El Paso Electric 10-Q 4 May 18 2018 Q1 Quarterly report Financial data

Document And Entity Information

Document And Entity Information - shares	3 Months Ended
	Mar. 31, 2018	Apr. 30, 2018
Document and Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Trading Symbol	ee
Entity Registrant Name	EL PASO ELECTRIC CO /TX/
Entity Central Index Key	31,978
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		40,664,754

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Utility plant:
Electric plant in service	$ 4,018,380	$ 3,982,095
Less accumulated depreciation and amortization	(1,332,076)	(1,320,175)
Net plant in service	2,686,304	2,661,920
Construction work in progress	165,608	146,059
Nuclear fuel; includes fuel in process of $69,533 and $59,689, respectively	204,778	194,933
Net nuclear fuel	120,549	120,458
Net utility plant	2,972,461	2,928,437
Current assets:
Cash and cash equivalents	2,574	6,990
Accounts receivable, principally trade, net of allowance for doubtful accounts of $1,820 and $2,300, respectively	76,681	88,585
Inventories, at cost	49,919	50,910
Prepayments and other	10,781	10,307
Total current assets	139,955	156,792
Deferred charges and other assets:
Decommissioning trust funds	284,082	286,866
Regulatory assets	92,619	96,036
Other	16,503	16,232
Total deferred charges and other assets	393,204	399,134
Total assets	3,505,620	3,484,363
Capitalization:
Common stock, stated value $1 per share, 100,000,000 shares authorized, 65,666,420 and 65,694,829 shares issued, and 162,268 and 133,859 restricted shares, respectively	65,829	65,829
Capital in excess of stated value	325,227	326,117
Retained earnings	1,180,114	1,159,667
Accumulated other comprehensive income (loss), net of tax	(32,321)	11,058
Total stockholders' equity subtotal before treasury stock	1,538,849	1,562,671
Treasury stock, 25,166,943 and 25,244,350 shares, respectively, at cost	(419,216)	(420,506)
Common stock equity	1,119,633	1,142,165
Long-term debt, net of current portion	1,196,110	1,195,988
Total capitalization	2,315,743	2,338,153
Current liabilities:
Short-term borrowings under the revolving credit facility	233,067	173,533
Accounts payable, principally trade	35,838	59,270
Taxes accrued	26,478	35,660
Interest accrued	19,060	12,470
Over-collection of fuel revenues	14,190	6,225
Other	35,764	29,067
Total current liabilities	364,397	316,225
Deferred credits and other liabilities:
Accumulated deferred income taxes	298,440	305,023
Accrued pension liability	81,576	83,838
Accrued post-retirement benefit liability	26,957	26,417
Asset retirement obligation	95,098	93,029
Regulatory liabilities	298,035	296,685
Other	25,374	24,993
Total deferred credits and other liabilities	825,480	829,985
Commitments and contingencies
Total capitalization and liabilities	3,505,620	3,484,363
Nuclear Fuel
Utility plant:
Less accumulated depreciation and amortization	$ (84,229)	$ (74,475)

Balance Sheets Parenthetical

Balance Sheets Parenthetical - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Nuclear fuel; fuel in process	$ 69,533	$ 59,689
Allowance for Doubtful Accounts Receivable, Current	$ 1,820	$ 2,300
Common Stock, Par or Stated Value Per Share	$ 1	$ 1
Common Stock, Shares Authorized	100,000,000	100,000,000
Common Stock, Shares Issued	65,666,420	65,694,829
Common stock, Restricted shares	162,268	133,859
Treasury Stock, Shares	25,166,943	25,244,350

Statements Of Operations

Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Operating revenues	$ 175,713	$ 171,335	$ 921,175	$ 900,462
Energy expenses:
Fuel	41,054	36,606	189,517	176,025
Purchased and interchanged power	11,134	13,673	57,143	63,754
Cost of services, energy services	52,188	50,279	246,660	239,779
Operating revenues net of energy expenses	123,525	121,056	674,515	660,683
Other operating expenses:
Operations and maintenance	80,160	79,187	321,254	317,518
Depreciation and amortization	23,814	21,934	92,723	82,958
Taxes other than income taxes	15,507	15,730	70,640	66,451
Utilities operating expense	119,481	116,851	484,617	466,927
Operating income	4,044	4,205	189,898	193,756
Other income (deductions):
Allowance for equity funds used during construction	920	815	3,130	5,502
Investment and interest income, net	5,155	9,263	34,745	35,959
Miscellaneous non-operating income	3,136	2,895	12,292	10,983
Miscellaneous non-operating deductions	(2,743)	(2,828)	(11,494)	(11,513)
Nonoperating income	6,468	10,145	38,673	40,931
Interest charges (credits):
Interest on long-term debt and revolving credit facility	17,988	18,367	72,591	73,312
Other interest	4,654	4,345	18,479	17,155
Capitalized interest	(1,214)	(1,294)	(4,942)	(5,042)
Allowance for borrowed funds used during construction	(898)	(791)	(3,082)	(4,116)
Interest expense	20,530	20,627	83,046	81,309
Income (loss) before income taxes	(10,018)	(6,277)	145,525	153,378
Income tax (benefit) expense	(3,052)	(2,288)	50,240	54,791
Net income (loss)	$ (6,966)	$ (3,989)	$ 95,285	$ 98,587
Basic earnings (loss) per share	$ (0.170)	$ (0.10)	$ 2.35	$ 2.43
Diluted earnings (loss) per share	(0.170)	(0.10)	2.34	2.43
Dividends declared per share of common stock	$ 0.335	$ 0.310	$ 1.340	$ 1.240
Weighted average number of shares outstanding	40,491,194	40,387,235	40,440,189	40,366,024
Weighted average number of shares and dilutive potential shares outstanding	40,491,194	40,387,235	40,563,625	40,435,689

Statements Of Comprehensive Ope

Statements Of Comprehensive Operations - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Net income (loss)	$ (6,966)	$ (3,989)	$ 95,285	$ 98,587
Unrecognized pension and post-retirement benefit costs:
Net gain (loss) arising during period	0	0	12,634	(20,053)
Prior service benefit	0	0	0	32,697
Reclassification adjustments included in net income for amortization of:
Prior service benefit	(2,416)	(2,416)	(9,657)	(8,157)
Net loss	1,575	1,694	6,657	5,436
Net unrealized gains/loss on marketable securities:
Net holding gains (losses) arising during period	(2,708)	7,721	14,846	13,975
Reclassification adjustments for net (gains) losses included in net income	518	(2,191)	(7,917)	(8,443)
Net losses on cash flow hedges:
Reclassification adjustment for interest expense included in net income	139	130	541	506
Total other comprehensive income (loss) before income taxes	(2,892)	4,938	17,104	15,961
Income tax benefit (expense) related to items of other comprehensive income (loss):
Unrecognized pension and post-retirement benefit costs	156	193	(3,652)	(4,124)
Net unrealized (gains) losses on marketable securities	435	(1,121)	(1,366)	(1,054)
Losses on cash flow hedges	(50)	(78)	(195)	(335)
Total income tax benefit (expense)	541	(1,006)	(5,213)	(5,513)
Other comprehensive income (loss), net of tax	(2,351)	3,932	11,891	10,448
Comprehensive income (loss)	$ (9,317)	$ (57)	$ 107,176	$ 109,035

Statements Of Cash Flows

Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net income (loss)	$ (6,966)	$ (3,989)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization of electric plant in service	23,814	21,934
Amortization of Nuclear Fuel	10,404	11,278
Deferred income taxes, net	(3,964)	(3,209)
Allowance for equity funds used during construction	(920)	(815)
Other amortization and accretion	5,240	4,988
Net loss (gain) on decommissioning trust funds	2,509	(2,191)
Other operating activities	81	(165)
Change in:
Accounts receivable	8,063	8,663
Inventories	1,418	(1,638)
Prepayments and other	(2,603)	(2,494)
Accounts payable	(23,324)	(13,766)
Taxes accrued	(7,552)	(4,843)
Interest accrued	6,590	6,097
Net over-collection of fuel revenues	7,965	8,530
Other current liabilities	6,697	648
Deferred charges and credits	(1,216)	(2,896)
Net cash provided by operating activities	26,236	26,132
Cash flows from investing activities:
Cash additions to utility property, plant and equipment	(66,924)	(53,867)
Cash additions to nuclear fuel	(9,257)	(10,873)
Insurance proceeds received for equipment	4,175	742
Capitalized interest and AFUDC:
Utility property, plant and equipment	(1,818)	(1,606)
Nuclear fuel and other	(1,214)	(1,294)
Allowance for equity funds used during construction	920	815
Decommissioning trust funds:
Purchases, including funding of $0.5 million and $1.1 million, respectively	(33,578)	(28,482)
Sales and maturities	31,663	26,055
Other investing activities	526	(236)
Net cash used for investing activities	(75,507)	(68,746)
Cash flows from financing activities:
Dividends paid	(13,615)	(12,565)
Borrowings under the revolving credit facility:
Proceeds	192,670	128,339
Payments	(133,136)	(75,735)
Other financing activities	(1,064)	(679)
Net cash provided by financing activities	44,855	39,360
Net decrease in cash and cash equivalents	(4,416)	(3,254)
Cash and cash equivalents at beginning of period	6,990	8,420
Cash and cash equivalents at end of period	$ 2,574	$ 5,166

Statements Of Cash Flows Parent

Statements Of Cash Flows Parenthetical - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Payments to fund Decommissioning Fund	$ 0.5	$ 1.1

Principles Of Preparation

Principles Of Preparation	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Principles Of Preparation	Principles of Preparation These condensed financial statements should be read in conjunction with the financial statements and notes thereto in the Annual Report of El Paso Electric Company on Form 10-K for the fiscal year ended December 31, 2017 (" 2017 Form 10-K"). Capitalized terms used in this report and not defined herein have the meaning ascribed to such terms in the 2017 Form 10-K. In the opinion of the Company’s management, the accompanying financial statements contain all adjustments necessary to present fairly the financial position of the Company at March 31, 2018 and December 31, 2017 ; the results of its operations and comprehensive operations for the three and twelve months ended March 31, 2018 and 2017 ; and its cash flows for the three months ended March 31, 2018 and 2017 . The results of operations and comprehensive operations for the three months ended March 31, 2018 and the cash flows for the three months ended March 31, 2018 are not necessarily indicative of the results to be expected for the full calendar year. Pursuant to the rules and regulations of the U.S. Securities Exchange Commission ("SEC"), certain financial information has been condensed and certain footnote disclosures have been omitted. Such information and disclosures are normally included in financial statements prepared in accordance with U.S. Generally Accepted Accounting Principles ("GAAP"). Reclassification . Certain amounts in the financial statements for 2017 have been reclassified to conform with the 2018 presentation. The Company implemented Accounting Standards Update ("ASU") 2017-07, Compensation - Retirement Benefits, and ASU 2016-15, Statement of Cash Flows, in the first quarter of 2018, retrospective to all periods presented in the Company's financial statements. See "New Accounting Standards Adopted" below for further details. Use of Estimates . The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. The Company evaluates its estimates on an on-going basis, including those related to depreciation, unbilled revenue, income taxes, fuel costs, pension and other post-retirement obligations and asset retirement obligations ("AROs"). Actual results could differ from those estimates. Revenues . 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 $20.0 million at March 31, 2018 and $22.2 million at December 31, 2017 . The Company presents revenues net of sales taxes in its statements of operations. Depreciation. The Company routinely evaluates the depreciable service lives, cost of removal and salvage values of its property, plant and equipment. Depreciation is provided on a straight-line basis over the estimated remaining lives of the assets (ranging in average from 5 to 48 years). When property subject to composite depreciation is retired or otherwise disposed of in the normal course of business, its cost together with the cost of removal, less salvage is charged to accumulated depreciation. For other property dispositions, the applicable cost and accumulated depreciation is removed from the balance sheet accounts and a gain or loss is recognized. New Accounting Standards Adopted In March 2016, the Financial Accounting Standards Board ("FASB") issued ASU 2016-09, Compensation - Stock Compensation (Topic 718) Improvements to Employee Share-Based Payment Accounting, to simplify the accounting for share-based payment transactions, including the income tax consequences, classification of awards either as equity or liabilities, and classification on the statements of cash flows. The Company adopted the new standard effective January 1, 2017. The adoption of the new standard did not have a material impact on the Company’s financial condition, results of operations or cash flows. The cumulative effect of the adoption of the new standard was to increase net operating loss carryforward deferred tax assets and retained earnings by $0.2 million on January 1, 2017. In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), to provide a framework that replaces the existing revenue recognition guidance, and has since modified the standard with several ASUs. The standard provides that an entity should recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. On January 1, 2018, the Company adopted the new accounting standard using the modified retrospective method. There was no cumulative effect adjustment at the initial application of the new standard. In addition, comparative information has not been restated and continues to be reported under the accounting standards in effect for those periods. The Company expects the ongoing impact of the new standard to be immaterial to net income. As required by the standard, revenues of $1.9 million related to reimbursed costs of energy efficiency programs approved by the Company's regulators are reported in operating revenues from customers prospectively, as opposed to being offset with associated costs within operations and maintenance. Related expenses of an equal amount are reported in operations and maintenance expenses. See Note B, Revenues, for additional information. In January 2016, the FASB issued ASU 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Liabilities, to enhance the reporting model for financial instruments by addressing certain aspects of recognition, measurement, presentation and disclosure. The Company adopted the new standard effective January 1, 2018. The adoption of ASU 2016-01 eliminates the requirements to classify investments in equity securities with readily determinable fair values into trading or available for sale and requires entities to measure equity investments at fair value and recognize any changes in fair value in the Statements of Operations. ASU 2016-01 requires a modified-retrospective approach and therefore comparative information has not been restated and continues to be reported under the accounting standards in effect for those periods. Upon adoption of the new standard, the Company recorded a cumulative effect adjustment, net of income taxes, to increase retained earnings by $41.0 million with an offset to accumulated other comprehensive income ("AOCI"). In addition, the Company recorded net losses of $3.8 million related to equity securities still held at March 31, 2018. In March 2018, the FASB issued ASU 2018-04, Investments - Debt Securities (Topic 320) and Regulated Operations (Topic 980), which provides clarification to ASU 2016-01. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230) Classification of Certain Cash Receipts and Cash Payments, to reduce diversity in practice in how certain cash receipts and cash payments are classified in the statement of cash flows. The Company adopted the new standard effective January 1, 2018. ASU 2016-15 was applied using a retrospective transition method to each period presented. Accordingly, the Company presented in the Statement of Cash Flows insurance proceeds received for equipment of $4.2 million and $0.7 million , respectively, for the three months ended March 31, 2018 and 2017 as cash inflows from investing activities. In March 2017, the FASB issued ASU 2017-07, Compensation-Retirement Benefits (Topic 715) Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. ASU 2017-07 amends Accounting Standards Codification 715, Compensation - Retirement Benefits, to require companies to present the service cost component of net benefit cost in the income statement line items where compensation cost is reported. Companies will present all other components of net benefit cost separately from the line item(s) that includes the service cost and outside of any subtotal of operating income. In addition, only the service cost component will be eligible for capitalization in assets. The Company adopted the new standard effective January 1, 2018. The amendments in ASU 2017-07 were applied retrospectively for the income statement presentation of the service cost component and the other components of net benefit costs. The Company elected to apply the practical expedient and used the amounts disclosed in its pension and other postretirement benefit plan note for the 2017 comparative period as the estimation basis for applying the retrospective presentation requirements. See Note J, Employee Benefits, for additional information. In March 2018, the FASB issued ASU 2018-05, Income Taxes (Topic 740) Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 ("SAB 118"), to add various SEC paragraphs for clarification due to The Federal Tax Cuts and Jobs Act of 2017 ("TCJA"). The Company adopted ASU 2018-05 upon issuance and implemented SAB 118 in December of 2017 in conjunction with the enactment of the TCJA. New Accounting Standards to be Adopted in the Future In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), to increase transparency and comparability among organizations by recognizing lease assets and lease liabilities on the balance sheet and requiring qualitative and quantitative disclosures on leasing agreements. ASU 2016-02 maintains a distinction between finance leases and operating leases similar to the distinction under previous lease guidance for capital leases and operating leases. The impact of leases reported in the Company's operating results and statement of cash flows is expected to be similar to previous GAAP. ASU 2016-02 requires the recognition in the statement of financial position, by the lessee, of a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. How leases are recorded in regard to financial position represents a significant change from previous GAAP guidance. The lessee is permitted to make an accounting policy election to not recognize lease assets and lease liabilities for short-term leases. Implementation of the standard will be required for reporting periods beginning after December 15, 2018. Adoption of the new lease accounting standard will require the Company to apply the new standard to the earliest period using a modified retrospective approach. The Company is currently in the process of evaluating the impact of the new standard, which includes continuing to monitor activities of the FASB, including the impact of the recently issued ASU 2018-01, and the proposed project to allow entities to adopt the standard with a cumulative effect adjustment as of the beginning of the adoption year, while maintaining prior year comparative financial information and disclosures as reported. ASU 2018-01, Land Easement Practical expedient for Transition to Topic 842, provides an optional practical expedient to not evaluate existing or expired land easements under Topic 842, if those land easements were not previously accounted for as leases under Accounting Standards Codification ("ASC") Topic 840. The Company currently anticipates that it will apply the practical expedient under ASU 2018-01 to its existing or expired land easements as part of its transition to Topic 842. The Company's evaluation process also includes evaluating the impact, if any, on changes to business processes, systems and controls to support recognition and disclosure under the new guidance; however, at this time the Company is unable to determine the impact this standard will have on the financial statements and related disclosures. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326). ASU 2016-13 changes how companies measure and recognize credit impairment for many financial assets. The new current expected credit loss model will require companies to immediately recognize an estimate of credit losses expected to occur over the remaining life of the financial assets that are in the scope of the standard. The ASU also makes targeted amendments to the current impairment model for available-for-sale debt securities. ASU 2016-13 will be required for reporting periods beginning after December 15, 2019. ASU 2016-13 will be applied in a modified retrospective approach through a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is implemented. The Company is currently assessing the future impact of ASU 2016-13. In February 2018, the FASB issued ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220), as a result of concerns raised due to the TCJA. More specifically, because the remeasurement of deferred taxes due to the change in the federal corporate income tax rate is required to be included in income from continuing operations, the tax effects of items within AOCI (referred to as stranded tax effects) do not reflect the appropriate tax rate. ASU 2018-02 generally allows companies to reclassify stranded taxes from AOCI to retained earnings. The amount of the adjustment would be the difference between the historical federal corporate income tax rate of 35% and the newly enacted 21% federal corporate income tax rate. The provisions of ASU 2018-02 are effective for fiscal years and interim periods within that reporting period beginning after December 15, 2018. Early adoption is permitted, including adoption in any interim periods for reporting periods for which financial statements have not been issued. The Company is currently evaluating the impact of ASU 2018-02 and its impact on regulated utilities. At March 31, 2018, stranded taxes in AOCI are approximately $7.2 million . Supplemental Cash Flow Disclosures (in thousands) Three Months Ended March 31, 2018 2017 Cash paid (received) for: Interest on long-term debt and borrowings under the revolving credit facility $ 11,967 $ 11,721 Income tax refunded, net (1,060 ) (697 ) Non-cash investing and financing activities: Changes in accrued plant additions (108 ) (3,335 ) Grants of restricted shares of common stock 513 540 Issuance of performance shares 1,499 932

