Summary Of Significant Accounting Policies (Policy)	12 Months Ended
Dec. 31, 2016
Accounting Policies [Abstract]
Business	Business Zions Bancorporation (“the Parent”) is a financial holding company headquartered in Salt Lake City, Utah, which owns and operates a commercial bank. The Parent and its subsidiaries (collectively “the Company”) provide a full range of banking and related services in 11 Western and Southwestern states through seven separately managed and branded units as follows: Zions Bank, in Utah, Idaho and Wyoming; California Bank & Trust (“CB&T”); Amegy Bank (“Amegy”), in Texas; National Bank of Arizona (“NBAZ”); Nevada State Bank (“NSB”); Vectra Bank Colorado (“Vectra”), in Colorado and New Mexico; and The Commerce Bank of Washington (“TCBW”) which operates under that name in Washington and under the name The Commerce Bank of Oregon (“TCBO”) in Oregon. Pursuant to a Board resolution adopted November 21, 2014, The Commerce Bank of Oregon merged into TCBW effective March 31, 2015. The Parent also owns and operates certain nonbank subsidiaries that engage in financial services.
Basis Of Financial Statement Presentation	Basis of Financial Statement Presentation The consolidated financial statements include the accounts of the Parent and its majority-owned subsidiaries (“the Company,” “we,” “our,” “us”). Unconsolidated investments where we have the ability to exercise significant influence over the operating and financial policies of the respective investee are accounted for using the equity method of accounting; those that are not consolidated or accounted for using the equity method of accounting are accounted for under cost or fair value accounting. All significant intercompany accounts and transactions have been eliminated in consolidation. The consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) and prevailing practices within the financial services industry. References to GAAP, including standards promulgated by the Financial Accounting Standards Board (“FASB”), are made according to sections of the Accounting Standards Codification (“ASC”). Changes to the ASC are made with Accounting Standards Updates (“ASU”) that include consensus issues of the Emerging Issues Task Force (“EITF”). In certain cases, ASUs are issued jointly with International Financial Reporting Standards. In preparing the consolidated financial statements, we are required to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates. Certain prior year amounts have been reclassified to conform with the current year presentation. These reclassifications did not affect net income or shareholders’ equity.
Reclassification Policy	Certain prior year amounts have been reclassified to conform with the current year presentation.
Variable Interest Entities	Variable Interest Entities A variable interest entity (“VIE”) is consolidated when a company is the primary beneficiary of the VIE. Current accounting guidance requires continuous analysis on a qualitative rather than a quantitative basis to determine the primary beneficiary of a VIE. At the commencement of our involvement and periodically thereafter, we consider our consolidation conclusions for all entities with which we are involved. As of December 31, 2016 and 2015, no VIEs have been consolidated in the Company’s financial statements.
Statement Of Cash Flows	Statement of Cash Flows For purposes of presentation in the consolidated statements of cash flows, “cash and cash equivalents” are defined as those amounts included in cash and due from banks in the consolidated balance sheets.
Security Resell Agreements	Security Resell Agreements Security resell agreements represent overnight and term agreements with the majority maturing within 30 days. These agreements are generally treated as collateralized financing transactions and are carried at amounts at which the securities were acquired plus accrued interest. Either the Company, or in some instances third parties on its behalf, take possession of the underlying securities. The fair value of such securities is monitored throughout the contract term to ensure that asset values remain sufficient to protect against counterparty default. We are permitted by contract to sell or repledge certain securities that we accept as collateral for security resell agreements. If sold, our obligation to return the collateral is recorded as “securities sold, not yet purchased” and included as a liability in “Federal funds and other short-term borrowings.” At December 31, 2016, we held $347 million of securities for which we were permitted by contract to sell or repledge. Security resell agreements averaged $464 million during 2016, and the maximum amount outstanding at any month-end during 2016 was $2.2 billion.
