Principles Of Preparation (Policy)	9 Months Ended
Principles Of Preparation (Policy)	Sep. 30, 2018
Accounting Policies [Abstract]
Reclassification, Policy	Reclassification. Certain amounts in the financial statements for 2017 have been reclassified to conform with the 2018 presentation. The Company implemented Accounting Standards Update ("ASU") 2017-07, Compensation - Retirement Benefits, and ASU 2016-15, Statement of Cash Flows, in the first quarter of 2018, retrospective to all periods presented in the Company's financial statements. See "New Accounting Standards Adopted" below for further details.
Use of Estimates, Policy	Use of Estimates. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. The Company evaluates its estimates on an on-going basis, including those related to depreciation, unbilled revenue, income taxes, fuel costs, pension and other post-retirement obligations and asset retirement obligations ("AROs"). Actual results could differ from those estimates.
Revenue Recognition, Policy	Revenues. The Company recognizes revenue associated with contracts with customers when performance obligations under the terms of the contract with the customer are satisfied. Revenue is measured as the amount of consideration the Company receives in exchange for transferring goods or providing services to the customer. Taxes collected concurrently with revenue producing activities are excluded from revenue. Unbilled revenues are recorded for estimated amounts of energy delivered in the period following the customer's last billing cycle to the end of the reporting period. Unbilled revenues are estimated based on monthly generation volumes and by applying an average revenue/kilowatt-hour ("kWh") to the number of estimated kWhs delivered but not billed. Accounts receivable included accrued unbilled revenues of $31.0 million at September 30, 2018 and $22.2 million at December 31, 2017. The Company presents revenues net of sales taxes in its statements of operations. The Company recognizes revenue when performance obligations under the terms of the contract with the customer are satisfied. Revenue is measured as the amount of consideration the Company receives in exchange for transferring goods or providing services to the customer. Taxes collected concurrently with revenue producing activities are excluded from revenue. The Company has elected the optional invoice practical expedient for Wholesale and Wheeling revenues, as the invoice amount will correspond directly to the value provided by the Company's performance to date. Retail. Retail contracts represent the Company's primary revenue source. The Company has determined that retail electric service to residential, commercial and industrial, and public authority customers represents an implied daily contract with the customer. The contract is comprised of an obligation to supply and distribute electricity and related capacity. Revenue is recognized, over time, equal to the product of the applicable tariff rates, as approved by the Public Utility Commission of Texas (the "PUCT") and the New Mexico Public Regulation Commission, (the "NMPRC"), and the volume of the electricity delivered to the customer, or through the passage of time based upon providing the service of standing ready. Unbilled revenues are recognized at month end based on estimated monthly generation volumes and by applying an average revenue per kWh to the number of estimated kWhs delivered but not billed to customers, and recorded as a receivable for the period following the last billing cycle to the end of the reporting period. Retail customers receive a bill monthly, with payment due sixteen days after issuance. Wholesale. Wholesale contracts primarily include forward power sales into markets outside the Company’s service territory when the Company has competitive generation capacity available, after meeting its regulated service obligations. Pricing is either fixed or based on an index rate with consideration potentially including variable components. Uncertainties regarding the variable consideration will be resolved when the transaction price is known at the point of delivering the energy. The obligation to deliver the electricity is satisfied over time as the customer receives and consumes the electricity. Wholesale customers are invoiced on the 10^th day of each month, with payment due by the 20^th day of the month. In the case of the sale of renewable energy certificates, the transaction price is allocated to the performance obligation to deliver the confirmed quantity of the certificates based on the stand alone selling price of each certificate. Revenue is recognized as control of the certificates is transferred to the customer. The customer is invoiced upon the completed transfer of the certificates, with payment due within ten business days. Wholesale also includes an annual agreement between the Company and one of its wholesale customers, Rio Grande Electric Cooperative ("RGEC"), which involves the provision of full requirements electric service from the Company to RGEC. The rates for this service are recalculated annually and require Federal Energy Regulatory Commission ("FERC") approval. Wheeling (transmission). Wheeling involves the Company providing point-to-point transmission service, which includes the receipt of capacity and energy at designated point(s) and the transfer of such capacity and energy to designated point(s) of delivery on either a firm or non-firm basis for periods of one year or less. The performance obligation to provide capacity and transmit energy is satisfied over time as the Company performs. Transmission customers are invoiced on a monthly basis, with payment due within twenty days of receipt of the invoice.