Revenues

Revenues	3 Months Ended
	Mar. 31, 2018
Revenue Recognition [Abstract]
Revenue Recognition Disclosure	Revenues On January 1, 2018, the Company adopted ASU 2014-09, Revenue from Contracts with Customers (Topic 606), for all of its contracts using the modified retrospective method. There was no cumulative effect adjustment at the initial application of the new standard. In addition, comparative information has not been restated and continues to be reported under the accounting standards in effect for those periods. The Company expects the ongoing impact of the new standard to be immaterial to net income and no significant changes in the Company's business processes and internal controls were necessary upon adoption of the new standard. The following table disaggregates revenue from contracts with customers, for the three and twelve months ended March 31, 2018 (in thousands): March 31, 2018 Three Months Ended Twelve Months Ended Retail $ 146,628 $ 826,148 Wholesale 24,143 72,641 Wheeling (transmission) 4,286 18,133 Total revenues from contracts with customers 175,057 916,922 Other 656 4,253 Total operating revenues $ 175,713 $ 921,175 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 Public Utility Commission of Texas (the "PUCT") and the New Mexico Public Regulation Commission, (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 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 Federal Energy Regulatory Commission (“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. Other. Other includes 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 . Other revenue also includes (i) late payment fees, (ii) leasing income, and (iii) the Company’s allocated share, based on ownership, of sales of surplus effluent water from Palo Verde Generating Station (“Palo Verde”). Accounts receivable. Accounts receivable is principally comprised of revenue from contracts with customers. The Company recognizes expense for accounts that are deemed uncollectible in operating expense. The Company recognized $0.5 million and $3.2 million of uncollectible expense for the three and twelve months ended March 31, 2018, respectively.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Notes)	3 Months Ended
	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Comprehensive Income (Loss) Note	Accumulated Other Comprehensive Income (Loss) Upon adoption of ASU 2016-01, Financial Instruments-Overall, the Company recorded, on January 1, 2018, a cumulative effect adjustment, net of income taxes, to increase retained earnings by $41.0 million with an offset to AOCI. Changes in Accumulated Other Comprehensive Income (Loss) (net of tax) by component are presented below (in thousands): Three Months Ended March 31, 2018 Three Months Ended March 31, 2017 Unrecognized Pension and Post-retirement Benefit Costs Net Unrealized Gains (Losses) on Debt Securities Net Losses on Cash Flow Hedges Accumulated Other Comprehensive Income (Loss) Unrecognized Pension and Post-retirement Benefit Costs Net Unrealized Gains (Losses) on Marketable Securities Net Losses on Cash Flow Hedges Accumulated Other Comprehensive Income (Loss) Balance at beginning of period as previously reported $ (17,790 ) $ 40,190 $ (11,342 ) $ 11,058 $ (23,928 ) $ 28,463 $ (11,651 ) $ (7,116 ) Cumulative effect adjustment — (41,028 ) — (41,028 ) — — — — Other comprehensive income before reclassifications — (2,159 ) — (2,159 ) — 6,165 — 6,165 Amounts reclassified from accumulated other comprehensive income (loss) (685 ) 404 89 (192 ) (529 ) (1,756 ) 52 (2,233 ) Balance at end of period $ (18,475 ) $ (2,593 ) $ (11,253 ) $ (32,321 ) $ (24,457 ) $ 32,872 $ (11,599 ) $ (3,184 ) Twelve Months Ended March 31, 2018 Twelve Months Ended March 31, 2017 Unrecognized Pension and Post-retirement Benefit Costs Net Unrealized Gains (Losses) on Marketable Securities Net Losses on Cash Flow Hedges Accumulated Other Comprehensive Income (Loss) Unrecognized Pension and Post-retirement Benefit Costs Net Unrealized Gains (Losses) on Marketable Securities Net Losses on Cash Flow Hedges Accumulated Other Comprehensive Income (Loss) Balance at beginning of period as previously reported $ (24,457 ) $ 32,872 $ (11,599 ) $ (3,184 ) $ (30,256 ) $ 28,394 $ (11,770 ) $ (13,632 ) Cumulative effect adjustment — (41,028 ) — (41,028 ) — — — — Other comprehensive income before reclassifications 7,951 11,927 — 19,878 7,363 11,327 — 18,690 Amounts reclassified from accumulated other comprehensive income (loss) (1,969 ) (6,364 ) 346 (7,987 ) (1,564 ) (6,849 ) 171 (8,242 ) Balance at end of period $ (18,475 ) $ (2,593 ) $ (11,253 ) $ (32,321 ) $ (24,457 ) $ 32,872 $ (11,599 ) $ (3,184 ) Amounts reclassified from Accumulated Other Comprehensive Income (Loss) for the three and twelve months ended March 31, 2018 and 2017 are as follows (in thousands): Details about Accumulated Other Comprehensive Income (Loss) Components Three Months Ended March 31, Twelve Months Ended March 31, Affected Line Item in the Statements of Operations 2018 2017 2018 2017 Amortization of pension and post-retirement benefit costs: Prior service benefit $ 2,416 $ 2,416 $ 9,657 $ 8,157 Miscellaneous non-operating income Net loss (1,575 ) (1,694 ) (6,657 ) (5,436 ) Miscellaneous non-operating deductions 841 722 3,000 2,721 Income (loss) before income taxes Income tax effect (156 ) (193 ) (1,031 ) (1,157 ) Income tax (benefit) expense 685 529 1,969 1,564 Net income (loss) Marketable securities: Net realized gain (loss) on sale of securities (518 ) 2,191 7,917 8,443 Investment and interest income, net (518 ) 2,191 7,917 8,443 Income (loss) before income taxes Income tax effect 114 (435 ) (1,553 ) (1,594 ) Income tax (benefit) expense (404 ) 1,756 6,364 6,849 Net income (loss) Loss on cash flow hedge: Amortization of loss (139 ) (130 ) (541 ) (506 ) Interest on long-term debt and revolving credit facility (139 ) (130 ) (541 ) (506 ) Income (loss) before income taxes Income tax effect 50 78 195 335 Income tax (benefit) expense (89 ) (52 ) (346 ) (171 ) Net income (loss) Total reclassifications $ 192 $ 2,233 $ 7,987 $ 8,242