Investment Securities	Investment Securities We classify our investment securities according to their purpose and holding period. Gains or losses on the sale of securities are recognized using the specific identification method and recorded in noninterest income. Held-to-maturity (“HTM”) debt securities are carried at amortized cost with purchase discounts or premiums accreted or amortized into interest income over the contractual life of the security. The Company has the intent and ability to hold such securities until maturity. Available-for-sale (“AFS”) securities are stated at fair value and generally consist of debt securities held for investment and marketable equity securities not accounted for under the equity method. Unrealized gains and losses of AFS securities, after applicable taxes, are recorded as a component of other comprehensive income (“OCI”). We review quarterly our investment securities portfolio for any declines in value that are considered to be other-than-temporary impairment (“OTTI”). The process, methodology and factors considered to evaluate securities for OTTI are discussed further in Note 5. Trading securities are stated at fair value and consist of securities acquired for short-term appreciation or other trading purposes. Realized and unrealized gains and losses are recorded in trading income, which is included in capital markets and foreign exchange. The fair values of investment securities, as estimated under current accounting guidance, are discussed in Note 20.
Loans And Allowance For Credit Losses	Loans and Allowance for Credit Losses Loans are reported at the principal amount outstanding, net of unearned income. Unearned income, which includes deferred fees net of deferred direct loan origination costs, is amortized to interest income over the life of the loan using the interest method. Interest income is recognized on an accrual basis. At the time of origination, we determine whether loans will be held for investment or held for sale. We may subsequently change our intent to hold loans for investment and reclassify them as held for sale. Loans held for sale are carried at the lower of aggregate cost or fair value. A valuation allowance is recorded when cost exceeds fair value based on reviews at the time of reclassification and periodically thereafter. Gains and losses are recorded in noninterest income based on the difference between sales proceeds and carrying value. We evaluate loans throughout their lives for signs of credit deterioration, which may impact the loan’s status, and potentially impact our accounting for that loan. Loan status categories include past due as to contractual payments, nonaccrual, impaired, and restructured (including trouble debt restructurings “TDRs”). Our accounting policies for these loan statuses and our estimation of the related allowance for loan losses (“ALLL”) are discussed further in Note 6. In the ordinary course of business, we transfer portions of loans under participation agreements to manage credit risk and our portfolio concentration. We evaluate the loan participations to determine if they meet the appropriate accounting guidance to qualify as sales. Certain purchased loans require separate accounting procedures that are also discussed in Note 6. The allowance for credit losses (“ACL”) includes the ALLL and the reserve for unfunded lending commitments (“RULC”), and represents our estimate of losses inherent in the loan portfolio that may be recognized from loans and lending commitments that are not recoverable. Further discussion of our estimation process for the ACL is included in Note 6.
Other Real Estate Owned	Other Real Estate Owned Other real estate owned (“OREO”) consists principally of commercial and residential real estate obtained in partial or total satisfaction of loan obligations. Amounts are recorded initially at fair value (less any selling costs) based on property appraisals at the time of transfer and subsequently at the lower of cost or fair value (less any selling costs).
Nonmarketable Securities	Other Noninterest-Bearing Investments These investments include investments in private equity funds (referred to in this document as private equity investments “PEIs”), venture capital securities, securities acquired for various debt and regulatory requirements, bank-owned life insurance, and certain other noninterest-bearing investments. See further discussions in Notes 5, 17 and 20. Certain PEIs and venture capital securities are accounted for under the equity method and reported at fair value. Changes in fair value and gains and losses from sales are recognized in noninterest income. The values assigned to the securities where no market quotations exist are based upon available information and may not necessarily represent amounts that will ultimately be realized. Such estimated amounts depend on future circumstances and will not be realized until the individual securities are liquidated. Bank-owned life insurance is accounted for at fair value based on the cash surrender values (“CSVs”) of the general account insurance policies. A third party service provides these values. Other PEIs and those acquired for various debt and regulatory requirements are accounted for at cost. Periodic reviews are conducted for impairment by comparing carrying values with estimates of fair value determined according to the previous discussion.
