Income Taxes	3 Months Ended
Mar. 31, 2017
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Income Taxes	Income Taxes The Company files income tax returns in the United States ("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, Arizona and New Mexico jurisdictions for years prior to 2012. The Company is currently under audit in Texas for tax years 2007 through 2011. For the three months ended March 31, 2017 and 2016, the Company’s effective tax rate was 36.5% and 35.2%, respectively. For the twelve months ended March 31, 2017 and 2016, the Company's effective tax rate was 35.7% and 29.7%, respectively. The Company's effective tax rate for all periods differs from the federal statutory tax rate of 35.0% primarily due to capital gains in the decommissioning trusts which are taxed at the federal rate of 20.0%, the allowance for equity funds used during construction ("AEFUDC"), state taxes and the issue discussed in the following paragraph. In the third quarter of 2016, the Company changed its accounting for state income taxes from the flow-through method to the normalization method in accordance with the PUCT's and NMPRC's most recent final orders. Under the flow-through method, the Company previously recorded deferred state income taxes and regulatory liabilities and assets offsetting such deferred state income taxes at the expected cash flow to be reflected in future rates. Upon implementation of normalization, the Company began amortizing the net regulatory asset for deferred state income taxes to deferred income tax expense over a 15 year period as allowed by the regulators. In the third quarter of 2016, the Company began recording deferred state income tax expense as required by normalization, retroactive to January 2016 as provided in the final orders. The impact of the change was additional deferred income tax expense of $1.0 million and $5.1 million for the three and twelve months ended March 31, 2017, respectively.

Income Taxes

3 Months Ended

Mar. 31, 2017

Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]

The Company files income tax returns in the United States ("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, Arizona and New Mexico jurisdictions for years prior to 2012. The Company is currently under audit in Texas for tax years 2007 through 2011.

For the three months ended March 31, 2017 and 2016, the Company’s effective tax rate was 36.5% and 35.2%, respectively. For the twelve months ended March 31, 2017 and 2016, the Company's effective tax rate was 35.7% and 29.7%, respectively. The Company's effective tax rate for all periods differs from the federal statutory tax rate of 35.0% primarily due to capital gains in the decommissioning trusts which are taxed at the federal rate of 20.0%, the allowance for equity funds used during construction ("AEFUDC"), state taxes and the issue discussed in the following paragraph.

In the third quarter of 2016, the Company changed its accounting for state income taxes from the flow-through method to the normalization method in accordance with the PUCT's and NMPRC's most recent final orders. Under the flow-through method, the Company previously recorded deferred state income taxes and regulatory liabilities and assets offsetting such deferred state income taxes at the expected cash flow to be reflected in future rates. Upon implementation of normalization, the Company began amortizing the net regulatory asset for deferred state income taxes to deferred income tax expense over a 15 year period as allowed by the regulators. In the third quarter of 2016, the Company began recording deferred state income tax expense as required by normalization, retroactive to January 2016 as provided in the final orders. The impact of the change was additional deferred income tax expense of $1.0 million and $5.1 million for the three and twelve months ended March 31, 2017, respectively.