Regulation

Regulation	3 Months Ended
	Mar. 31, 2018
Regulated Operations [Abstract]
Regulation	Regulation General The rates and services of the Company are regulated by incorporated municipalities in Texas, the PUCT, the NMPRC, and the FERC. Municipal orders, ordinances and other agreements regarding rates and services adopted by Texas municipalities are subject to review and approval by the PUCT. The FERC has jurisdiction over the Company's wholesale (sales for resale) transactions, transmission service and compliance with federally-mandated reliability standards. The decisions of the PUCT, the NMPRC and the FERC are subject to judicial review. Texas Regulatory Matters 2015 Texas Retail Rate Case Filing. On August 10, 2015, the Company filed with the City of El Paso, other municipalities incorporated in its Texas service territory, and the PUCT in Docket No. 44941, a request for an annual increase in non-fuel base revenues ("2015 Texas Retail Rate Case"). On July 21, 2016, the parties to PUCT Docket No. 44941 filed the Joint Motion to Implement Uncontested Amended and Restated Stipulation and Agreement which was unopposed by the parties. On August 25, 2016, the PUCT issued the PUCT Final Order in Docket No. 44941 ("2016 PUCT Final Order"). The 2016 PUCT Final Order provided for: (i) an annual non-fuel base rate increase, lower annual depreciation expense, a revised return on equity for allowance for funds used during construction ("AFUDC") purposes, and the inclusion of substantially all new plant in service in rate base; (ii) an additional annual non-fuel base rate increase of $3.7 million related to Four Corners Generating Station ("Four Corners") costs, which was collected through a surcharge that terminated on July 11, 2017 ; (iii) removing the separate rate treatment for residential customers with solar systems that the Company had proposed in its August 10, 2015 filing; (iv) allowing the Company to recover $3.1 million in rate case expenses through a separate surcharge; and (v) allowing the Company to recover revenues associated with the relate back of rates to consumption on and after January 12, 2016 through March 31, 2016 through a separate surcharge. Interim rates associated with the annual non-fuel base rate increase became effective on April 1, 2016 . The additional surcharges associated with the incremental Four Corners costs, rate case expenses and the relate back of rates to consumption on and after January 12, 2016 through March 31, 2016 , were implemented on October 1, 2016 . For financial reporting purposes, the Company deferred any recognition of the Company's request in its 2015 Texas Retail Rate Case until it received the 2016 PUCT Final Order on August 25, 2016 . Accordingly, it reported in the third quarter of 2016 the cumulative effect of the 2016 PUCT Final Order, which related back to January 12, 2016. 2017 Texas Retail Rate Case Filing. On February 13, 2017, the Company filed with the City of El Paso, other municipalities incorporated in the Company's Texas service territory and the PUCT in Docket No. 46831, a request for an increase in non-fuel base revenues ("2017 Texas Retail Rate Case"). On November 2, 2017, the Company filed the Joint Motion to Implement Uncontested Stipulation and Agreement with the Administrative Law Judges for the 2017 Texas Retail Rate Case. On December 18, 2017, the PUCT issued the PUCT Final Order in Docket No. 46831 ("2017 PUCT Final Order"), which provides, among other things, for the following: (i) an annual non-fuel base rate increase of $14.5 million ; (ii) a return on equity of 9.65% ; (iii) all new plant in service as filed in the Company's rate filing package was prudent and used and useful and therefore is included in rate base; (iv) recovery of the costs of decommissioning Four Corners in the amount of $5.5 million over a seven year period beginning August 1, 2017; (v) the Company to recover reasonable rate case expenses of approximately $3.4 million through a separate surcharge over a three year period; and (vi) a requirement that the Company file a refund tariff if the federal statutory income tax rate, as it relates to the Company, is decreased before the Company files its next rate case. The 2017 PUCT Final Order also established baseline revenue requirements for recovery of future transmission and distribution investment costs and includes a minimum monthly bill of $30.00 for new residential customers with distributed generation, such as private rooftop solar. Additionally, the 2017 PUCT Final Order allows for the annual recovery of $2.1 million of nuclear decommissioning funding and establishes annual depreciation expense that is approximately $1.9 million lower than the annual amount requested by the Company in its initial filing. Finally, the 2017 PUCT Final Order allows for the Company to recover revenues associated with the relate back of rates to consumption on and after July 18, 2017 through a separate surcharge. New base rates, including additional surcharges associated with rate case expenses and the relate back of rates to consumption on and after July 18, 2017 through December 31, 2017 were implemented in January 2018. For financial reporting purposes, the Company deferred any recognition of the Company's request in its 2017 Texas Retail Rate Case until it received the 2017 PUCT Final Order on December 18, 2017. Accordingly, it reported in the fourth quarter of 2017 the cumulative effect of the 2017 PUCT Final Order, which related back to July 18, 2017. The 2017 PUCT Final Order required the Company to file a refund tariff if the federal statutory income tax rate, as it relates to the Company, were decreased before the Company files its next rate case. Following the enactment of the TCJA on December 22, 2017, and in compliance with the 2017 PUCT Final Order, on March 1, 2018, the Company filed with the PUCT and each of its municipalities a proposed refund tariff designed to reduce base charges for Texas customers equivalent to the expected annual decrease of $22.7 million in federal income tax expense resulting from the tax law changes. This filing was assigned PUCT Docket No. 48124. On March 27, 2018, the PUCT approved the Company's proposed refund tariff on an interim basis, subject to refund or surcharge, for customer billing effective April 1, 2018. Each of the Company's municipalities also implemented the Company's proposed tax credits on an interim basis effective April 1, 2018. The refund will be reflected in rates over a period of a year and will be updated annually until new base rates are implemented pursuant to the Company's next rate case filing. No party requested a hearing in the case before the PUCT by the deadline of April 16, 2018, and on April 18, 2018, the PUCT Staff filed its final recommendation supporting approval of the Company's application. The refund tariff case is pending with the refund tariff subject to final action by the incorporated municipalities in the Company's Texas service territory and a final order from the PUCT. Energy Efficiency Cost Recovery Factor . On May 1, 2017, the Company filed its annual application, which was assigned PUCT Docket No. 47125, to establish its energy efficiency cost recovery factor for 2018. In addition to projected energy efficiency costs for 2018 and a reconciliation of collections to prior year actual costs, the Company requested approval of an incentive bonus for the 2016 energy efficiency program results in accordance with PUCT rules. Interim rates were approved effective January 1, 2018. The Company, the PUCT Staff, and the City of El Paso reached an agreement that includes an incentive bonus of $0.8 million . The agreement was filed on January 25, 2018, and was approved by the PUCT on February 15, 2018. On May 1, 2018, the Company filed its annual application, which was assigned PUCT Docket No. 48332, to establish its energy efficiency cost recovery factor for 2019. In addition to projected energy efficiency costs for 2019 and a reconciliation to prior year actual costs, the Company requested approval of a $1.1 million incentive bonus for the 2017 energy efficiency program results in accordance with PUCT rules. Fuel and Purchased Power Costs. The Company's actual fuel costs, including purchased power energy costs, are recovered from customers through a fixed fuel factor. The PUCT has adopted the fuel cost recovery rule ("Texas Fuel Rule") that allows the Company to seek periodic adjustments to its fixed fuel factor. The Company can seek to revise its fixed fuel factor based upon the approved formula at least four months after its last revision except in the month of December. The Texas Fuel Rule requires the Company to request to refund fuel costs in any month when the over-recovery balance exceeds a threshold material amount and it expects fuel costs to continue to be materially over-recovered. The Texas Fuel Rule also permits the Company to seek to surcharge fuel under-recoveries in any month the balance exceeds a threshold material amount and it expects fuel cost recovery to continue to be materially under-recovered. Fuel over and under-recoveries are considered material when they exceed 4% of the previous twelve months' fuel costs. All such fuel revenue and expense activities are subject to periodic final review by the PUCT in periodic fuel reconciliation proceedings. On November 30, 2016, the Company filed a request, which was assigned PUCT Docket No. 46610, to increase its fixed fuel factor by approximately 28.8% to reflect increased fuel expenses primarily related to an increase in the price of natural gas used to generate power. The increase in the fixed fuel factor was effective on an interim basis January 1, 2017 and approved by the PUCT on January 10, 2017. On October 13, 2017, the Company filed a request, which was assigned PUCT Docket No. 47692, to decrease the Texas fixed fuel factor by approximately 19% to reflect decreased fuel expenses primarily related to a decrease in the price of natural gas used to generate power. The decrease in the Texas fixed fuel factor became effective beginning with the November 2017 billing month. On April 13, 2018, the Company filed a request with the PUCT, which was assigned Docket No. 48264, to decrease the Texas fixed fuel factor by approximately 29% to reflect decreased fuel expenses primarily related to a decrease in the price of natural gas used to generate power. On April 25, 2018, the Company's proposed fuel factors were approved on an interim basis effective for the first billing cycle of the May 2018 billing month. If no party to the case requests a hearing by May 14, 2018, the Company's fuel factors will become final as provided by the PUCT's rules and no further action by the PUCT is required. The Texas fixed fuel factor will continue thereafter until changed by the PUCT. As of March 31, 2018, the Company had a net fuel over-recovery balance of approximately $13.3 million in Texas. Fuel Reconciliation Proceeding . On September 27, 2016, the Company filed an application with the PUCT, designated as PUCT Docket No. 46308, to reconcile $436.6 million of Texas fuel and purchased power expenses incurred during the period of April 1, 2013 through March 31, 2016 . On June 29, 2017, the PUCT approved a settlement in this proceeding. The settlement provides for the reconciliation of fuel and purchased power costs incurred from April 1, 2013 through March 31, 2016 . Additionally, the settlement modifies and tightens the Palo Verde performance rewards measurement bands beginning with the 2018 performance period. The financial results for the twelve months ended March 31, 2018 include a $5.0 million , pre-tax increase to income reflecting the settlement of the Texas fuel reconciliation proceeding. This amount represents Palo Verde performance rewards associated with the 2013 to 2015 performance periods net of disallowed fuel and purchased power costs as approved in the settlement. Texas jurisdictional fuel and purchased power costs subject to prudence review are costs from April 1, 2016 through March 31, 2018, that total approximately $271.3 million . Community Solar. On June 8, 2015, the Company filed a petition with the PUCT to initiate a community solar program that includes the construction and ownership of a 3 Megawatts ("MW") solar photovoltaic system located at Montana Power Station ("MPS"). Participation is on a voluntary basis, and customers contract for a set capacity (kW) amount and receive all energy produced. This case was assigned PUCT Docket No. 44800. The Company filed a settlement agreement among all parties on July 1, 2016, approving the program, and the PUCT approved the settlement agreement and program on September 1, 2016. On April 19, 2017, the Company announced that the entire 3 MW program was fully subscribed by approximately 1,500 Texas customers. The community solar facility began commercial operation on May 31, 2017 . On March 20, 2018, the Company filed a petition with the PUCT and each of its regulatory authorities to expand its community solar program to include 2 MW of solar powered generation from the 10 MW solar photovoltaic facility located at Newman Power Station ("Newman") and to reduce rates under the community solar tariff. The case before the PUCT was assigned Docket No. 48181 and is currently pending. Other Required Approvals . The Company has obtained other required approvals for tariffs and other approvals required by the Texas Public Utility Regulatory Act and the PUCT. New Mexico Regulatory Matters 2015 New Mexico Rate Case Filing . On May 11, 2015, the Company filed a request with the NMPRC, in Case No. 15-00127-UT, for an annual increase in non-fuel base rates. On June 8, 2016, the NMPRC issued the NMPRC Final Order in Case No. 15-00127-UT ("NMPRC Final Order"), which approved an annual increase in non-fuel base rates of approximately $0.6 million , an increase of approximately $0.5 million in other service fees and a decrease in the Company's allowed return on equity to 9.48% . The NMPRC Final Order concluded that all of the Company's new plant in service was reasonable and necessary and therefore would be recoverable in rates. The Company's rates were approved by the NMPRC effective July 1, 2016, and implemented at such time. Future New Mexico Rate Case Filing. On April 12, 2017, the NMPRC issued an order in Case No. 15-00109-UT requiring the Company to make a rate filing in New Mexico no later than July 31, 2019, using an appropriate historical test year period. New Mexico Order Commencing Review of the Effects of the TCJA on Regulated New Mexico Utilities. On January 24, 2018, the NMPRC initiated a proceeding in Case No. 18-00016-UT into the impact of the TCJA on New Mexico regulated utilities. On February 23, 2018, the Company responded to a NMPRC Staff inquiry regarding the proceeding. On April 4, 2018, the NMPRC issued an order requiring the Company to file a proposed interim rate rider to adjust the Company’s New Mexico base revenues in amounts equivalent to the Company’s reduced income tax expense for New Mexico customers resulting from the TCJA, to be implemented on or before May 1, 2018. The NMPRC order further requires that the Company record and track a regulatory liability for the excess accumulated deferred income taxes created by the change in the federal corporate income tax rate, consistent with the effective date of the TCJA, and subject to amortization determined by the NMPRC in the Company’s next general rate case. The Company recorded such a regulatory liability during the quarter ended December 31, 2017. On April 16, 2018, after consultation with the New Mexico Attorney General pursuant to the NMPRC order, the Company filed an interim rate rider, with a proposed effective date of May 1, 2018. The annualized credits expected to be refunded to New Mexico customers approximate $4.9 million . On April 25, 2018, the NMPRC approved the Company's interim rate rider to be implemented in customer bills beginning May 1, 2018. Fuel and Purchased Power Costs. Historically, fuel and purchased power costs were recovered through base rates and a Fuel and Purchased Power Cost Adjustment Clause ("FPPCAC") that accounts for changes in the costs of fuel relative to the amount included in base rates. Effective July 1, 2016, with the implementation of the NMPRC Final Order, fuel and purchased power costs are no longer recovered through base rates but are recovered through the FPPCAC. The Company's request to reconcile its fuel and purchased power costs for the period January 1, 2013 through December 31, 2014, also was approved in Case No. 15-00127-UT. New Mexico jurisdictional costs subject to prudence review are costs from January 1, 2015 through March 31, 2018 that total approximately $181.0 million . At March 31, 2018 , the Company had a net fuel over-recovery balance of approximately $0.9 million in New Mexico. As required, the Company filed a request to continue use of its FPPCAC with the NMPRC on January 5, 2018, which was assigned NMPRC Case No. 18-00006-UT. Hearings in the case are scheduled to begin in July 2018. 5 MW Holloman Air Force Base ("HAFB") Facility Certificate of Convenience and Necessity ("CCN") . On October 7, 2015, in NMPRC Case No. 15-00185-UT, the NMPRC issued a final order approving a CCN for a 5 MW solar power generation facility located on HAFB in the Company's service territory in New Mexico. The Company and HAFB negotiated a retail contract, which includes a power sales agreement for the facility, to replace the existing load retention agreement which was approved by final order issued October 5, 2016 in NMPRC Case No. 16-00224-UT. Construction of the solar generation facility is expected to be completed in the third quarter of 2018. New Mexico Efficient Use of Energy Recovery Factor. On July 1, 2016 , the Company filed its annual application requesting approval of its 2017 Energy Efficiency and Load Management Plan and to establish energy efficiency cost recovery factors for 2017. In addition to projected energy efficiency costs for 2017, the Company requested approval of a $0.4 million incentive for 2017 energy efficiency programs in accordance with NMPRC rules. This case was assigned Case No. 16-00185-UT. On February 22, 2017 , the NMPRC issued a final order approving the Company’s 2017 Energy Efficiency and Load Management Plan and authorizing recovery in 2017 of a base incentive of $0.4 million . The Company’s energy efficiency cost recovery factors were approved and effective in customer bills beginning on March 1, 2017. On July 1, 2016 , the Company filed its 2015 Annual Report for Energy Efficiency Programs, which included an incentive for verified 2015 program performance of $0.3 million , which was approved in Case No. 13-00176-UT. The Company recorded the $0.3 million approved incentive in operating revenues in the first quarter of 2017. In addition, on June 30, 2017 , the Company filed its 2016 Annual Report for Energy Efficiency Programs, which included an incentive for verified 2016 program performance of $0.4 million that was approved in Case No. 13-00176-UT. The Company recorded the $0.4 million approved incentive in operating revenues in the third quarter of 2017. Community Solar . On April 24, 2018, the Company filed a petition with the NMPRC to initiate a community solar program to include construction and ownership of a 2 MW solar photovoltaic system located in Doña Ana County near the City of Las Cruces. Customer participation will be on a voluntary basis, and customers will contract for a set capacity (kW) amount and receive all energy produced by their subscribed capacity. The Company cannot predict the outcome of this petition at this time. Other Required Approvals . The Company has obtained other required approvals for tariffs and other approvals as required by the New Mexico Public Utility Act and the NMPRC. Federal Regulatory Matters Inquiry Regarding the Effect of the TCJA on Commission-Jurisdictional Rates and Order to Show Cause. On March 15, 2018, the FERC issued two show cause orders under Section 206 of the Federal Power Act and Rule 209(a) of the FERC’s Rules of Practice and Procedure, directing 48 individual public utilities with stated transmission rates or transmission formula rates with a fixed line item of 35% for the federal income tax component to, within 60 days of the orders, either (1) propose revisions to their transmission rates under their open access transmission tariffs or transmission owner tariffs on file with the FERC, or (2) show cause why they should not be required to do so. The Company is included in the list of public utilities impacted by the FERC orders and is currently evaluating a response to the orders. Other Required Approvals. The Company has obtained required approvals for rates, tariffs and other approvals as required by the FERC.