Depreciation, Depletion, and Amortization, Policy	Depreciation. The Company routinely evaluates the depreciable service lives, cost of removal and salvage values of its property, plant and equipment. Depreciation is provided on a straight-line basis over the estimated remaining lives of the assets (ranging in average from 5 to 48 years). When property subject to composite depreciation is retired or otherwise disposed of in the normal course of business, its cost together with the cost of removal, less salvage is charged to accumulated depreciation. For other property dispositions, the applicable cost and accumulated depreciation is removed from the balance sheet accounts and a gain or loss is recognized.
New Accounting Standards, Policy	New Accounting Standards Adopted In March 2016, the Financial Accounting Standards Board ("FASB") issued ASU 2016-09, Compensation - Stock Compensation (Topic 718) Improvements to Employee Share-Based Payment Accounting, to simplify the accounting for share-based payment transactions, including the income tax consequences, classification of awards either as equity or liabilities, and classification on the statements of cash flows. The Company adopted the new standard effective January 1, 2017. The adoption of the new standard did not have a material impact on the Company’s financial condition, results of operations or cash flows. The cumulative effect of the adoption of the new standard was to increase net operating loss carryforward ("NOL carryforward") deferred tax assets and retained earnings by $0.2 million on January 1, 2017. In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), to provide a framework that replaces the existing revenue recognition guidance, and has since modified the standard with several ASUs. The standard provides that an entity should recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. On January 1, 2018, the Company adopted the new accounting standard using the modified retrospective method. There was no cumulative effect adjustment at the initial application of the new standard. In addition, comparative information has not been restated and continues to be reported under the accounting standards in effect for those periods. The Company expects the ongoing impact of the new standard to be immaterial to net income. As required by the standard, revenues of $5.9 million related to reimbursed costs of energy efficiency programs approved by the Company's regulators are reported in year-to-date operating revenues from customers prospectively, as opposed to being offset with associated costs within operations and maintenance. Related expenses of an equal amount are reported in operations and maintenance expenses. See Note B, Revenues, for additional information. In January 2016, the FASB issued ASU 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Liabilities, to enhance the reporting model for financial instruments by addressing certain aspects of recognition, measurement, presentation and disclosure. The Company adopted the new standard effective January 1, 2018. The adoption of ASU 2016-01 eliminates the requirements to classify investments in equity securities with readily determinable fair values into trading or available for sale and requires entities to measure equity investments at fair value and recognize any changes in fair value in the Statements of Operations. ASU 2016-01 requires a modified retrospective approach and therefore comparative information has not been restated and continues to be reported under the accounting standards in effect for those periods. Upon adoption of the new standard, the Company recorded a cumulative effect adjustment, net of income taxes, to increase retained earnings by $41.0 million with an offset to accumulated other comprehensive income ("AOCI"). In addition, the Company recorded net gains of $3.7 million related to equity securities still held at September 30, 2018. In March 2018, the FASB issued ASU 2018-04, Investments - Debt Securities (Topic 320) and Regulated Operations (Topic 980), which provides clarification to ASU 2016-01. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230) Classification of Certain Cash Receipts and Cash Payments, to reduce diversity in practice in how certain cash receipts and cash payments are classified in the statement of cash flows. The Company adopted the new standard effective January 1, 2018. ASU 2016-15 was applied using a retrospective transition method to each period presented. Accordingly, the Company presented in the Statement of Cash Flows insurance proceeds received for equipment of $5.4 million and $8.1 million, respectively, for the nine months ended September 30, 2018 and 2017 as cash inflows from investing activities. In March 2017, the FASB issued ASU 2017-07, Compensation-Retirement Benefits (Topic 715) Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. ASU 2017-07 amends Accounting Standards Codification ("ASC") 715, Compensation - Retirement Benefits, to require companies to present the service cost component of net benefit cost in the income statement line items where compensation cost is reported. Companies will present all other components of net benefit cost separately from the line item(s) that includes the service cost and outside of any subtotal of operating income. In addition, only the service cost component will be eligible for capitalization in assets. The Company adopted the new standard effective January 1, 2018. The amendments in ASU 2017-07 were applied retrospectively for the income statement presentation of the service cost component and the other components of net benefit costs. The Company elected to apply the practical expedient and used the amounts disclosed in its pension and other postretirement benefit plan note for the 2017 comparative period as the estimation basis for applying the retrospective presentation requirements. See Note J, Employee Benefits, for additional information. In March 2018, the FASB issued ASU 2018-05, Income Taxes (Topic 740) Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 ("SAB 118"), to add various SEC paragraphs for clarification due to the federal legislation commonly referred to as the Tax Cuts and Jobs Act of 2017 ("TCJA"). The Company adopted ASU 2018-05 upon issuance and implemented SAB 118 in December of 2017 in conjunction with the enactment of the TCJA. In August 2018, the FASB issued ASU 2018-15, Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract (Topic 350). ASU 2018-15 aligns the requirements for capitalizing implementation costs for a cloud computing arrangement with the requirements for capitalizing implementation costs for an internal use software license. Implementation costs for a cloud computing arrangement will be capitalized or expensed based on the nature of the costs and the project’s stage in which they are incurred by applying the existing guidance for internal use software implementation costs. Capitalized costs for a cloud computing arrangement will be presented on the same line of the balance sheet as any related prepaid amounts for the arrangement, while amortization of those costs will be presented on the same line of the income statement as the related hosting fees. Early adoption is permitted, and entities may apply the guidance either prospectively to eligible costs incurred on or after the effective date or retrospectively. The Company early-adopted this guidance in the third quarter of 2018, on a prospective basis, and the adoption did not have a material impact on the Company’s financial condition, results of operations or cash flows. New Accounting Standards to be Adopted in the Future In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), to increase transparency and comparability among organizations by recognizing lease assets and lease liabilities on the balance sheet and requiring qualitative and quantitative disclosures on leasing agreements. ASU 2016-02 maintains a distinction between finance leases and operating leases similar to the distinction under previous lease guidance for capital leases and operating leases. The impact of leases reported in the Company's operating results and statement of cash flows is expected to be similar to previous GAAP. ASU 2016-02 requires the recognition in the statement of financial position, by the lessee, of a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. How leases are recorded in regard to financial position represents a significant change from previous GAAP guidance. The lessee is permitted to make an accounting policy election to not recognize lease assets and lease liabilities for short-term leases. Adoption of the new lease accounting standard will require the Company to apply the new standard to the earliest period using a modified retrospective approach. The Company continues to monitor activities of the FASB, including the impact of ASU 2018-01, Land Easement Practical expedient for Transition to Topic 842, ASU 2018-11, Targeted Improvements, and the proposed update related to narrow scope improvements for lessors. ASU 2018-11 allows entities to adopt the standard with a cumulative effect adjustment as of the beginning of the adoption year, while maintaining prior year comparative financial information and disclosures as reported. ASU 2018-01 provides an optional practical expedient to not evaluate existing or expired land easements under Topic 842, if those land easements were not previously accounted for as leases under ASC Topic 840. The Company currently anticipates that it will apply the practical expedient under ASU 2018-01 to its existing or expired land easements as part of its transition to Topic 842. The Company's implementation process is continuing and includes finalizing the following activities: the analysis of the Company's lease population, the implementation of a new lease accounting system, the determination of the transition method and practical expedients that will be applied, and evaluating the impact, if any, on changes to business processes, systems and controls to support recognition and disclosure under the new guidance. The Company will adopt this guidance effective January 1, 2019. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326). ASU 2016-13 changes how companies measure and recognize credit impairment for many financial assets. The new expected credit loss model will require companies to immediately recognize an estimate of credit losses expected to occur over the remaining life of the financial assets that are in the scope of the standard. The ASU also makes targeted amendments to the current impairment model for available-for-sale debt securities. ASU 2016-13 will be required for reporting periods beginning after December 15, 2019. ASU 2016-13 will be applied in a modified retrospective approach through a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is implemented. The Company is currently assessing the future impact of ASU 2016-13. In February 2018, the FASB issued ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220), as a result of concerns raised due to the TCJA. More specifically, because the remeasurement of deferred taxes due to the change in the federal corporate income tax rate is required to be included in income from continuing operations, the tax effects of items within AOCI (referred to as stranded tax effects) do not reflect the appropriate tax rate. ASU 2018-02 generally allows companies to reclassify stranded taxes from AOCI to retained earnings. The amount of the adjustment would be the difference between the historical federal corporate income tax rate of 35% and the newly enacted 21% federal corporate income tax rate. The provisions of ASU 2018-02 are effective for fiscal years and interim periods within that reporting period beginning after December 15, 2018. Early adoption is permitted, including adoption in any interim periods for reporting periods for which financial statements have not been issued. The Company is currently evaluating the impact of ASU 2018-02 and its impact on regulated utilities. At September 30, 2018, stranded taxes in AOCI are approximately $7.2 million.