Premises And Equipment	Premises, Equipment and Software, Net Premises, equipment and software, are stated at cost, net of accumulated depreciation and amortization. Depreciation, computed primarily on the straight-line method, is charged to operations over the estimated useful lives of the properties, generally 25 to 40 years for buildings, 3 to 10 years for furniture and equipment, and 3 to 10 years for software, including capitalized costs related to the Company’s Core Transformation project to install new lending and deposit systems. Leasehold improvements are amortized over the terms of the respective leases or the estimated useful lives of the improvements, whichever is shorter.
Business Combinations	Business Combinations Business combinations are accounted for under the acquisition method of accounting. Upon initially obtaining control, we recognize 100% of all acquired assets and all assumed liabilities regardless of the percentage owned. The assets and liabilities are recorded at their estimated fair values, with goodwill being recorded when such fair values are less than the cost of acquisition. Certain transaction and restructuring costs are expensed as incurred. Changes to estimated fair values from a business combination are recognized as an adjustment to goodwill over the measurement period, which cannot exceed one year from the acquisition date. Results of operations of the acquired business are included in our statement of income from the date of acquisition.
Goodwill And Identifiable Intangible Assets	Goodwill and Identifiable Intangible Assets Goodwill and intangible assets deemed to have indefinite lives are not amortized. We subject these assets to annual specified impairment tests as of the beginning of the fourth quarter and more frequently if changing conditions warrant. Core deposit assets and other intangibles with finite useful lives are generally amortized on an accelerated basis using an estimated useful life of up to 12 years.
Derivative Instruments	Derivative Instruments We use derivative instruments, including interest rate swaps and floors and basis swaps, as part of our overall interest rate risk management strategy. These instruments enable us to manage to desired asset and liability duration and to reduce interest rate risk exposure by matching estimated repricing periods of interest-sensitive assets and liabilities. We also execute derivative instruments with commercial banking customers to facilitate their risk management strategies. These derivatives are immediately hedged by offsetting derivatives with third parties such that we minimize our net risk exposure as a result of such transactions. We record all derivatives at fair value in the balance sheet as either other assets or other liabilities. See further discussion in Note 7.
Commitments And Letters Of Credit	Commitments and Letters of Credit In the ordinary course of business, we enter into commitments to extend credit, commercial letters of credit, and standby letters of credit. Such financial instruments are recorded in the financial statements when they become payable. The credit risk associated with these commitments is evaluated in a manner similar to the ALLL. The RULC is presented separately in the balance sheet.
Revenue Recognition, Policy [Policy Text Block]	Revenue Recognition Service charges and fees on deposit accounts are recognized in accordance with published deposit account agreements for customer accounts or contractual agreements for commercial accounts. Other service charges, commissions and fees include interchange fees, bank services, and other fees, which are generally recognized when earned.
Share-Based Compensation	Share-Based Compensation Share-based compensation generally includes grants of stock options, restricted stock, restricted stock units (“RSUs”), and other awards to employees and nonemployee directors. We recognize compensation expense in the statement of income based on the fair value of the associated share-based awards. See further discussion in Note 16.
Income Taxes	Income Taxes Deferred tax assets and liabilities are determined based on temporary differences between financial statement asset and liability amounts and their respective tax bases, and are measured using enacted tax laws and rates. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Deferred tax assets are recognized subject to management’s judgment that realization is more likely than not. Unrecognized tax benefits for uncertain tax positions relate primarily to state tax contingencies. See further discussion in Note 14.
Net Earnings Per Common Share	Net Earnings Per Common Share Net earnings per common share is based on net earnings applicable to common shareholders, which is net of preferred stock dividends. Basic net earnings per common share is based on the weighted average outstanding common shares during each year. Unvested share-based awards with rights to receive nonforfeitable dividends are considered participating securities and included in the computation of basic earnings per share. Diluted net earnings per common share is based on the weighted average outstanding common shares during each year, including common stock equivalents. Stock options, restricted stock, RSUs, and stock warrants are converted to common stock equivalents using the treasury stock method. Diluted net earnings per common share excludes common stock equivalents whose effect is antidilutive. See further discussion in Note 15.