Palo Verde

Palo Verde	3 Months Ended
	Mar. 31, 2018
Palo Verde [Abstract]
Palo Verde	Palo Verde Spent Fuel and Waste Disposal. Pursuant to the Nuclear Waste Policy Act of 1982, as amended in 1987, the U.S. Department of Energy ("DOE") is legally obligated to accept and dispose of all spent nuclear fuel and other high-level radioactive waste generated by all domestic power reactors by 1998. The DOE's obligations are reflected in a contract for Disposal of Spent Nuclear Fuel and/or High Level Radioactive Waste with each nuclear power plant. The DOE failed to begin accepting spent nuclear fuel by 1998. Pursuant to the terms of the August 18, 2014 settlement agreement, and as amended with the DOE, Arizona Public Service Company ("APS") files annual claims for the period July 1 of the then-previous year to June 30 of the then-current year on behalf of itself and those utilities that share in power and energy entitlements, and bear certain allocated costs, with respect to Palo Verde pursuant to the Arizona Nuclear Power Project Participation Agreement dated August 23, 1973, as amended ("ANPP Participation Agreement"). The Company's share of costs recovered in 2017 and 2018 is presented below (in thousands): Amount Credited to Customers through Fuel Period Credited Costs Recovery Period Amount Refunded Adjustment Clauses to Customers July 2015 - June 2016 $ 1,779 $ 1,432 March 2017 July 2016 - June 2017 1,413 1,121 March 2018 Palo Verde Operations and Maintenance Expense . Included in "operations and maintenance" in the Company's Statements of Operations are expenses associated with Palo Verde as follows (in thousands): 2018 2017 Three months ended March 31, $ 22,175 $ 21,608 Twelve months ended March 31, 99,931 96,179

Common Stock

Common Stock	3 Months Ended
	Mar. 31, 2018
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Common Stock and Earnings Per Share Information	Common Stock Dividends. The Company paid $13.6 million and $12.6 million in quarterly cash dividends during the three months ended March 31, 2018 and 2017 , respectively. The Company paid a total of $54.3 million and $50.3 million in quarterly cash dividends during the twelve months ended March 31, 2018 and 2017 , respectively. Basic and Diluted Earnings Per Share . The basic and diluted earnings per share are presented below (in thousands except for share data): Three Months Ended March 31, Twelve Months Ended March 31, 2018 2017 2018 2017 Weighted average number of common shares outstanding: Basic number of common shares outstanding 40,491,194 40,387,235 40,440,189 40,366,024 Dilutive effect of unvested performance awards — — 123,436 69,665 Diluted number of common shares outstanding 40,491,194 40,387,235 40,563,625 40,435,689 Basic net income (loss) per common share: Net income (loss) $ (6,966 ) $ (3,989 ) $ 95,285 $ 98,587 Income allocated to participating restricted stock (48 ) (45 ) (353 ) (349 ) Net income (loss) available to common shareholders $ (7,014 ) $ (4,034 ) $ 94,932 $ 98,238 Diluted net income (loss) per common share: Net income (loss) $ (6,966 ) $ (3,989 ) $ 95,285 $ 98,587 Income reallocated to participating restricted stock (48 ) (45 ) (353 ) (349 ) Net income (loss) available to common shareholders $ (7,014 ) $ (4,034 ) $ 94,932 $ 98,238 Basic net income (loss) per common share: Distributed earnings $ 0.335 $ 0.31 $ 1.34 $ 1.24 Undistributed earnings (losses) (0.505 ) (0.41 ) 1.01 1.19 Basic net income (loss) per common share $ (0.170 ) $ (0.10 ) $ 2.35 $ 2.43 Diluted net income (loss) per common share: Distributed earnings $ 0.335 $ 0.31 $ 1.34 $ 1.24 Undistributed earnings (losses) (0.505 ) (0.41 ) 1.00 1.19 Diluted net income (loss) per common share $ (0.170 ) $ (0.10 ) $ 2.34 $ 2.43 The number of restricted stock awards and performance shares at 100% performance level excluded from the calculation of the diluted number of common shares outstanding because their effect was antidilutive is presented below: Three Months Ended Twelve Months Ended March 31, March 31, 2018 2017 2018 2017 Restricted stock awards 72,218 78,025 66,288 58,344 Performance shares (a) 45,977 — 11,494 47,246 ________________________ (a) Certain performance shares were excluded from the computation of diluted earnings per share as no payouts would have been required based upon performance at the end of each corresponding period.

Income Taxes

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.

Commitments, Contingencies And

Commitments, Contingencies And Uncertainties	3 Months Ended
	Mar. 31, 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. Environmental Litigation and Investigations . Since July 2011, the U.S. Department of Justice, on behalf of the U.S. Environmental Protection Agency, and APS have been engaged in substantive settlement negotiations in an effort to resolve certain of the pending matters related to Four Corners. The allegations being addressed through settlement negotiations are that APS failed to obtain the necessary permits and install the controls necessary under the U.S. Clean Air Act ("CAA") to reduce sulfur dioxide, nitrogen oxides, and particulate matter, and that defendants failed to obtain an operating permit under Title V of the CAA that reflects applicable requirements imposed by law. On June 24, 2015, the parties filed with the U.S. District Court for New Mexico a settlement agreement resolving this matter. On August 17, 2015, the U.S. District Court for New Mexico entered the settlement agreement. The agreement imposes a total civil penalty payable by the co-owners of Four Corners collectively in the amount of $1.5 million , and it requires the co-owners to pay $6.7 million for environmental mitigation projects. At March 31, 2018, the Company has accrued its remaining unpaid share of approximately $0.2 million related to this matter.

Litigation

Litigation	3 Months Ended
	Mar. 31, 2018
Disclosure Text Block [Abstract]
Litigation Matters Disclosure	Litigation The Company is involved in various legal, environmental, tax and regulatory proceedings before various courts, regulatory commissions and governmental agencies regarding matters arising in the ordinary course of business. In many of these matters, the Company has excess casualty liability insurance that covers the various claims, actions and complaints. The Company regularly analyzes current information and, as necessary, makes provisions in its financial statements for probable liabilities for the eventual disposition of these matters. While the outcome of these matters cannot be predicted with certainty, based upon a review of the matters and applicable insurance coverage, the Company believes that none of these matters will have a material adverse effect on the financial position, results of operations or cash flows of the Company. The Company expenses legal costs, including expenses related to loss contingencies, as they are incurred. See Notes D and H above and Notes C and K of the Notes to Financial Statements in the 2017 Form 10-K for discussion of the effects of government legislation and regulation on the Company.

Employee Benefits

Employee Benefits	3 Months Ended
	Mar. 31, 2018
Retirement Benefits, Description [Abstract]
Employee Benefits	Employee Benefits The Company adopted ASU 2017-07, Compensation-Retirement Benefits, effective January 1, 2018. Upon adoption of the new standard, the service cost is included in "Operations and maintenance" in the Company's Statements of Operations. The expected return on plan assets is included in "Investment and interest income, net". The amortization of prior service benefit and amortization of gains are included in "Miscellaneous non-operating income". The amortization of prior service cost and amortization of losses are included in "Miscellaneous non-operating deductions". The interest cost component of net periodic benefit cost is included in "Other interest". The provisions in ASU 2017-07 were applied retrospectively for the income statement presentation of the service cost component and the other components of net benefit costs. The Company elected to apply the practical expedient and used the amounts disclosed in its pension and other postretirement benefit plan note for the 2017 comparative period as the estimation basis for applying the retrospective presentation requirements. The Company reclassified $2.1 million to "Operations and maintenance" in the Company’s Statement of Operations for the three months ended March 31, 2017 by increasing (i) "Investment and interest income, net" by $5.3 million , (ii) "Miscellaneous non-operating income" by $2.8 million , (iii) "Miscellaneous non-operating deductions" by $2.1 million , and (iv) "Other interest" by $3.9 million . As a result of the reclassifications, "Operations and maintenance" increased to $2.9 million in service cost from the $0.8 million in net periodic benefit cost previously reported. The Company reclassified $7.5 million to "Operations and maintenance" in the Company’s Statement of Operations for the twelve months ended March 31, 2017 by increasing (i) "Investment and interest income, net" by $20.8 million , (ii) "Miscellaneous non-operating income" by $10.3 million , (iii) "Miscellaneous non-operating deductions" by $7.5 million , and (iv) "Other interest" by $16.0 million . As a result of the reclassifications, "Operations and maintenance" increased to $11.0 million in service cost from the $3.5 million in net periodic benefit cost previously reported. Retirement Plans The net periodic benefit cost recognized for the three and twelve months ended March 31, 2018 and 2017 , is made up of the components listed below as determined using the projected unit credit actuarial cost method (in thousands): Three Months Ended Twelve Months Ended March 31, March 31, 2018 2017 2018 2017 Components of net periodic benefit cost: Service cost $ 2,758 $ 2,270 $ 9,006 $ 8,366 Interest cost 3,223 3,248 13,034 13,022 Expected return on plan assets (5,315 ) (4,808 ) (19,696 ) (18,975 ) Amortization of: Net loss 2,100 2,089 8,465 7,540 Prior service benefit (878 ) (878 ) (3,506 ) (3,506 ) Net periodic benefit cost $ 1,888 $ 1,921 $ 7,303 $ 6,447 During the three months ended March 31, 2018 , the Company contributed $2.9 million of its projected $9.4 million 2018 annual contribution to its retirement plans. Other Postretirement Benefits The net periodic benefit recognized for the three and twelve months ended March 31, 2018 and 2017 , is made up of the components listed below (in thousands): Three Months Ended Twelve Months Ended March 31, March 31, 2018 2017 2018 2017 Components of net periodic benefit: Service cost $ 700 $ 588 $ 2,348 $ 2,642 Interest cost 565 678 2,610 2,972 Expected return on plan assets (613 ) (470 ) (2,050 ) (1,845 ) Amortization of: Prior service benefit (1,538 ) (1,538 ) (6,151 ) (4,651 ) Net gain (525 ) (395 ) (1,808 ) (2,104 ) Net periodic benefit $ (1,411 ) $ (1,137 ) $ (5,051 ) $ (2,986 ) During the three months ended March 31, 2018 , the Company contributed $0.1 million of its projected $0.5 million 2018 annual contribution to its other postretirement benefits plan.

