Income Taxes	3 Months Ended
Mar. 31, 2018
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Income Taxes	Income Taxes The Company files income tax returns in the U.S. federal jurisdiction and in the states of Texas, New Mexico and Arizona. The Company is no longer subject to tax examination by the taxing authorities in the federal, Texas, Arizona, and New Mexico jurisdictions for years prior to 2013. For the three months ended March 31, 2018 and 2017, the Company’s effective tax rate was 30.5% and 36.5%, respectively. For the twelve months ended March 31, 2018 and 2017, the Company's effective tax rate was 34.5% and 35.7%, respectively. The federal statutory tax rate is 21% in 2018 and 35% in 2017. The Company's effective tax rate for the three months ended March 31, 2018, differs from the federal statutory tax rate primarily due to the tax benefit of stock incentive plans and other permanent differences that increase the effective tax rate when the Company incurs a net loss. The Company's effective tax rates for the three months ended March 31, 2017, and the twelve months ended March 31, 2018 and 2017, differs from the federal statutory tax rate primarily due to state income taxes offset by capital gains in the decommissioning trusts, which are taxed at the federal income tax rate of 20%, and the tax benefit of stock incentive plans. The results for the three and twelve months ended March 31, 2018, contain provisional estimates of the impact of the TCJA. These amounts are considered provisional because they use estimates for which tax returns have not yet been filed and because estimated amounts may be impacted by future regulatory and accounting guidance if and when issued. The Company will adjust these provisional amounts as further information becomes available and as we refine our calculations. As permitted by recent guidance issued by the SEC, these adjustments will occur during a reasonable “measurement period” not to exceed twelve months from the date of enactment. In February 2018, the FASB issued ASU 2018-02. The Company is currently evaluating the impact of ASU 2018-02 and its impact on regulated utilities. See Note A, Principles of Preparation - New Accounting Standards to be Adopted in the Future, for additional information.

Income Taxes

The Company files income tax returns in the U.S. federal jurisdiction and in the states of Texas, New Mexico and Arizona. The Company is no longer subject to tax examination by the taxing authorities in the federal, Texas, Arizona, and New Mexico jurisdictions for years prior to 2013.

For the three months ended March 31, 2018 and 2017, the Company’s effective tax rate was 30.5% and 36.5%, respectively. For the twelve months ended March 31, 2018 and 2017, the Company's effective tax rate was 34.5% and 35.7%, respectively. The federal statutory tax rate is 21% in 2018 and 35% in 2017. The Company's effective tax rate for the three months ended March 31, 2018, differs from the federal statutory tax rate primarily due to the tax benefit of stock incentive plans and other permanent differences that increase the effective tax rate when the Company incurs a net loss. The Company's effective tax rates for the three months ended March 31, 2017, and the twelve months ended March 31, 2018 and 2017, differs from the federal statutory tax rate primarily due to state income taxes offset by capital gains in the decommissioning trusts, which are taxed at the federal income tax rate of 20%, and the tax benefit of stock incentive plans.

The results for the three and twelve months ended March 31, 2018, contain provisional estimates of the impact of the TCJA. These amounts are considered provisional because they use estimates for which tax returns have not yet been filed and because estimated amounts may be impacted by future regulatory and accounting guidance if and when issued. The Company will adjust these provisional amounts as further information becomes available and as we refine our calculations. As permitted by recent guidance issued by the SEC, these adjustments will occur during a reasonable “measurement period” not to exceed twelve months from the date of enactment.

In February 2018, the FASB issued ASU 2018-02. The Company is currently evaluating the impact of ASU 2018-02 and its impact on regulated utilities. See Note A, Principles of Preparation - New Accounting Standards to be Adopted in the Future, for additional information.