Summary Of Significant Accounting Policies (Policy)

12 Months Ended

Dec. 31, 2016

Zions Bancorporation (“the Parent”) is a financial holding company headquartered in Salt Lake City, Utah, which owns and operates a commercial bank. The Parent and its subsidiaries (collectively “the Company”) provide a full range of banking and related services in 11 Western and Southwestern states through seven separately managed and branded units as follows: Zions Bank, in Utah, Idaho and Wyoming; California Bank & Trust (“CB&T”); Amegy Bank (“Amegy”), in Texas; National Bank of Arizona (“NBAZ”); Nevada State Bank (“NSB”); Vectra Bank Colorado (“Vectra”), in Colorado and New Mexico; and The Commerce Bank of Washington (“TCBW”) which operates under that name in Washington and under the name The Commerce Bank of Oregon (“TCBO”) in Oregon. Pursuant to a Board resolution adopted November 21, 2014, The Commerce Bank of Oregon merged into TCBW effective March 31, 2015. The Parent also owns and operates certain nonbank subsidiaries that engage in financial services.

Basis Of Financial Statement Presentation

Basis of Financial Statement Presentation

The consolidated financial statements include the accounts of the Parent and its majority-owned subsidiaries (“the Company,” “we,” “our,” “us”). Unconsolidated investments where we have the ability to exercise significant influence over the operating and financial policies of the respective investee are accounted for using the equity method of accounting; those that are not consolidated or accounted for using the equity method of accounting are accounted for under cost or fair value accounting. All significant intercompany accounts and transactions have been eliminated in consolidation.

The consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) and prevailing practices within the financial services industry. References to GAAP, including standards promulgated by the Financial Accounting Standards Board (“FASB”), are made according to sections of the Accounting Standards Codification (“ASC”). Changes to the ASC are made with Accounting Standards Updates (“ASU”) that include consensus issues of the Emerging Issues Task Force (“EITF”). In certain cases, ASUs are issued jointly with International Financial Reporting Standards.

In preparing the consolidated financial statements, we are required to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates. Certain prior year amounts have been reclassified to conform with the current year presentation. These reclassifications did not affect net income or shareholders’ equity.

Reclassification Policy

Certain prior year amounts have been reclassified to conform with the current year presentation.

Variable Interest Entities

A variable interest entity (“VIE”) is consolidated when a company is the primary beneficiary of the VIE. Current accounting guidance requires continuous analysis on a qualitative rather than a quantitative basis to determine the primary beneficiary of a VIE. At the commencement of our involvement and periodically thereafter, we consider our consolidation conclusions for all entities with which we are involved. As of December 31, 2016 and 2015, no VIEs have been consolidated in the Company’s financial statements.

Statement Of Cash Flows

Statement of Cash Flows

For purposes of presentation in the consolidated statements of cash flows, “cash and cash equivalents” are defined as those amounts included in cash and due from banks in the consolidated balance sheets.

Security Resell Agreements

Security resell agreements represent overnight and term agreements with the majority maturing within 30 days. These agreements are generally treated as collateralized financing transactions and are carried at amounts at which the securities were acquired plus accrued interest. Either the Company, or in some instances third parties on its behalf, take possession of the underlying securities. The fair value of such securities is monitored throughout the contract term to ensure that asset values remain sufficient to protect against counterparty default. We are permitted by contract to sell or repledge certain securities that we accept as collateral for security resell agreements. If sold, our obligation to return the collateral is recorded as “securities sold, not yet purchased” and included as a liability in “Federal funds and other short-term borrowings.” At December 31, 2016, we held $347 million of securities for which we were permitted by contract to sell or repledge. Security resell agreements averaged $464 million during 2016, and the maximum amount outstanding at any month-end during 2016 was $2.2 billion.

Investment Securities

We classify our investment securities according to their purpose and holding period. Gains or losses on the sale of securities are recognized using the specific identification method and recorded in noninterest income.