Financial Instruments And Inves

Financial Instruments And Investments (Notes)	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Financial Instruments and Investments	Financial Instruments and Investments The FASB guidance requires the Company to disclose estimated fair values for its financial instruments. The Company has determined that cash and temporary investments, investment in debt securities, accounts receivable, decommissioning trust funds, long-term debt, short-term borrowings under the Company's Revolving Credit Facility ("RCF"), accounts payable and customer deposits meet the definition of financial instruments. The carrying amounts of cash and temporary investments, accounts receivable, accounts payable and customer deposits approximate fair value because of the short maturity of these items. Investments in debt securities and decommissioning trust funds are carried at estimated fair value. Long-Term Debt and Short-Term Borrowings Under the RCF. The fair values of the Company's long-term debt and short-term borrowings under the RCF are based on estimated market prices for similar issues and are presented below (in thousands): March 31, 2018 December 31, 2017 Carrying Amount Estimated Fair Value Carrying Amount Estimated Fair Value Pollution Control Bonds $ 157,699 $ 166,779 $ 157,676 $ 169,186 Senior Notes 993,514 1,147,504 993,426 1,211,922 RGRT Senior Notes (1) 44,897 46,620 44,886 47,070 RCF (1) 233,067 233,067 173,533 173,533 Total $ 1,429,177 $ 1,593,970 $ 1,369,521 $ 1,601,711 _______________ (1) Nuclear fuel financing, as of March 31, 2018 and December 31, 2017 , is funded through $45 million Rio Grande Resources Trust ("RGRT") Senior Notes and $89.1 million and $88.5 million , respectively, under the RCF. As of March 31, 2018 and December 31, 2017 , $144.0 million and $85.0 million , respectively, was outstanding under the RCF for working capital or general corporate purposes. The interest rate on the Company's borrowings under the RCF is reset throughout the quarter reflecting c urrent market rates. Consequently, the carrying value approximates fair value. Marketable Securities. The Company's marketable securities, included in decommissioning trust funds in the balance sheets, are reported at fair value which was $284.1 million and $286.9 million at March 31, 2018 and December 31, 2017 , respectively. The investments in the Company's Palo Verde nuclear decommissioning trust funds ("NDT") are classified as available for sale debt securities, equity securities and cash and cash equivalents. These investments are recorded at their estimated fair value in accordance with FASB guidance for certain investments in debt and equity securities. On January 1, 2018, the Company adopted ASU 2016-01, Financial Instruments-Overall, which eliminates the requirements to classify investments in equity securities with readily determinable fair values as trading or available for sale and requires entities to recognize changes in fair value for these securities in net income as reported in the Statements of Operations. ASU 2016-01 requires a modified-retrospective approach and therefore, comparative information has not been restated and continues to be reported under the accounting standards in effect for those periods. The reported fair values include gross unrealized losses on securities classified as available for sale whose impairment the Company has deemed to be temporary. The tables below present the gross unrealized losses and the fair value of these securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position (in thousands): March 31, 2018 Less than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Description of Securities (1) : Federal Agency Mortgage Backed Securities $ 5,995 $ (114 ) $ 10,376 $ (407 ) $ 16,371 $ (521 ) U.S. Government Bonds 34,018 (940 ) 18,325 (1,215 ) 52,343 (2,155 ) Municipal Debt Obligations 4,227 (109 ) 7,031 (581 ) 11,258 (690 ) Corporate Debt Obligations 20,855 (609 ) 3,769 (301 ) 24,624 (910 ) Total $ 65,095 $ (1,772 ) $ 39,501 $ (2,504 ) $ 104,596 $ (4,276 ) _________________ (1) Includes 147 securities. December 31, 2017 Less than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Description of Securities (2) : Federal Agency Mortgage Backed Securities $ 4,700 $ (46 ) $ 10,099 $ (165 ) $ 14,799 $ (211 ) U.S. Government Bonds 28,866 (416 ) 18,186 (969 ) 47,052 (1,385 ) Municipal Debt Obligations 4,290 (73 ) 9,736 (742 ) 14,026 (815 ) Corporate Debt Obligations 10,685 (107 ) 4,475 (331 ) 15,160 (438 ) Total Debt Securities 48,541 (642 ) 42,496 (2,207 ) 91,037 (2,849 ) Domestic Equity Securities 962 (210 ) — — 962 (210 ) Total $ 49,503 $ (852 ) $ 42,496 $ (2,207 ) $ 91,999 $ (3,059 ) _________________ (2) Includes 146 securities. The Company monitors the length of time specific securities trade below their cost basis along with the amount and percentage of the unrealized loss in determining if a decline in fair value below recorded cost of debt securities classified as available for sale is considered to be other than temporary. The Company recognizes impairment losses on certain of its available for sale debt securities deemed to be other than temporary. In accordance with the FASB guidance, these impairment losses are recognized in net income, and a lower cost basis is established for these securities. In addition, the Company will research the future prospects of individual securities as necessary. The Company does not anticipate expending monies held in trust before 2044 or a later period when decommissioning of Palo Verde begins. For the three and twelve months ended March 31, 2018 and 2017 , the Company recognized other than temporary impairment losses on its available-for-sale securities as follows (in thousands): Three Months Ended Twelve Months Ended March 31, March 31, 2018 2017 2018 2017 Unrealized holding losses included in pre-tax income $ — $ — $ — $ (196 ) Investments categorized as available for sale securities also include gross unrealized gains which have not been recognized in the Company's net income. The table below presents the unrecognized gross unrealized gains and the fair value of these securities, aggregated by investment category (in thousands): March 31, 2018 December 31, 2017 Fair Value Unrealized Gains Fair Value Unrealized Gains Description of Securities: Federal Agency Mortgage Backed Securities $ 9,469 $ 143 $ 5,933 $ 203 U.S. Government Bonds 2,205 92 11,129 256 Municipal Debt Obligations 1,073 92 2,558 109 Corporate Debt Obligations 11,444 546 19,514 1,067 Total Debt Securities 24,191 873 39,134 1,635 Domestic Equity Securities — — 120,065 45,587 International Equity Securities — — 28,804 5,908 Cash and Cash Equivalents — — 6,864 — Total $ 24,191 $ 873 $ 194,867 $ 53,130 The Company's marketable securities include investments in mortgage backed securities, municipal, corporate and federal debt obligations. The contractual year for maturity of these available-for-sale debt securities as of March 31, 2018 , is as follows (in thousands): Total 2018 2019 2023 through 2027 2028 and Beyond Federal Agency Mortgage Backed Securities $ 25,840 $ — $ 16 $ 261 $ 25,563 U.S. Government Bonds 54,548 1,894 28,263 11,415 12,976 Municipal Debt Obligations 12,331 76 5,483 5,735 1,037 Corporate Debt Obligations 36,068 213 17,518 7,151 11,186 The Company's available for sale securities in the NDT are sold from time to time and the Company uses the specific identification basis to determine the amount to reclassify from AOCI into net income. The proceeds from the sale of these securities during the three and twelve months ended March 31, 2018 and 2017 , and the related effects on pre-tax income are as follows (in thousands): Three Months Ended Twelve Months Ended March 31, March 31, 2018 2017 2018 2017 Proceeds from sales or maturities of available-for-sale securities $ 11,757 $ 26,055 $ 82,739 $ 93,245 Gross realized gains included in pre-tax income $ 9 $ 2,587 $ 9,195 $ 9,968 Gross realized losses included in pre-tax income (527 ) (396 ) (1,278 ) (1,329 ) Gross unrealized losses included in pre-tax income — — — (196 ) Net gains included in pre-tax income $ (518 ) $ 2,191 $ 7,917 $ 8,443 Upon the adoption of ASU 2016-01, Financial Instruments-Overall, on January 1, 2018, the Company records, on a modified-retrospective basis, changes in fair market value for equity securities held in the NDT in the Statements of Operations. The unrealized gains and losses recognized during the three months ended March 31, 2018 and related effects on pre-tax income are as follows (in thousands): Three Months Ended March 31, 2018 Net gains and (losses) recognized on equity securities $ (1,991 ) Less: Net gains and (losses) recognized on equity securities sold 1,790 Unrealized gains and (losses) recognized on equity securities still held at reporting date $ (3,781 ) Fair Value Measurements. The FASB guidance requires the Company to provide expanded quantitative disclosures for financial assets and liabilities recorded on the balance sheet at fair value. Financial assets carried at fair value include the Company's decommissioning trust investments and investments in debt securities which are included in deferred charges and other assets on the Balance Sheets. The Company has no liabilities that are measured at fair value on a recurring basis. The FASB guidance establishes a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value into three levels as follows: • Level 1 – Observable inputs that reflect quoted market prices for identical assets and liabilities in active markets. Financial assets utilizing Level 1 inputs include the nuclear decommissioning trust investments in active exchange-traded equity securities, mutual funds and U.S. Treasury securities that are in a highly liquid and active market. The Institutional Funds are valued using the Net Asset Value ("NAV") provided by the administrator of the fund. The NAV price is quoted on a restrictive market although the underlying investments are traded on active markets. The NAV used for determining the fair value of the Institutional Funds-International Equity investments have readily determinable fair values. Accordingly, such fund values are categorized as Level 1. • Level 2 – Inputs other than quoted market prices included in Level 1 that are observable for the asset or liability either directly or indirectly. Financial assets utilizing Level 2 inputs include the nuclear decommissioning trust investments in fixed income securities. The fair value of these financial instruments is based on evaluated prices that reflect observable market information, such as actual trade information of similar securities, adjusted for observable differences. • Level 3 – Unobservable inputs using data that is not corroborated by market data and primarily based on internal Company analysis using models and various other analysis. Financial assets utilizing Level 3 inputs are the Company's investment in debt securities. The securities in the NDT are valued using prices and other relevant information generated by market transactions involving identical or comparable securities. The FASB guidance identifies this valuation technique as the "market approach" with observable inputs. The Company analyzes available-for-sale securities to determine if losses are other than temporary. The fair value of the NDT and investments in debt securities at March 31, 2018 and December 31, 2017 , and the level within the three levels of the fair value hierarchy defined by the FASB guidance are presented in the table below (in thousands): Description of Securities Fair Value as of March 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Trading Securities: Investments in Debt Securities $ 1,726 $ — $ — $ 1,726 Equity Securities: Domestic $ 119,344 $ 119,344 $ — $ — International 28,542 28,542 — — Total Equity Securities 147,886 147,886 — — Available for Sale Debt Securities: Federal Agency Mortgage Backed Securities 25,840 — 25,840 — U.S. Government Bonds 54,548 54,548 — — Municipal Debt Obligations 12,331 — 12,331 — Corporate Debt Obligations 36,068 — 36,068 — Total Available for Sale Debt Securities 128,787 54,548 74,239 — Cash and Cash Equivalents 7,409 7,409 — — Total $ 284,082 $ 209,843 $ 74,239 $ — Description of Securities Fair Value as of December 31, 2017 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Trading Securities: Investments in Debt Securities $ 1,735 $ — $ — $ 1,735 Available for Sale: Federal Agency Mortgage Backed Securities $ 20,732 $ — $ 20,732 $ — U.S. Government Bonds 58,181 58,181 — — Municipal Debt Obligations 16,584 — 16,584 — Corporate Debt Obligations 34,674 — 34,674 — Subtotal, Debt Securities 130,171 58,181 71,990 — Domestic 121,027 121,027 — — International 28,804 28,804 — — Subtotal, Equity Securities 149,831 149,831 — — Cash and Cash Equivalents 6,864 6,864 — — Total $ 286,866 $ 214,876 $ 71,990 $ — There were no transfers in or out of Level 1 and Level 2 fair value measurements categories due to changes in observable inputs during the three and twelve months ended March 31, 2018 and 2017 . There were no purchases, sales, issuances and settlements related to the assets in the Level 3 fair value measurement category during the three and twelve months ended March 31, 2018 and 2017 .

Principles Of Preparation (Poli

Principles Of Preparation (Policy)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Reclassification, Policy	Reclassification . Certain amounts in the financial statements for 2017 have been reclassified to conform with the 2018 presentation. The Company implemented Accounting Standards Update ("ASU") 2017-07, Compensation - Retirement Benefits, and ASU 2016-15, Statement of Cash Flows, in the first quarter of 2018, retrospective to all periods presented in the Company's financial statements. See "New Accounting Standards Adopted" below for further details.
Use of Estimates, Policy	Use of Estimates . The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. The Company evaluates its estimates on an on-going basis, including those related to depreciation, unbilled revenue, income taxes, fuel costs, pension and other post-retirement obligations and asset retirement obligations ("AROs"). Actual results could differ from those estimates.
Revenue Recognition, Policy	Revenues . 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 $20.0 million at March 31, 2018 and $22.2 million at December 31, 2017 . The Company presents revenues net of sales taxes in its statements of operations. 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 Public Utility Commission of Texas (the "PUCT") and the New Mexico Public Regulation Commission, (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 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 Federal Energy Regulatory Commission (“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.
Depreciation, Depletion, and Amortization, Policy	Depreciation. The Company routinely evaluates the depreciable service lives, cost of removal and salvage values of its property, plant and equipment. Depreciation is provided on a straight-line basis over the estimated remaining lives of the assets (ranging in average from 5 to 48 years). When property subject to composite depreciation is retired or otherwise disposed of in the normal course of business, its cost together with the cost of removal, less salvage is charged to accumulated depreciation. For other property dispositions, the applicable cost and accumulated depreciation is removed from the balance sheet accounts and a gain or loss is recognized.
New Accounting Standards, Policy	New Accounting Standards Adopted In March 2016, the Financial Accounting Standards Board ("FASB") issued ASU 2016-09, Compensation - Stock Compensation (Topic 718) Improvements to Employee Share-Based Payment Accounting, to simplify the accounting for share-based payment transactions, including the income tax consequences, classification of awards either as equity or liabilities, and classification on the statements of cash flows. The Company adopted the new standard effective January 1, 2017. The adoption of the new standard did not have a material impact on the Company’s financial condition, results of operations or cash flows. The cumulative effect of the adoption of the new standard was to increase net operating loss carryforward deferred tax assets and retained earnings by $0.2 million on January 1, 2017. In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), to provide a framework that replaces the existing revenue recognition guidance, and has since modified the standard with several ASUs. The standard provides that an entity should recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. On January 1, 2018, the Company adopted the new accounting standard using the modified retrospective method. There was no cumulative effect adjustment at the initial application of the new standard. In addition, comparative information has not been restated and continues to be reported under the accounting standards in effect for those periods. The Company expects the ongoing impact of the new standard to be immaterial to net income. As required by the standard, revenues of $1.9 million related to reimbursed costs of energy efficiency programs approved by the Company's regulators are reported in operating revenues from customers prospectively, as opposed to being offset with associated costs within operations and maintenance. Related expenses of an equal amount are reported in operations and maintenance expenses. See Note B, Revenues, for additional information. In January 2016, the FASB issued ASU 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Liabilities, to enhance the reporting model for financial instruments by addressing certain aspects of recognition, measurement, presentation and disclosure. The Company adopted the new standard effective January 1, 2018. The adoption of ASU 2016-01 eliminates the requirements to classify investments in equity securities with readily determinable fair values into trading or available for sale and requires entities to measure equity investments at fair value and recognize any changes in fair value in the Statements of Operations. ASU 2016-01 requires a modified-retrospective approach and therefore comparative information has not been restated and continues to be reported under the accounting standards in effect for those periods. Upon adoption of the new standard, the Company recorded a cumulative effect adjustment, net of income taxes, to increase retained earnings by $41.0 million with an offset to accumulated other comprehensive income ("AOCI"). In addition, the Company recorded net losses of $3.8 million related to equity securities still held at March 31, 2018. In March 2018, the FASB issued ASU 2018-04, Investments - Debt Securities (Topic 320) and Regulated Operations (Topic 980), which provides clarification to ASU 2016-01. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230) Classification of Certain Cash Receipts and Cash Payments, to reduce diversity in practice in how certain cash receipts and cash payments are classified in the statement of cash flows. The Company adopted the new standard effective January 1, 2018. ASU 2016-15 was applied using a retrospective transition method to each period presented. Accordingly, the Company presented in the Statement of Cash Flows insurance proceeds received for equipment of $4.2 million and $0.7 million , respectively, for the three months ended March 31, 2018 and 2017 as cash inflows from investing activities. In March 2017, the FASB issued ASU 2017-07, Compensation-Retirement Benefits (Topic 715) Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. ASU 2017-07 amends Accounting Standards Codification 715, Compensation - Retirement Benefits, to require companies to present the service cost component of net benefit cost in the income statement line items where compensation cost is reported. Companies will present all other components of net benefit cost separately from the line item(s) that includes the service cost and outside of any subtotal of operating income. In addition, only the service cost component will be eligible for capitalization in assets. The Company adopted the new standard effective January 1, 2018. The amendments in ASU 2017-07 were applied retrospectively for the income statement presentation of the service cost component and the other components of net benefit costs. The Company elected to apply the practical expedient and used the amounts disclosed in its pension and other postretirement benefit plan note for the 2017 comparative period as the estimation basis for applying the retrospective presentation requirements. See Note J, Employee Benefits, for additional information. In March 2018, the FASB issued ASU 2018-05, Income Taxes (Topic 740) Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 ("SAB 118"), to add various SEC paragraphs for clarification due to The Federal Tax Cuts and Jobs Act of 2017 ("TCJA"). The Company adopted ASU 2018-05 upon issuance and implemented SAB 118 in December of 2017 in conjunction with the enactment of the TCJA. New Accounting Standards to be Adopted in the Future In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), to increase transparency and comparability among organizations by recognizing lease assets and lease liabilities on the balance sheet and requiring qualitative and quantitative disclosures on leasing agreements. ASU 2016-02 maintains a distinction between finance leases and operating leases similar to the distinction under previous lease guidance for capital leases and operating leases. The impact of leases reported in the Company's operating results and statement of cash flows is expected to be similar to previous GAAP. ASU 2016-02 requires the recognition in the statement of financial position, by the lessee, of a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. How leases are recorded in regard to financial position represents a significant change from previous GAAP guidance. The lessee is permitted to make an accounting policy election to not recognize lease assets and lease liabilities for short-term leases. Implementation of the standard will be required for reporting periods beginning after December 15, 2018. Adoption of the new lease accounting standard will require the Company to apply the new standard to the earliest period using a modified retrospective approach. The Company is currently in the process of evaluating the impact of the new standard, which includes continuing to monitor activities of the FASB, including the impact of the recently issued ASU 2018-01, and the proposed project to allow entities to adopt the standard with a cumulative effect adjustment as of the beginning of the adoption year, while maintaining prior year comparative financial information and disclosures as reported. ASU 2018-01, Land Easement Practical expedient for Transition to Topic 842, provides an optional practical expedient to not evaluate existing or expired land easements under Topic 842, if those land easements were not previously accounted for as leases under Accounting Standards Codification ("ASC") Topic 840. The Company currently anticipates that it will apply the practical expedient under ASU 2018-01 to its existing or expired land easements as part of its transition to Topic 842. The Company's evaluation process also includes evaluating the impact, if any, on changes to business processes, systems and controls to support recognition and disclosure under the new guidance; however, at this time the Company is unable to determine the impact this standard will have on the financial statements and related disclosures. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326). ASU 2016-13 changes how companies measure and recognize credit impairment for many financial assets. The new current expected credit loss model will require companies to immediately recognize an estimate of credit losses expected to occur over the remaining life of the financial assets that are in the scope of the standard. The ASU also makes targeted amendments to the current impairment model for available-for-sale debt securities. ASU 2016-13 will be required for reporting periods beginning after December 15, 2019. ASU 2016-13 will be applied in a modified retrospective approach through a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is implemented. The Company is currently assessing the future impact of ASU 2016-13. In February 2018, the FASB issued ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220), as a result of concerns raised due to the TCJA. More specifically, because the remeasurement of deferred taxes due to the change in the federal corporate income tax rate is required to be included in income from continuing operations, the tax effects of items within AOCI (referred to as stranded tax effects) do not reflect the appropriate tax rate. ASU 2018-02 generally allows companies to reclassify stranded taxes from AOCI to retained earnings. The amount of the adjustment would be the difference between the historical federal corporate income tax rate of 35% and the newly enacted 21% federal corporate income tax rate. The provisions of ASU 2018-02 are effective for fiscal years and interim periods within that reporting period beginning after December 15, 2018. Early adoption is permitted, including adoption in any interim periods for reporting periods for which financial statements have not been issued. The Company is currently evaluating the impact of ASU 2018-02 and its impact on regulated utilities. At March 31, 2018, stranded taxes in AOCI are approximately $7.2 million .

