NATURE OF OPERATIONS AND FINANCIAL STATEMENT PRESENTATION (Policies)	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	BASIS OF PRESENTATION TEP's Condensed Consolidated Financial Statements and disclosures are presented in accordance with Generally Accepted Accounting Principles (GAAP) in the United States of America, including specific accounting guidance for regulated operations and the Securities and Exchange Commission's (SEC) interim reporting requirements. The Condensed Consolidated Financial Statements include the accounts of TEP and its subsidiaries. In the consolidation process, accounts of the parent and subsidiaries are combined and intercompany balances and transactions are eliminated. TEP jointly owns several generation and transmission facilities with both affiliated and non-affiliated entities. TEP's proportionate share of jointly-owned facilities is recorded in Utility Plant on the Condensed Consolidated Balance Sheets, and its proportionate share of the operating costs associated with these facilities is included in the Condensed Consolidated Statements of Income. These Condensed Consolidated Financial Statements exclude some information and footnotes required by GAAP and the SEC for annual financial statement reporting and should be read in conjunction with the Consolidated Financial Statements and footnotes in TEP's 2017 Annual Report on Form 10-K/A. The Condensed Consolidated Financial Statements are unaudited, but, in management's opinion, include all normal, recurring adjustments necessary for a fair statement of the results for the interim periods presented. Because weather and other factors cause seasonal fluctuations in sales, TEP's quarterly operating results are not indicative of annual operating results.
Variable interest entities	Variable Interest Entities TEP regularly reviews contracts to determine if it has a variable interest in an entity, if that entity is a Variable Interest Entity (VIE), and if it is the primary beneficiary of the VIE. The primary beneficiary is required to consolidate the VIE when the variable interest holder has: (i) the power to direct activities that most significantly impact the economic performance of the VIE; and (ii) the obligation to absorb losses or the right to receive benefits that could potentially be significant to the VIE. TEP routinely enters into long-term renewable Power Purchase Agreements (PPA) with various entities. Some of these entities are VIEs due to the long-term fixed price component in the agreements. These PPAs effectively transfer commodity price risk to TEP, the buyer of the power, creating a variable interest. TEP has determined it is not a primary beneficiary of the VIEs as it lacks the power to direct the activities that most significantly impact the economic performance of the VIEs. TEP reconsiders whether it is a primary beneficiary of the VIEs on a quarterly basis. As of September 30, 2018, the carrying amount of assets and liabilities in the balance sheet that relates to variable interests under long-term PPAs is predominantly related to working capital accounts and generally represents the amounts owed by TEP for the deliveries associated with the current billing cycle. TEP's maximum exposure to loss is limited to the cost of replacing the power if the providers do not meet the production guarantee. However, the exposure to loss is mitigated as the Company would likely recover these costs through cost recovery mechanisms. See Note 2 for additional information related to cost recovery mechanisms.
Asset retirement obligation	ASSET RETIREMENT OBLIGATIONS TEP records the fair value of a liability for a legal obligation to retire a long-lived tangible asset in the period in which the liability is incurred. The Asset Retirement Obligation (ARO) accrual is primarily related to generation and photovoltaic assets and is included in Regulatory and Other Liabilities—Other on the Condensed Consolidated Balance Sheets.
Revenue from contract with customer	TEP earns the majority of its revenues from the sale of power to retail and wholesale customers based on regulator-approved tariff rates. Most of the Company's contracts have a single performance obligation, the delivery of power. TEP satisfies the performance obligation over time as power is delivered and control is transferred to the customer. The Company bills for power sales based on the reading of electric meters on a systematic basis throughout the month. In general, TEP's contracts have payment terms of 10 to 20 days from the date the bill is rendered. TEP considers any payment not received by the due date delinquent and charges the customer a late payment fee. No component of the transaction price is allocated to unsatisfied performance obligations. TEP has certain contracts with variable transaction pricing that require it to estimate the resulting variable consideration. TEP's variable consideration includes revenues that are subject to refund and performance incentive revenues. TEP estimates variable consideration at the most likely amount to which the Company expects to be entitled and recognizes a refund liability until TEP is certain that the Company will be entitled to the consideration. The Company includes estimated amounts of variable consideration in the transaction price to the extent it is probable that changes in its estimate will not result in significant reversals of revenue in subsequent periods.
