
Average Loans And Leases	2,338	2,263
Average Deposits	$ 4,211	4,011
Number of Branches | Branches	50
Vectra Bank Colorado [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 30	30
Provision for Loan and Lease Losses	(3)	(3)
Interest Income (Expense), after Provision for Loan Loss	33	33
Noninterest Income, Other Operating Income	6	6
Noninterest Expense	25	23
Income (Loss) from Continuing Operations before Income Taxes, Domestic	14	16
Average Loans And Leases	2,535	2,453
Average Deposits	2,791	2,783
TCBW [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	11	9
Provision for Loan and Lease Losses	(1)	(2)
Interest Income (Expense), after Provision for Loan Loss	12	11
Noninterest Income, Other Operating Income	1	1
Noninterest Expense	5	5
Income (Loss) from Continuing Operations before Income Taxes, Domestic	8	7
Average Loans And Leases	877	733
Average Deposits	1,100	953
Other Segment [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	(13)	(27)
Provision for Loan and Lease Losses	(1)	1
Interest Income (Expense), after Provision for Loan Loss	(12)	(28)
Noninterest Income, Other Operating Income	25	10
Noninterest Expense	39	37
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(26)	(55)
Average Loans And Leases	123	110
Average Deposits	$ 942	(90)
Utah [Member] | Zions Bank [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	98
Idaho [Member] | Zions Bank [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	23
WYOMING | Zions Bank [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	1
Colorado [Member] | Vectra Bank Colorado [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	36
New Mexico [Member] | Vectra Bank Colorado [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	1
WASHINGTON | TCBW [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	1
OREGON | TCBW [Member]
Segment Reporting Information [Line Items]
Number of Branches | Branches	1
Consolidated Entities [Member]
Segment Reporting Information [Line Items]
Interest Income (Expense), Net	$ 489	453
Provision for Loan and Lease Losses	23	42
Interest Income (Expense), after Provision for Loan Loss	466	411
Noninterest Income, Other Operating Income	132	117
Noninterest Expense	414	396
Income (Loss) from Continuing Operations before Income Taxes, Domestic	184	132
Average Loans And Leases	42,566	41,003
Average Deposits	$ 52,212	$ 49,555