Revenue Recognition (Policies)	12 Months Ended
Revenue Recognition (Policies)	Dec. 31, 2018
Revenue Recognition [Abstract]
Revenue Recognition, Policy	Operating Revenues. The Company accrues revenues for services rendered, including unbilled electric service revenues. Fuel and purchase power expenses are stated at actual cost incurred. 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 $21.6 million and $22.2 million as of December 31, 2018 and 2017, respectively. The Company presents revenues net of sales taxes in its statements of operations. The Company’s Texas retail customers are billed under base rates and a fixed fuel factor approved by the Public Utility Commission of Texas ("PUCT"). The Company’s New Mexico retail customers are billed under base rates and a fuel adjustment clause that is adjusted monthly, as approved by the New Mexico Public Regulation Commission ("NMPRC"). The Company's FERC sales for resale customers are billed under formula base rates and fuel factors and a fuel adjustment clause that is adjusted monthly. The Company’s recovery of fuel and purchased power expenses is subject to periodic reconciliations of actual fuel and purchased power expenses incurred to actual fuel revenues collected. The difference between fuel and purchased power expenses incurred and fuel revenues charged to customers is reflected as over/under-collection of fuel revenues, which is included in regulatory liabilities/assets - current in the balance sheets. See Part II, Item 8, Financial Statements and Supplementary Data, Note D of Notes to Financial Statements for further discussion. 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 PUCT and 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 monthly 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 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.
Revenue Recognition for Alternative Revenue Programs, Policy	alternative revenue program revenue relating to the Company’s potential bonus awards from the PUCT and the NMPRC mandated energy efficiency programs. Both the PUCT and the NMPRC allow for the potential to earn an incentive bonus if the Company achieves its approved energy efficiency goals under the applicable programs. The Company recognizes revenue related to the energy efficiency program incentives at the point in time that the amount is objectively determinable generally based upon an approved order from the regulator, is probable of recovery, and if it is expected to be collected within 24 months.