Recently adopted new accounting pronouncements	RECENTLY ADOPTED ACCOUNTING PRONOUNCEMENTS Revenue from Contracts with Customers Effective January 1, 2018, TEP adopted accounting guidance that requires recognition of revenue when a customer obtains control of promised goods or services in an amount that reflects the consideration to which the company expects to be entitled. The Company continues to recognize revenue for tariff-based sales to retail and wholesale customers, which represent TEP’s primary source of revenue, as power is delivered. TEP adopted the new guidance using the modified retrospective approach. There was no adjustment identified or recorded to the opening balance of retained earnings on adoption. The Company applied the new revenue guidance to contracts with customers that were not completed at the date of initial application, January 1, 2018. The new guidance requires disclosure of the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. See Note 3 for additional disclosure related to TEP's operating revenues. Compensation—Retirement Benefits Effective January 1, 2018, TEP adopted accounting guidance that requires an employer to disaggregate the service cost component from the other components of net periodic benefit cost. TEP no longer capitalizes the non-service cost components of net periodic benefit cost as part of inventory or plant in service and presents non-service costs in Other, Net on the Condensed Consolidated Statements of Income. The adoption of this change in accounting principle did not have a material impact on TEP's financial position or results of operations. Derivatives and Hedging Effective January 1, 2018, TEP early adopted accounting guidance that simplifies the application of hedge accounting through changes to both the designation and measurement guidance and is intended to enable the Company to better portray the economics of its risk management activities in its financial statements. The adoption of this change in accounting principle did not have a material impact on TEP's financial statements and disclosures. Reclassification of Certain Tax Effects Effective January 1, 2018, TEP early adopted accounting guidance that permits reclassification of certain tax effects resulting from the TCJA from accumulated other comprehensive income to retained earnings. TEP applied the guidance as of the beginning of the period of adoption. On adoption, TEP recorded a one-time reclassification of $1 million from Accumulated Other Comprehensive Loss to Retained Earnings on the Condensed Consolidated Balance Sheets as a result of income tax effects due to the reduction in the U.S. federal statutory tax rate. See Note 11 for additional disclosure related to the TCJA. LEASES In February 2016, the FASB issued an ASU that will require the recognition of leased assets and liabilities by lessees for those leases classified as operating leases with a lease term greater than 12 months and disclosure of additional quantitative and qualitative information about leasing arrangements. The standard is effective for periods beginning January 1, 2019, and is to be applied using a modified retrospective approach or an optional transition method with practical expedient options. The modified retrospective approach requires the restatement of prior periods and presentation of lease disclosures under the new guidance for all periods presented. The optional transition method allows entities to continue to apply the current lease guidance in the comparative periods presented in the year of adoption and apply the transition provisions of the new guidance on the effective date of the new guidance. Early adoption is permitted. In January 2018, the FASB issued an additional optional practical expedient that permits entities to not evaluate existing land easements that were not previously accounted for as leases. TEP plans to adopt this guidance effective January 1, 2019 and will apply the transition provisions of the new standard at the adoption date. The Company expects to elect a package of practical expedients that allows it to not reassess whether any expired or existing contract is a lease or contains a lease, the lease classification of any expired or existing leases, and the initial direct costs for any existing leases. In addition, TEP will elect the practical expedient related to land easements, and the new lease guidance will be applied on a prospective basis to all new or modified land easements. TEP’s leasing activities accounted for as operating leases primarily relate to leases for land and rail cars. The adoption of the new guidance will affect the Company’s Condensed Consolidated Balance Sheets as the Company will be required to record lease assets and lease liabilities related to these operating lease arrangements. TEP is currently identifying and implementing changes to processes and controls and preparing the expanded lease disclosures. The Company does not expect the new ASU to materially affect its balance sheet, results of operations or cash flows. Management continues to monitor standard-setting activities that may affect the transition requirements of the new lease standard. INTERNAL-USE SOFTWARE In August 2018, the FASB issued an ASU that will clarify accounting for implementation costs incurred in a cloud computing arrangement that is a service contract. Under the new guidance, customers will apply the same criteria for capitalizing implementation costs as they would for an arrangement that has a software license. The ASU also provides specific requirements for the classification and presentation of the capitalized implementation costs and the related amortization of those costs. The standard is effective for periods beginning January 1, 2020, and should be applied either retrospectively or prospectively after the date of adoption. Early adoption is permitted. TEP plans to early adopt the new standard prospectively effective January 1, 2019 and does not expect the ASU to have a material impact on its financial statements and disclosures.