Held-to-maturity (“HTM”) debt securities are carried at amortized cost with purchase discounts or premiums accreted or amortized into interest income over the contractual life of the security. The Company has the intent and ability to hold such securities until maturity.

Available-for-sale (“AFS”) securities are stated at fair value and generally consist of debt securities held for investment and marketable equity securities not accounted for under the equity method. Unrealized gains and losses of AFS securities, after applicable taxes, are recorded as a component of other comprehensive income (“OCI”).

We review quarterly our investment securities portfolio for any declines in value that are considered to be other-than-temporary impairment (“OTTI”). The process, methodology and factors considered to evaluate securities for OTTI are discussed further in Note 5.

Trading securities are stated at fair value and consist of securities acquired for short-term appreciation or other trading purposes. Realized and unrealized gains and losses are recorded in trading income, which is included in capital markets and foreign exchange.

The fair values of investment securities, as estimated under current accounting guidance, are discussed in Note 20.

Loans And Allowance For Credit Losses

Loans and Allowance for Credit Losses

Loans are reported at the principal amount outstanding, net of unearned income. Unearned income, which includes deferred fees net of deferred direct loan origination costs, is amortized to interest income over the life of the loan using the interest method. Interest income is recognized on an accrual basis.

At the time of origination, we determine whether loans will be held for investment or held for sale. We may subsequently change our intent to hold loans for investment and reclassify them as held for sale. Loans held for sale are carried at the lower of aggregate cost or fair value. A valuation allowance is recorded when cost exceeds fair value based on reviews at the time of reclassification and periodically thereafter. Gains and losses are recorded in noninterest income based on the difference between sales proceeds and carrying value.

We evaluate loans throughout their lives for signs of credit deterioration, which may impact the loan’s status, and potentially impact our accounting for that loan. Loan status categories include past due as to contractual payments, nonaccrual, impaired, and restructured (including trouble debt restructurings “TDRs”). Our accounting policies for these loan statuses and our estimation of the related allowance for loan losses (“ALLL”) are discussed further in Note 6.

In the ordinary course of business, we transfer portions of loans under participation agreements to manage credit risk and our portfolio concentration. We evaluate the loan participations to determine if they meet the appropriate accounting guidance to qualify as sales. Certain purchased loans require separate accounting procedures that are also discussed in Note 6.

The allowance for credit losses (“ACL”) includes the ALLL and the reserve for unfunded lending commitments (“RULC”), and represents our estimate of losses inherent in the loan portfolio that may be recognized from loans and lending commitments that are not recoverable. Further discussion of our estimation process for the ACL is included in Note 6.

Other Real Estate Owned

Other real estate owned (“OREO”) consists principally of commercial and residential real estate obtained in partial or total satisfaction of loan obligations. Amounts are recorded initially at fair value (less any selling costs) based on property appraisals at the time of transfer and subsequently at the lower of cost or fair value (less any selling costs).

Nonmarketable Securities

Other Noninterest-Bearing Investments

These investments include investments in private equity funds (referred to in this document as private equity investments “PEIs”), venture capital securities, securities acquired for various debt and regulatory requirements, bank-owned life insurance, and certain other noninterest-bearing investments. See further discussions in Notes 5, 17 and 20.

Certain PEIs and venture capital securities are accounted for under the equity method and reported at fair value. Changes in fair value and gains and losses from sales are recognized in noninterest income. The values assigned to the securities where no market quotations exist are based upon available information and may not necessarily represent amounts that will ultimately be realized. Such estimated amounts depend on future circumstances and will not be realized until the individual securities are liquidated.

Bank-owned life insurance is accounted for at fair value based on the cash surrender values (“CSVs”) of the general account insurance policies. A third party service provides these values.

Other PEIs and those acquired for various debt and regulatory requirements are accounted for at cost. Periodic reviews are conducted for impairment by comparing carrying values with estimates of fair value determined according to the previous discussion.