Revenue Recognition (Policies)

Revenue Recognition (Policies)	3 Months Ended
	Mar. 31, 2018
Revenue Recognition [Abstract]
Revenue Recognition, Policy	Revenues . 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 $20.0 million at March 31, 2018 and $22.2 million at December 31, 2017 . The Company presents revenues net of sales taxes in its statements of operations. 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 Public Utility Commission of Texas (the "PUCT") and the New Mexico Public Regulation Commission, (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 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 Federal Energy Regulatory Commission (“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.

Revenues Alternative Revenue Pr

Revenues Alternative Revenue Programs (Policies)	3 Months Ended
	Mar. 31, 2018
Revenue Recognition [Abstract]
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 .

Principles Of Preparation (Tabl

Principles Of Preparation (Tables)	3 Months Ended
	Mar. 31, 2018
Supplemental Cash Flow Information [Abstract]
Cash Flow, Supplemental Disclosures	Supplemental Cash Flow Disclosures (in thousands) Three Months Ended March 31, 2018 2017 Cash paid (received) for: Interest on long-term debt and borrowings under the revolving credit facility $ 11,967 $ 11,721 Income tax refunded, net (1,060 ) (697 ) Non-cash investing and financing activities: Changes in accrued plant additions (108 ) (3,335 ) Grants of restricted shares of common stock 513 540 Issuance of performance shares 1,499 932

Revenues (Table)

Revenues (Table)	3 Months Ended
	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Disaggregation of Revenue	The following table disaggregates revenue from contracts with customers, for the three and twelve months ended March 31, 2018 (in thousands): March 31, 2018 Three Months Ended Twelve Months Ended Retail $ 146,628 $ 826,148 Wholesale 24,143 72,641 Wheeling (transmission) 4,286 18,133 Total revenues from contracts with customers 175,057 916,922 Other 656 4,253 Total operating revenues $ 175,713 $ 921,175

Accumulated Other Comprehensi24

Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss)	Upon adoption of ASU 2016-01, Financial Instruments-Overall, the Company recorded, on January 1, 2018, a cumulative effect adjustment, net of income taxes, to increase retained earnings by $41.0 million with an offset to AOCI. Changes in Accumulated Other Comprehensive Income (Loss) (net of tax) by component are presented below (in thousands): Three Months Ended March 31, 2018 Three Months Ended March 31, 2017 Unrecognized Pension and Post-retirement Benefit Costs Net Unrealized Gains (Losses) on Debt Securities Net Losses on Cash Flow Hedges Accumulated Other Comprehensive Income (Loss) Unrecognized Pension and Post-retirement Benefit Costs Net Unrealized Gains (Losses) on Marketable Securities Net Losses on Cash Flow Hedges Accumulated Other Comprehensive Income (Loss) Balance at beginning of period as previously reported $ (17,790 ) $ 40,190 $ (11,342 ) $ 11,058 $ (23,928 ) $ 28,463 $ (11,651 ) $ (7,116 ) Cumulative effect adjustment — (41,028 ) — (41,028 ) — — — — Other comprehensive income before reclassifications — (2,159 ) — (2,159 ) — 6,165 — 6,165 Amounts reclassified from accumulated other comprehensive income (loss) (685 ) 404 89 (192 ) (529 ) (1,756 ) 52 (2,233 ) Balance at end of period $ (18,475 ) $ (2,593 ) $ (11,253 ) $ (32,321 ) $ (24,457 ) $ 32,872 $ (11,599 ) $ (3,184 ) Twelve Months Ended March 31, 2018 Twelve Months Ended March 31, 2017 Unrecognized Pension and Post-retirement Benefit Costs Net Unrealized Gains (Losses) on Marketable Securities Net Losses on Cash Flow Hedges Accumulated Other Comprehensive Income (Loss) Unrecognized Pension and Post-retirement Benefit Costs Net Unrealized Gains (Losses) on Marketable Securities Net Losses on Cash Flow Hedges Accumulated Other Comprehensive Income (Loss) Balance at beginning of period as previously reported $ (24,457 ) $ 32,872 $ (11,599 ) $ (3,184 ) $ (30,256 ) $ 28,394 $ (11,770 ) $ (13,632 ) Cumulative effect adjustment — (41,028 ) — (41,028 ) — — — — Other comprehensive income before reclassifications 7,951 11,927 — 19,878 7,363 11,327 — 18,690 Amounts reclassified from accumulated other comprehensive income (loss) (1,969 ) (6,364 ) 346 (7,987 ) (1,564 ) (6,849 ) 171 (8,242 ) Balance at end of period $ (18,475 ) $ (2,593 ) $ (11,253 ) $ (32,321 ) $ (24,457 ) $ 32,872 $ (11,599 ) $ (3,184 )
Reclassification out of Accumulated Other Comprehensive Income	Amounts reclassified from Accumulated Other Comprehensive Income (Loss) for the three and twelve months ended March 31, 2018 and 2017 are as follows (in thousands): Details about Accumulated Other Comprehensive Income (Loss) Components Three Months Ended March 31, Twelve Months Ended March 31, Affected Line Item in the Statements of Operations 2018 2017 2018 2017 Amortization of pension and post-retirement benefit costs: Prior service benefit $ 2,416 $ 2,416 $ 9,657 $ 8,157 Miscellaneous non-operating income Net loss (1,575 ) (1,694 ) (6,657 ) (5,436 ) Miscellaneous non-operating deductions 841 722 3,000 2,721 Income (loss) before income taxes Income tax effect (156 ) (193 ) (1,031 ) (1,157 ) Income tax (benefit) expense 685 529 1,969 1,564 Net income (loss) Marketable securities: Net realized gain (loss) on sale of securities (518 ) 2,191 7,917 8,443 Investment and interest income, net (518 ) 2,191 7,917 8,443 Income (loss) before income taxes Income tax effect 114 (435 ) (1,553 ) (1,594 ) Income tax (benefit) expense (404 ) 1,756 6,364 6,849 Net income (loss) Loss on cash flow hedge: Amortization of loss (139 ) (130 ) (541 ) (506 ) Interest on long-term debt and revolving credit facility (139 ) (130 ) (541 ) (506 ) Income (loss) before income taxes Income tax effect 50 78 195 335 Income tax (benefit) expense (89 ) (52 ) (346 ) (171 ) Net income (loss) Total reclassifications $ 192 $ 2,233 $ 7,987 $ 8,242

Palo Verde Palo Verde (Tables)

Palo Verde Palo Verde (Tables)	3 Months Ended
	Mar. 31, 2018
Jointly Owned Utility Plant Interests [Line Items]
Company Share of Recovered Costs	Amount Credited to Customers through Fuel Period Credited Costs Recovery Period Amount Refunded Adjustment Clauses to Customers July 2015 - June 2016 $ 1,779 $ 1,432 March 2017 July 2016 - June 2017 1,413 1,121 March 2018
Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Schedule of Jointly Owned Utility Plants	Palo Verde Operations and Maintenance Expense . Included in "operations and maintenance" in the Company's Statements of Operations are expenses associated with Palo Verde as follows (in thousands): 2018 2017 Three months ended March 31, $ 22,175 $ 21,608 Twelve months ended March 31, 99,931 96,179

Common Stock (Tables)

Common Stock (Tables)	3 Months Ended
	Mar. 31, 2018
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Schedule of Earnings Per Share, Basic and Diluted	Basic and Diluted Earnings Per Share . The basic and diluted earnings per share are presented below (in thousands except for share data): Three Months Ended March 31, Twelve Months Ended March 31, 2018 2017 2018 2017 Weighted average number of common shares outstanding: Basic number of common shares outstanding 40,491,194 40,387,235 40,440,189 40,366,024 Dilutive effect of unvested performance awards — — 123,436 69,665 Diluted number of common shares outstanding 40,491,194 40,387,235 40,563,625 40,435,689 Basic net income (loss) per common share: Net income (loss) $ (6,966 ) $ (3,989 ) $ 95,285 $ 98,587 Income allocated to participating restricted stock (48 ) (45 ) (353 ) (349 ) Net income (loss) available to common shareholders $ (7,014 ) $ (4,034 ) $ 94,932 $ 98,238 Diluted net income (loss) per common share: Net income (loss) $ (6,966 ) $ (3,989 ) $ 95,285 $ 98,587 Income reallocated to participating restricted stock (48 ) (45 ) (353 ) (349 ) Net income (loss) available to common shareholders $ (7,014 ) $ (4,034 ) $ 94,932 $ 98,238 Basic net income (loss) per common share: Distributed earnings $ 0.335 $ 0.31 $ 1.34 $ 1.24 Undistributed earnings (losses) (0.505 ) (0.41 ) 1.01 1.19 Basic net income (loss) per common share $ (0.170 ) $ (0.10 ) $ 2.35 $ 2.43 Diluted net income (loss) per common share: Distributed earnings $ 0.335 $ 0.31 $ 1.34 $ 1.24 Undistributed earnings (losses) (0.505 ) (0.41 ) 1.00 1.19 Diluted net income (loss) per common share $ (0.170 ) $ (0.10 ) $ 2.34 $ 2.43
Schedule of Antidilutive Securities Excluded From Computation Of Earnings Per Share	The number of restricted stock awards and performance shares at 100% performance level excluded from the calculation of the diluted number of common shares outstanding because their effect was antidilutive is presented below: Three Months Ended Twelve Months Ended March 31, March 31, 2018 2017 2018 2017 Restricted stock awards 72,218 78,025 66,288 58,344 Performance shares (a) 45,977 — 11,494 47,246 ________________________ (a) Certain performance shares were excluded from the computation of diluted earnings per share as no payouts would have been required based upon performance at the end of each corresponding period.

Employee Benefits (Tables)

Employee Benefits (Tables)	3 Months Ended
	Mar. 31, 2018
Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Schedule of Net Benefit Costs	The net periodic benefit cost recognized for the three and twelve months ended March 31, 2018 and 2017 , is made up of the components listed below as determined using the projected unit credit actuarial cost method (in thousands): Three Months Ended Twelve Months Ended March 31, March 31, 2018 2017 2018 2017 Components of net periodic benefit cost: Service cost $ 2,758 $ 2,270 $ 9,006 $ 8,366 Interest cost 3,223 3,248 13,034 13,022 Expected return on plan assets (5,315 ) (4,808 ) (19,696 ) (18,975 ) Amortization of: Net loss 2,100 2,089 8,465 7,540 Prior service benefit (878 ) (878 ) (3,506 ) (3,506 ) Net periodic benefit cost $ 1,888 $ 1,921 $ 7,303 $ 6,447
Other Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Schedule of Net Benefit Costs	The net periodic benefit recognized for the three and twelve months ended March 31, 2018 and 2017 , is made up of the components listed below (in thousands): Three Months Ended Twelve Months Ended March 31, March 31, 2018 2017 2018 2017 Components of net periodic benefit: Service cost $ 700 $ 588 $ 2,348 $ 2,642 Interest cost 565 678 2,610 2,972 Expected return on plan assets (613 ) (470 ) (2,050 ) (1,845 ) Amortization of: Prior service benefit (1,538 ) (1,538 ) (6,151 ) (4,651 ) Net gain (525 ) (395 ) (1,808 ) (2,104 ) Net periodic benefit $ (1,411 ) $ (1,137 ) $ (5,051 ) $ (2,986 )

