Commitments, Contingencies And Uncertainties	9 Months Ended
Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments, Contingencies And Uncertainties	Commitments, Contingencies and Uncertainties For a full discussion of commitments and contingencies, see Note K of the Notes to Financial Statements in the 2017 Form 10-K. In addition, see Notes D and E above and Notes C and E of the Notes to Financial Statements in the 2017 Form 10-K regarding matters related to wholesale power sales contracts and transmission contracts subject to regulation and Palo Verde, including decommissioning, spent nuclear fuel and waste disposal, and liability and insurance matters. Power Purchase and Sale Contracts To supplement its own generation and operating reserve requirements and to meet required renewable portfolio standards, the Company engages in power purchase arrangements that may vary in duration and amount based on an evaluation of the Company's resource needs, the economics of the transactions and specific renewable portfolio requirements. For a discussion of power purchase and sale contracts that the Company has entered into with various counterparties, see Note K of the Notes to Financial Statements in the 2017 Form 10-K. Environmental Matters General. The Company is subject to extensive laws, regulations and permit requirements with respect to air and greenhouse gas emissions, water discharges, soil and water quality, waste management and disposal, natural resources and other environmental matters by federal, state, regional, tribal and local authorities. Failure to comply with such laws, regulations and requirements can result in actions by authorities or other third parties that might seek to impose on the Company administrative, civil and/or criminal penalties or other sanctions. In addition, releases of pollutants or contaminants into the environment can result in costly cleanup liabilities. These laws, regulations and requirements are subject to change through modification or reinterpretation, or the introduction of new laws and regulations, and as a result, the Company may face additional capital and operating costs to comply. National Ambient Air Quality Standards ("NAAQS"). Under the Clean Air Act ("CAA"), the U.S Environmental Protection Agency ("EPA") sets NAAQS for six criteria pollutants considered harmful to public health and the environment, including particulate matter, nitrogen oxide, carbon monoxide, ozone and sulfur dioxide. On October 1, 2015, the EPA released a final rule tightening the primary and secondary NAAQS for ground-level ozone from its 2008 standard levels of 75 parts per billion ("ppb") to 70 ppb. The EPA published the Final Rule on June 4, 2018, designating El Paso County, Texas, as “attainment/unclassifiable” under the 2015 ozone NAAQS and designating a section of southern Dona Ana County, New Mexico as “nonattainment.” In August, several petitions for review of the Final Rule were filed in the U.S. Court of Appeals for the D.C. Circuit. One of these petitions, filed by the City of Sunland Park, New Mexico, specifically challenges the “attainment/unclassifiable” designation of El Paso County, Texas. The Company and other intervenors filed and were granted motions to intervene in the challenges to EPA’s 2015 ozone NAAQS designations. States, including New Mexico, that contain any areas designated as nonattainment are required to complete development of implementation plans in the 2020-2021 timeframe. Most nonattainment areas are expected to have until 2020 or 2023 to meet the primary (health) standard, with the exact attainment date varying based on the ozone level in the area. The Company continues to evaluate what impact these final and proposed NAAQS could have on its operations. If the Company is required to install additional equipment to control emissions at its facilities, the NAAQS, individually or in the aggregate, could have a material impact on its operations and financial results. Climate Change. The U.S. federal government has either considered, proposed and/or finalized legislation or regulations limiting greenhouse gas ("GHG") emissions, including carbon dioxide. In particular, the U.S. Congress has considered legislation to restrict or regulate GHG emissions. In October 2015, the EPA published a rule establishing guidelines for states to regulate CO2 emissions from existing power plants, known as the Clean Power Plan ("CPP"). Legal challenges to the CPP are ongoing. On August 31, 2018, the EPA published a proposal to replace the CPP called the Affordable Clean Energy ("ACE") rule. At this time the Company cannot determine the impact that the CPP, the ACE rule, and related proposals and legal challenges may have on our financial position, results of operations or cash flows.

Commitments, Contingencies and Uncertainties

For a full discussion of commitments and contingencies, see Note K of the Notes to Financial Statements in the 2017 Form 10-K. In addition, see Notes D and E above and Notes C and E of the Notes to Financial Statements in the 2017 Form 10-K regarding matters related to wholesale power sales contracts and transmission contracts subject to regulation and Palo Verde, including decommissioning, spent nuclear fuel and waste disposal, and liability and insurance matters.

Power Purchase and Sale Contracts

To supplement its own generation and operating reserve requirements and to meet required renewable portfolio standards, the Company engages in power purchase arrangements that may vary in duration and amount based on an evaluation of the Company's resource needs, the economics of the transactions and specific renewable portfolio requirements. For a discussion of power purchase and sale contracts that the Company has entered into with various counterparties, see Note K of the Notes to Financial Statements in the 2017 Form 10-K.

Environmental Matters

General. The Company is subject to extensive laws, regulations and permit requirements with respect to air and greenhouse gas emissions, water discharges, soil and water quality, waste management and disposal, natural resources and other environmental matters by federal, state, regional, tribal and local authorities. Failure to comply with such laws, regulations and requirements can result in actions by authorities or other third parties that might seek to impose on the Company administrative, civil and/or criminal penalties or other sanctions. In addition, releases of pollutants or contaminants into the environment can result in costly cleanup liabilities. These laws, regulations and requirements are subject to change through modification or reinterpretation, or the introduction of new laws and regulations, and as a result, the Company may face additional capital and operating costs to comply.

National Ambient Air Quality Standards ("NAAQS"). Under the Clean Air Act ("CAA"), the U.S Environmental Protection Agency ("EPA") sets NAAQS for six criteria pollutants considered harmful to public health and the environment, including particulate matter, nitrogen oxide, carbon monoxide, ozone and sulfur dioxide. On October 1, 2015, the EPA released a final rule tightening the primary and secondary NAAQS for ground-level ozone from its 2008 standard levels of 75 parts per billion ("ppb") to 70 ppb. The EPA published the Final Rule on June 4, 2018, designating El Paso County, Texas, as “attainment/unclassifiable” under the 2015 ozone NAAQS and designating a section of southern Dona Ana County, New Mexico as “nonattainment.” In August, several petitions for review of the Final Rule were filed in the U.S. Court of Appeals for the D.C. Circuit. One of these petitions, filed by the City of Sunland Park, New Mexico, specifically challenges the “attainment/unclassifiable” designation of El Paso County, Texas. The Company and other intervenors filed and were granted motions to intervene in the challenges to EPA’s 2015 ozone NAAQS designations.

States, including New Mexico, that contain any areas designated as nonattainment are required to complete development of implementation plans in the 2020-2021 timeframe. Most nonattainment areas are expected to have until 2020 or 2023 to meet the primary (health) standard, with the exact attainment date varying based on the ozone level in the area. The Company continues to evaluate what impact these final and proposed NAAQS could have on its operations. If the Company is required to install additional equipment to control emissions at its facilities, the NAAQS, individually or in the aggregate, could have a material impact on its operations and financial results.

Climate Change. The U.S. federal government has either considered, proposed and/or finalized legislation or regulations limiting greenhouse gas ("GHG") emissions, including carbon dioxide. In particular, the U.S. Congress has considered legislation to restrict or regulate GHG emissions. In October 2015, the EPA published a rule establishing guidelines for states to regulate CO2 emissions from existing power plants, known as the Clean Power Plan ("CPP"). Legal challenges to the CPP are ongoing. On August 31, 2018, the EPA published a proposal to replace the CPP called the Affordable Clean Energy ("ACE") rule. At this time the Company cannot determine the impact that the CPP, the ACE rule, and related proposals and legal challenges may have on our financial position, results of operations or cash flows.