Income Taxes	9 Months Ended
Sep. 30, 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 September 30, 2018 and 2017, the Company’s effective tax rate was 23.7% and 35.3%, respectively. For the nine months ended September 30, 2018 and 2017, the Company’s effective tax rate was 23.4% and 35.3%, respectively. For the twelve months ended September 30, 2018 and 2017, the Company's effective tax rate was 22.8% and 35.0%, respectively. The federal statutory tax rate is 21% in 2018 and 35% in 2017. The Company's effective tax rates for all periods in 2018 and 2017 differ from the federal statutory tax rate primarily due to capital gains in the decommissioning trusts which are taxed at the federal rate of 20%, the tax benefit of stock incentive plans, the allowance for equity funds used during construction ("AEFUDC"), and state taxes. The results for the three, nine and twelve months ended September 30, 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 September 30, 2018 and 2017, the Company’s effective tax rate was 23.7% and 35.3%, respectively. For the nine months ended September 30, 2018 and 2017, the Company’s effective tax rate was 23.4% and 35.3%, respectively. For the twelve months ended September 30, 2018 and 2017, the Company's effective tax rate was 22.8% and 35.0%, respectively. The federal statutory tax rate is 21% in 2018 and 35% in 2017. The Company's effective tax rates for all periods in 2018 and 2017 differ from the federal statutory tax rate primarily due to capital gains in the decommissioning trusts which are taxed at the federal rate of 20%, the tax benefit of stock incentive plans, the allowance for equity funds used during construction ("AEFUDC"), and state taxes.

The results for the three, nine and twelve months ended September 30, 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.