Financial Instruments And Inv28

Financial Instruments And Investments (Tables)	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Fair Values Of Long-Term Debt And Short-Term Borrowings Under the RCF	The fair values of the Company's long-term debt and short-term borrowings under the RCF are based on estimated market prices for similar issues and are presented below (in thousands): March 31, 2018 December 31, 2017 Carrying Amount Estimated Fair Value Carrying Amount Estimated Fair Value Pollution Control Bonds $ 157,699 $ 166,779 $ 157,676 $ 169,186 Senior Notes 993,514 1,147,504 993,426 1,211,922 RGRT Senior Notes (1) 44,897 46,620 44,886 47,070 RCF (1) 233,067 233,067 173,533 173,533 Total $ 1,429,177 $ 1,593,970 $ 1,369,521 $ 1,601,711 _______________ (1) Nuclear fuel financing, as of March 31, 2018 and December 31, 2017 , is funded through $45 million Rio Grande Resources Trust ("RGRT") Senior Notes and $89.1 million and $88.5 million , respectively, under the RCF. As of March 31, 2018 and December 31, 2017 , $144.0 million and $85.0 million , respectively, was outstanding under the RCF for working capital or general corporate purposes. The interest rate on the Company's borrowings under the RCF is reset throughout the quarter reflecting c urrent market rates. Consequently, the carrying value approximates fair value.
Unrecognized Gross Unrealized Gains (Losses) And the Fair Value	The tables below present the gross unrealized losses and the fair value of these securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position (in thousands): March 31, 2018 Less than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Description of Securities (1) : Federal Agency Mortgage Backed Securities $ 5,995 $ (114 ) $ 10,376 $ (407 ) $ 16,371 $ (521 ) U.S. Government Bonds 34,018 (940 ) 18,325 (1,215 ) 52,343 (2,155 ) Municipal Debt Obligations 4,227 (109 ) 7,031 (581 ) 11,258 (690 ) Corporate Debt Obligations 20,855 (609 ) 3,769 (301 ) 24,624 (910 ) Total $ 65,095 $ (1,772 ) $ 39,501 $ (2,504 ) $ 104,596 $ (4,276 ) _________________ (1) Includes 147 securities. December 31, 2017 Less than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Description of Securities (2) : Federal Agency Mortgage Backed Securities $ 4,700 $ (46 ) $ 10,099 $ (165 ) $ 14,799 $ (211 ) U.S. Government Bonds 28,866 (416 ) 18,186 (969 ) 47,052 (1,385 ) Municipal Debt Obligations 4,290 (73 ) 9,736 (742 ) 14,026 (815 ) Corporate Debt Obligations 10,685 (107 ) 4,475 (331 ) 15,160 (438 ) Total Debt Securities 48,541 (642 ) 42,496 (2,207 ) 91,037 (2,849 ) Domestic Equity Securities 962 (210 ) — — 962 (210 ) Total $ 49,503 $ (852 ) $ 42,496 $ (2,207 ) $ 91,999 $ (3,059 ) _________________ (2) Includes 146 securities.
Unrealized Holding Losses Included In Pre-Tax Income	For the three and twelve months ended March 31, 2018 and 2017 , the Company recognized other than temporary impairment losses on its available-for-sale securities as follows (in thousands): Three Months Ended Twelve Months Ended March 31, March 31, 2018 2017 2018 2017 Unrealized holding losses included in pre-tax income $ — $ — $ — $ (196 )
Marketable Securities Fair Value and Unrecognized Gross Unrealized Gain	The table below presents the unrecognized gross unrealized gains and the fair value of these securities, aggregated by investment category (in thousands): March 31, 2018 December 31, 2017 Fair Value Unrealized Gains Fair Value Unrealized Gains Description of Securities: Federal Agency Mortgage Backed Securities $ 9,469 $ 143 $ 5,933 $ 203 U.S. Government Bonds 2,205 92 11,129 256 Municipal Debt Obligations 1,073 92 2,558 109 Corporate Debt Obligations 11,444 546 19,514 1,067 Total Debt Securities 24,191 873 39,134 1,635 Domestic Equity Securities — — 120,065 45,587 International Equity Securities — — 28,804 5,908 Cash and Cash Equivalents — — 6,864 — Total $ 24,191 $ 873 $ 194,867 $ 53,130
Contractual Year For Maturity Of Available-For-Sale Securities	The contractual year for maturity of these available-for-sale debt securities as of March 31, 2018 , is as follows (in thousands): Total 2018 2019 2023 through 2027 2028 and Beyond Federal Agency Mortgage Backed Securities $ 25,840 $ — $ 16 $ 261 $ 25,563 U.S. Government Bonds 54,548 1,894 28,263 11,415 12,976 Municipal Debt Obligations 12,331 76 5,483 5,735 1,037 Corporate Debt Obligations 36,068 213 17,518 7,151 11,186
Sale of Securities And The Related Effects On Pre-Tax Income	The proceeds from the sale of these securities during the three and twelve months ended March 31, 2018 and 2017 , and the related effects on pre-tax income are as follows (in thousands): Three Months Ended Twelve Months Ended March 31, March 31, 2018 2017 2018 2017 Proceeds from sales or maturities of available-for-sale securities $ 11,757 $ 26,055 $ 82,739 $ 93,245 Gross realized gains included in pre-tax income $ 9 $ 2,587 $ 9,195 $ 9,968 Gross realized losses included in pre-tax income (527 ) (396 ) (1,278 ) (1,329 ) Gross unrealized losses included in pre-tax income — — — (196 ) Net gains included in pre-tax income $ (518 ) $ 2,191 $ 7,917 $ 8,443
Unrealized Gain (Loss) on Investments	The unrealized gains and losses recognized during the three months ended March 31, 2018 and related effects on pre-tax income are as follows (in thousands): Three Months Ended March 31, 2018 Net gains and (losses) recognized on equity securities $ (1,991 ) Less: Net gains and (losses) recognized on equity securities sold 1,790 Unrealized gains and (losses) recognized on equity securities still held at reporting date $ (3,781 )
Fair Value, Measurement Inputs, Disclosure	The fair value of the NDT and investments in debt securities at March 31, 2018 and December 31, 2017 , and the level within the three levels of the fair value hierarchy defined by the FASB guidance are presented in the table below (in thousands): Description of Securities Fair Value as of March 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Trading Securities: Investments in Debt Securities $ 1,726 $ — $ — $ 1,726 Equity Securities: Domestic $ 119,344 $ 119,344 $ — $ — International 28,542 28,542 — — Total Equity Securities 147,886 147,886 — — Available for Sale Debt Securities: Federal Agency Mortgage Backed Securities 25,840 — 25,840 — U.S. Government Bonds 54,548 54,548 — — Municipal Debt Obligations 12,331 — 12,331 — Corporate Debt Obligations 36,068 — 36,068 — Total Available for Sale Debt Securities 128,787 54,548 74,239 — Cash and Cash Equivalents 7,409 7,409 — — Total $ 284,082 $ 209,843 $ 74,239 $ — Description of Securities Fair Value as of December 31, 2017 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Trading Securities: Investments in Debt Securities $ 1,735 $ — $ — $ 1,735 Available for Sale: Federal Agency Mortgage Backed Securities $ 20,732 $ — $ 20,732 $ — U.S. Government Bonds 58,181 58,181 — — Municipal Debt Obligations 16,584 — 16,584 — Corporate Debt Obligations 34,674 — 34,674 — Subtotal, Debt Securities 130,171 58,181 71,990 — Domestic 121,027 121,027 — — International 28,804 28,804 — — Subtotal, Equity Securities 149,831 149,831 — — Cash and Cash Equivalents 6,864 6,864 — — Total $ 286,866 $ 214,876 $ 71,990 $ —

Principles Of Preparation (Accr

Principles Of Preparation (Accrued Unbilled Revenues) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Accrued unbilled revenues	$ 20	$ 22.2

Principles Of Preparation (Depr

Principles Of Preparation (Depreciation) (Details)	3 Months Ended
	Mar. 31, 2018
Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	48 years

Principles Of Preparation Narra

Principles Of Preparation Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Jan. 01, 2018	Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Proceeds from insurance settlement, investing activities	$ 4,175	$ 742
Federal statutory tax rate	21.00%	35.00%	35.00%
Decrease in accumulated other comprehensive income	$ (32,321)		$ 11,058
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in operating loss carryforward deferred tax assets					$ 200
Increase in retained earnings					$ 200
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of reimbursable expense	1,900
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings				$ 41,000
Decrease in accumulated other comprehensive income				$ (41,000)
Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Proceeds from insurance settlement, investing activities	4,175	$ 742
Potential Impact of Adopting Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in accumulated other comprehensive income	(7,200)
Decommisioning Fund Investments | US Treasury and Government | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unrealized loss on securities	$ 3,800

Principles Of Preparation (Supp

Principles Of Preparation (Supplemental Cash Flow Disclosures) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Supplemental Cash Flow Information [Abstract]
Interest on long-term debt and borrowings under the revolving credit facility	$ 11,967	$ 11,721
Proceeds from income tax refunds	(1,060)	(697)
Changes in accrued plant additions	(108)	(3,335)
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock issued	513	540
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock issued	$ 1,499	$ 932

Disaggregation of Revenues (Det

Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 175,057		$ 916,922
Other	656		4,253
Total operating revenues	175,713	$ 171,335	921,175	$ 900,462
Retail
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	146,628		826,148
Wholesale
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	24,143		72,641
Transmission
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 4,286		$ 18,133

Revenues Narrative (Details)

Revenues Narrative (Details)	3 Months Ended
	Mar. 31, 2018
Recovery period per ASC 980-605-25 | Maximum
Disaggregation of Revenue [Line Items]
Surcharge Or Refund Period	24 months
Retail
Disaggregation of Revenue [Line Items]
Customer Payment Terms	16 days
Wholesale
Disaggregation of Revenue [Line Items]
Number Of Customers	1
Wholesale | Maximum
Disaggregation of Revenue [Line Items]
Customer Payment Terms	10 days
Transmission | Maximum
Disaggregation of Revenue [Line Items]
Customer Payment Terms	20 days
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year

Revenues Accounts Receivable (D

Revenues Accounts Receivable (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2018
Revenue Recognition [Abstract]
Provision for Doubtful Accounts	$ 0.5	$ 3.2

Accumulated Other Comprehensi36

Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Jan. 01, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	$ 11,058
Accumulated Other Comprehensive Income (Loss), ending balance	(32,321)		$ (32,321)
Accumulated Defined Benefit Plans Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					$ 0	$ 0
Accumulated Net Unrealized Investment Gain (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					(41,028)	0
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					0	0
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					$ (41,028)	$ 0
Accumulated Defined Benefit Plans Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(17,790)	$ (23,928)	(24,457)	$ (30,256)
Other Comprehensive Income (Loss) before Reclassifications	0	0	7,951	7,363
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(685)	(529)	(1,969)	(1,564)
Accumulated Other Comprehensive Income (Loss), ending balance	(18,475)	(24,457)	(18,475)	(24,457)
Accumulated Net Unrealized Investment Gain (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	40,190	28,463	32,872	28,394
Other Comprehensive Income (Loss) before Reclassifications	(2,159)	6,165	11,927	11,327
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	404	(1,756)	(6,364)	(6,849)
Accumulated Other Comprehensive Income (Loss), ending balance	(2,593)	32,872	(2,593)	32,872
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(11,342)	(11,651)	(11,599)	(11,770)
Other Comprehensive Income (Loss) before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	89	52	346	171
Accumulated Other Comprehensive Income (Loss), ending balance	(11,253)	(11,599)	(11,253)	(11,599)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	11,058	(7,116)	(3,184)	(13,632)
Other Comprehensive Income (Loss) before Reclassifications	(2,159)	6,165	19,878	18,690
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(192)	(2,233)	(7,987)	(8,242)
Accumulated Other Comprehensive Income (Loss), ending balance	$ (32,321)	$ (3,184)	$ (32,321)	$ (3,184)

Accumulated Other Comprehensi37

Accumulated Other Comprehensive Income Amounts Reclassified (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (loss) before income taxes	$ (10,018)	$ (6,277)	$ 145,525	$ 153,378
Income tax expense (benefit)	3,052	2,288	(50,240)	(54,791)
Net income (loss)	(6,966)	(3,989)	95,285	98,587
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	2,416	2,416	9,657	8,157
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(1,575)	(1,694)	(6,657)	(5,436)
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	841	722	3,000	2,721
Reclassification from AOCI, Current Period, Tax	(156)	(193)	(1,031)	(1,157)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	685	529	1,969	1,564
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(404)	1,756	6,364	6,849
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(89)	(52)	(346)	(171)
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	192	2,233	7,987	8,242
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net realized gain on sale of securities	(518)	2,191	7,917	8,443
Income (loss) before income taxes	(518)	2,191	7,917	8,443
Income tax expense (benefit)	114	(435)	(1,553)	(1,594)
Net income (loss)	(404)	1,756	6,364	6,849
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (loss) before income taxes	(139)	(130)	(541)	(506)
Income tax expense (benefit)	50	78	195	335
Net income (loss)	(89)	(52)	(346)	(171)
Interest Rate Contract | Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest on long-term debt and revolving credit facility	$ (139)	$ (130)	$ (541)	$ (506)

Regulation (Narrative) (Details

Regulation (Narrative) (Details)

May 01, 2018USD ($)

Apr. 25, 2018USD ($)

Apr. 13, 2018

Mar. 15, 2018

Mar. 01, 2018USD ($)

Feb. 15, 2018USD ($)

Jan. 01, 2018

Dec. 18, 2017USD ($)

Nov. 01, 2017

Aug. 01, 2017

Jun. 29, 2017

Feb. 22, 2017USD ($)

Jan. 10, 2017

Sep. 27, 2016USD ($)

Aug. 25, 2016USD ($)

Jul. 01, 2016USD ($)

Jun. 08, 2016USD ($)

Mar. 31, 2018USD ($)MW

Sep. 30, 2017USD ($)

Jun. 30, 2017USD ($)

Mar. 31, 2017USD ($)

Mar. 31, 2018USD ($)

Dec. 31, 2017USD ($)

Mar. 20, 2018MW

Public Utilities, Requested Rate Decrease, Amount

$ 22,700,000

Nuclear Decommissioning Funding, Annual Recovery

$ 33,578,000

$ 28,482,000

Over-collection of fuel revenues

$ 14,190,000

$ 14,190,000

$ 6,225,000

Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent

21.00%

35.00%

35.00%

PUCT

Over-collection of fuel revenues

$ 13,300,000

$ 13,300,000

Reconcilable Fuel Expense

$ 271,300,000

Fuel Reconciliation Period

24 months

Fuel Over And Under Materiality Threshold Percentage

4.00%

4.00%

PUCT | Community Solar At Montana Power Station

Electric Capacity | MW

3

Number of customers subscribed

1,500

1,500

Property, Plant and Equipment, Operational Date

May 31, 2017

PUCT | Community Solar at Newman

Electric capacity, requested | MW

2

PUCT | Solar Facility At Newman Power Station

Electric Capacity | MW

10

NMPRC

Over-collection of fuel revenues

$ 900,000

$ 900,000

Reconcilable Fuel Expense

$ 181,000,000

Fuel Reconciliation Period

39 months

NMPRC | Holloman

Electric Capacity | MW

5

FERC

Show Cause Orders issued, Number

2

Number of public utilities affected

48

2016 Energy Efficiency Programs | PUCT

Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program

$ 800,000

2016 Energy Efficiency Programs | NMPRC

Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program

$ 400,000

Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program

$ 400,000

Fixed Fuel Factor | PUCT

Increase (Decrease) In Fixed Fuel Factor, Percentage

(19.00%)

Fixed Fuel Factor | PUCT | Subsequent Event

Public Utilities, Requested Rate Increase (Decrease), Percentage

(29.00%)

2017 Energy Efficiency Programs | PUCT | Subsequent Event

Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program

$ 1,100,000

2017 Energy Efficiency Programs | NMPRC

Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program

$ 400,000

Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program

$ 400,000

2015 Energy Efficiency Programs | NMPRC

Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program

$ 300,000

Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program

$ 300,000

Public Utilities, Regulatory Rate Making Impact | PUCT

Recovery of rate case expenses

$ 3,100,000

Period To Relate Back New Rates

80 days

Fuel Reconciliation Period

167 days

Public Utilities, Regulatory Rate Making Impact | NMPRC

Public Utilities, Approved Return on Equity, Percentage

9.48%

Public Utilities, Regulatory Rate Making Impact | Non-Fuel Base Rate | PUCT | Four Corners Generating Station