Premises And Equipment

Premises, Equipment and Software, Net

Premises, equipment and software, are stated at cost, net of accumulated depreciation and amortization. Depreciation, computed primarily on the straight-line method, is charged to operations over the estimated useful lives of the properties, generally 25 to 40 years for buildings, 3 to 10 years for furniture and equipment, and 3 to 10 years for software, including capitalized costs related to the Company’s Core Transformation project to install new lending and deposit systems. Leasehold improvements are amortized over the terms of the respective leases or the estimated useful lives of the improvements, whichever is shorter.

Business Combinations

Business combinations are accounted for under the acquisition method of accounting. Upon initially obtaining control, we recognize 100% of all acquired assets and all assumed liabilities regardless of the percentage owned. The assets and liabilities are recorded at their estimated fair values, with goodwill being recorded when such fair values are less than the cost of acquisition. Certain transaction and restructuring costs are expensed as incurred. Changes to estimated fair values from a business combination are recognized as an adjustment to goodwill over the measurement period, which cannot exceed one year from the acquisition date. Results of operations of the acquired business are included in our statement of income from the date of acquisition.

Goodwill And Identifiable Intangible Assets

Goodwill and Identifiable Intangible Assets

Goodwill and intangible assets deemed to have indefinite lives are not amortized. We subject these assets to annual specified impairment tests as of the beginning of the fourth quarter and more frequently if changing conditions warrant. Core deposit assets and other intangibles with finite useful lives are generally amortized on an accelerated basis using an estimated useful life of up to 12 years.

Derivative Instruments

We use derivative instruments, including interest rate swaps and floors and basis swaps, as part of our overall interest rate risk management strategy. These instruments enable us to manage to desired asset and liability duration and to reduce interest rate risk exposure by matching estimated repricing periods of interest-sensitive assets and liabilities. We also execute derivative instruments with commercial banking customers to facilitate their risk management strategies. These derivatives are immediately hedged by offsetting derivatives with third parties such that we minimize our net risk exposure as a result of such transactions. We record all derivatives at fair value in the balance sheet as either other assets or other liabilities. See further discussion in Note 7.

Commitments And Letters Of Credit

Commitments and Letters of Credit

In the ordinary course of business, we enter into commitments to extend credit, commercial letters of credit, and standby letters of credit. Such financial instruments are recorded in the financial statements when they become payable. The credit risk associated with these commitments is evaluated in a manner similar to the ALLL. The RULC is presented separately in the balance sheet.

Revenue Recognition, Policy [Policy Text Block]

Revenue Recognition

Service charges and fees on deposit accounts are recognized in accordance with published deposit account agreements for customer accounts or contractual agreements for commercial accounts. Other service charges, commissions and fees include interchange fees, bank services, and other fees, which are generally recognized when earned.

Share-Based Compensation

Share-based compensation generally includes grants of stock options, restricted stock, restricted stock units (“RSUs”), and other awards to employees and nonemployee directors. We recognize compensation expense in the statement of income based on the fair value of the associated share-based awards. See further discussion in Note 16.

Income Taxes

Deferred tax assets and liabilities are determined based on temporary differences between financial statement asset and liability amounts and their respective tax bases, and are measured using enacted tax laws and rates. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Deferred tax assets are recognized subject to management’s judgment that realization is more likely than not. Unrecognized tax benefits for uncertain tax positions relate primarily to state tax contingencies. See further discussion in Note 14.

Net Earnings Per Common Share

Net earnings per common share is based on net earnings applicable to common shareholders, which is net of preferred stock dividends. Basic net earnings per common share is based on the weighted average outstanding common shares during each year. Unvested share-based awards with rights to receive nonforfeitable dividends are considered participating securities and included in the computation of basic earnings per share. Diluted net earnings per common share is based on the weighted average outstanding common shares during each year, including common stock equivalents. Stock options, restricted stock, RSUs, and stock warrants are converted to common stock equivalents using the treasury stock method. Diluted net earnings per common share excludes common stock equivalents whose effect is antidilutive. See further discussion in Note 15.