Public Utilities, Approved Rate Increase (Decrease), Amount

$ 3,700,000

Regulatory Current Asset, End Date for Recovery

Jul. 11, 2017

Public Utilities, Regulatory Rate Making Impact | Non-Fuel Base Rate | NMPRC

Public Utilities, Approved Rate Increase (Decrease), Amount

$ 600,000

Public Utilities, Regulatory Rate Making Impact | Utility Cost Of Service | NMPRC

Public Utilities, Approved Rate Increase (Decrease), Amount

$ 500,000

PUCT Docket No. 46831

Recovery of rate case expenses

$ 3,400,000

Surcharge Or Refund Period

3 years

Minimum Distributive Generation Charge

$ 30

Nuclear Decommissioning Funding, Annual Recovery

2,100,000

Requested Depreciation Expense, Decrease

$ 1,900,000

Public Utilities, Approved Return on Equity, Percentage

9.65%

PUCT Docket No. 46831 | Four Corners decommissioning [Member]

Decommissioning Costs Increase (Decrease)

$ 5,500,000

Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided

7 years

PUCT Docket No. 46831 | Non-Fuel Base Rate

Public Utilities, Approved Rate Increase (Decrease), Amount

$ 14,500,000

PUCT Docket No. 46610 | Fixed Fuel Factor

Increase (Decrease) In Fixed Fuel Factor, Percentage

28.80%

PUCT Docket No. 46308

Reconcilable Fuel Expense

$ 436,600,000

Fuel Reconciliation Period

36 months

36 months

Fuel reconciliation effect, net amount

$ 5,000,000

Palo Verde Performance Award Period

3 years

NMPRC Case No. 15-00127-UT

Fuel Reconciliation Period

24 months

NMPRC Case No. 18-00016-UT | Subsequent Event

Public Utilities, Approved Rate Increase (Decrease), Amount

$ (4,900,000)

Maximum | FERC

Period to comply with order from regulatory agency

60 days

Palo Verde (Narrative) (Detail

Palo Verde (Narrative) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Oct. 31, 2017	Mar. 31, 2017	Oct. 31, 2016	Aug. 18, 2014	Dec. 19, 2012	Mar. 31, 2018	Mar. 31, 2017
Jointly Owned Utility Plant Interests [Line Items]
Refund passed through to customers							$ 7,965	$ 8,530
Suit Against Department of Energy for Spent Fuel | Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Settlement Agreement, Counterparty's Name						United States Department of Energy
Loss Contingency, Settlement Agreement, Date					August 18, 2014
Refund passed through to customers	$ 1,121		$ 1,432
Proceeds from Legal Settlements	$ 1,413		$ 1,779
Suit Against Department of Energy for Spent Fuel | Minimum
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered		July 1, 2016		July 1, 2015
Suit Against Department of Energy for Spent Fuel | Maximum
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered		June 30, 2017		June 30, 2016

Palo Verde Operations and Maint

Palo Verde Operations and Maintenance Expense (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Jointly Owned Utility Plant Interests [Line Items]
Utilities Operating Expense, Maintenance and Operations	$ 80,160	$ 79,187	$ 321,254	$ 317,518
Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Utilities Operating Expense, Maintenance and Operations	$ 22,175	$ 21,608	$ 99,931	$ 96,179

Common Stock (Narrative) (Detai

Common Stock (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Dividends, Common Stock [Abstract]
Dividends, Common Stock, Cash	$ 13,615	$ 12,565	$ 54,300	$ 50,300

Common Stock (Basic And Diluted

Common Stock (Basic And Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Basic number of common shares outstanding	40,491,194	40,387,235	40,440,189	40,366,024
Dilutive effect of unvested performance awards	0	0	123,436	69,665
Diluted number of common shares outstanding	40,491,194	40,387,235	40,563,625	40,435,689
Net income (loss)	$ (6,966)	$ (3,989)	$ 95,285	$ 98,587
Income allocated to participating restricted stock	(48)	(45)	(353)	(349)
Net income (loss) available to common shareholders, basic	(7,014)	(4,034)	94,932	98,238
Income reallocated to participating restricted stock	(48)	(45)	(353)	(349)
Net income (loss) available to common shareholders, diluted	$ (7,014)	$ (4,034)	$ 94,932	$ 98,238
Earnings Per Share, Basic, Distributed	$ 0.335	$ 0.31	$ 1.34	$ 1.24
Earnings (losses) Per Share, Basic, Undistributed	(0.505)	(0.41)	1.01	1.19
Basic net income (loss) per common share	(0.170)	(0.10)	2.35	2.43
Earnings Per Share, Diluted, Distributed	0.335	0.31	1.34	1.24
Earnings (losses) Per Share, Diluted, Undistributed	(0.505)	(0.41)	1	1.19
Diluted net income (loss) per common share	$ (0.170)	$ (0.10)	$ 2.34	$ 2.43

Common Stock (Antidilutive Secu

Common Stock (Antidilutive Securities Excluded From Computation Of Earnings Per Share) (Details) - shares		3 Months Ended		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Performance shares payout based upon performance		0	0	0	0
Performance shares payout level		100.00%	100.00%	100.00%	100.00%
Restricted Stock
Antidilutive Securities Excluded from the Computation of Diluted Earnings Per Share, Amount		72,218	78,025	66,288	58,344
Performance Shares
Antidilutive Securities Excluded from the Computation of Diluted Earnings Per Share, Amount	[1]	45,977	0	11,494	47,246
[1]	Certain performance shares were excluded from the computation of diluted earnings per share as no payouts would have been required based upon performance at the end of each corresponding period.

Income Taxes (Income Tax Rates)

Income Taxes (Income Tax Rates) (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Income Tax [Line Items]
Effective tax rate	30.50%	36.50%	34.50%		35.70%
Federal statutory tax rate	21.00%	35.00%		35.00%
Income Tax Rate Capital Gains	20.00%	20.00%	20.00%		20.00%
Maximum
Income Tax [Line Items]
Period in which adjustments related to the Federal Tax Cuts and Jobs Act of 2017 needed to be made	12 months

Income Taxes (Details)

Income Taxes (Details) - Latest Tax Year	3 Months Ended
	Mar. 31, 2018
Internal Revenue Service (IRS)
Income Tax [Line Items]
Tax Year Closed	2,012
TEXAS
Income Tax [Line Items]
Tax Year Closed	2,012
NEW MEXICO
Income Tax [Line Items]
Tax Year Closed	2,012
ARIZONA
Income Tax [Line Items]
Tax Year Closed	2,012

Commitments, Contingencies An46

Commitments, Contingencies And Uncertainties (Narrative) (Details) - USD ($) $ in Millions	Aug. 17, 2015	Mar. 31, 2018
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies		$ 0.2
Four Corners Coal Plant Participants
Loss Contingencies [Line Items]
Loss Contingency, Civil Penalty Amount	$ 1.5
Loss Contingency Environmental Mitigation Project Cost	$ 6.7

Employee Benefits ASU Narrative

Employee Benefits ASU Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance	$ 80,160	$ 79,187	$ 321,254	$ 317,518
Investment and interest income, net	5,155	9,263	34,745	35,959
Miscellaneous non-operating income	3,136	2,895	12,292	10,983
Miscellaneous non-operating deductions	2,743	2,828	11,494	11,513
Other interest	$ 4,654	4,345	$ 18,479	17,155
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance		2,100		7,500
Investment and interest income, net		5,300		20,800
Miscellaneous non-operating income		2,800		10,300
Miscellaneous non-operating deductions		2,100		7,500
Other interest		3,900		16,000
Service Cost Component		2,900		11,000
Net periodic benefit cost previously reported in operating income (including non-service cost components)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance		$ 800		$ 3,500

Employee Benefits (Retirement P

Employee Benefits (Retirement Plans And Other Postretirement Benefits) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,758	$ 2,270	$ 9,006	$ 8,366
Interest cost	3,223	3,248	13,034	13,022
Expected return on plan assets	(5,315)	(4,808)	(19,696)	(18,975)
Amortization of net (gain) loss	2,100	2,089	8,465	7,540
Amortization of prior service benefit	(878)	(878)	(3,506)	(3,506)
Net periodic benefit cost (benefit)	1,888	1,921	7,303	6,447
Other Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	700	588	2,348	2,642
Interest cost	565	678	2,610	2,972
Expected return on plan assets	(613)	(470)	(2,050)	(1,845)
Amortization of net (gain) loss	(525)	(395)	(1,808)	(2,104)
Amortization of prior service benefit	(1,538)	(1,538)	(6,151)	(4,651)
Net periodic benefit cost (benefit)	$ (1,411)	$ (1,137)	$ (5,051)	$ (2,986)

Employee Benefits (Narrative) (

Employee Benefits (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)
Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Employer Contribution	$ 2.9
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year	9.4
Other Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Employer Contribution	0.1
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year	$ 0.5

Financial Instruments And Inv50

Financial Instruments And Investments (Carrying Amount and Fair Value Amounts) (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		$ 1,429,177	$ 1,369,521
Carrying (Reported) Amount, Fair Value Disclosure | Pollution Control Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		157,699	157,676
Carrying (Reported) Amount, Fair Value Disclosure | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		993,514	993,426
Carrying (Reported) Amount, Fair Value Disclosure | RGRT Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1]	44,897	44,886
Carrying (Reported) Amount, Fair Value Disclosure | Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1]	233,067	173,533
Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		1,593,970	1,601,711
Estimate of Fair Value Measurement | Pollution Control Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		166,779	169,186
Estimate of Fair Value Measurement | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		1,147,504	1,211,922
Estimate of Fair Value Measurement | RGRT Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1]	46,620	47,070
Estimate of Fair Value Measurement | Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1]	$ 233,067	$ 173,533
[1]	Nuclear fuel financing, as of March 31, 2018 and December 31, 2017, is funded through $45 million Rio Grande Resources Trust ("RGRT") Senior Notes and $89.1 million and $88.5 million, respectively, under the RCF. As of March 31, 2018 and December 31, 2017, $144.0 million and $85.0 million, respectively, was outstanding under the RCF for working capital or general corporate purposes. The interest rate on the Company's borrowings under the RCF is reset throughout the quarter reflecting current market rates. Consequently, the carrying value approximates fair value.

Financial Instruments And Inv51

Financial Instruments And Investments (Long-Term Debt and Short-Term Borrowings - Narratives) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments	$ 284,082	$ 286,866
Nuclear Fuel Financing
Financial Instruments and Investments [Line Items]
Line of Credit Facility, Amount Outstanding	89,100	88,500
Working Capital and General Purpose
Financial Instruments and Investments [Line Items]
Line of Credit Facility, Amount Outstanding	144,000	85,000
RGRT Senior Notes
Financial Instruments and Investments [Line Items]
Long-term Debt, Gross	$ 45,000	$ 45,000

Financial Instruments And Inv52

Financial Instruments And Investments (Marketable Securities Fair Value And Unrealized Losses) (Details) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2018USD ($)securities		Dec. 31, 2017USD ($)securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months	[1]			$ 49,503
Marketable Securities Unrealized Losses Less Than 12 Months	[1]			(852)
Marketable Securities Fair Value 12 Months or Longer	[1]			42,496
Marketable Securities Unrealized Losses 12 Months or Longer	[1]			(2,207)
Marketable Securities Fair Value Total	[1]			91,999
Marketable Securities Unrealized Losses Total	[1]			$ (3,059)
Number of securities | securities		147		146
Debt Securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months		$ 65,095	[2]	$ 48,541	[1]
Marketable Securities Unrealized Losses Less Than 12 Months		(1,772)	[2]	(642)	[1]
Marketable Securities Fair Value 12 Months or Longer		39,501	[2]	42,496	[1]
Marketable Securities Unrealized Losses 12 Months or Longer		(2,504)	[2]	(2,207)	[1]
Marketable Securities Fair Value Total		104,596	[2]	91,037	[1]
Marketable Securities Unrealized Losses Total		(4,276)	[2]	(2,849)	[1]
Federal Agency Mortgage Backed Securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months		5,995	[2]	4,700	[1]
Marketable Securities Unrealized Losses Less Than 12 Months		(114)	[2]	(46)	[1]
Marketable Securities Fair Value 12 Months or Longer		10,376	[2]	10,099	[1]
Marketable Securities Unrealized Losses 12 Months or Longer		(407)	[2]	(165)	[1]
Marketable Securities Fair Value Total		16,371	[2]	14,799	[1]
Marketable Securities Unrealized Losses Total		(521)	[2]	(211)	[1]
U.S. Government Bonds
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months		34,018	[2]	28,866	[1]
Marketable Securities Unrealized Losses Less Than 12 Months		(940)	[2]	(416)	[1]
Marketable Securities Fair Value 12 Months or Longer		18,325	[2]	18,186	[1]
Marketable Securities Unrealized Losses 12 Months or Longer		(1,215)	[2]	(969)	[1]
Marketable Securities Fair Value Total		52,343	[2]	47,052	[1]
Marketable Securities Unrealized Losses Total		(2,155)	[2]	(1,385)	[1]
Municipal Obligations
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months		4,227	[2]	4,290	[1]
Marketable Securities Unrealized Losses Less Than 12 Months		(109)	[2]	(73)	[1]
Marketable Securities Fair Value 12 Months or Longer		7,031	[2]	9,736	[1]
Marketable Securities Unrealized Losses 12 Months or Longer		(581)	[2]	(742)	[1]
Marketable Securities Fair Value Total		11,258	[2]	14,026	[1]
Marketable Securities Unrealized Losses Total		(690)	[2]	(815)	[1]
Corporate Obligations
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months		20,855	[2]	10,685	[1]
Marketable Securities Unrealized Losses Less Than 12 Months		(609)	[2]	(107)	[1]
Marketable Securities Fair Value 12 Months or Longer		3,769	[2]	4,475	[1]
Marketable Securities Unrealized Losses 12 Months or Longer		(301)	[2]	(331)	[1]
Marketable Securities Fair Value Total		24,624	[2]	15,160	[1]
Marketable Securities Unrealized Losses Total		$ (910)	[2]	(438)	[1]
Domestic Equity Securities | Equity Securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months	[1]			962
Marketable Securities Unrealized Losses Less Than 12 Months	[1]			(210)
Marketable Securities Fair Value 12 Months or Longer	[1]			0
Marketable Securities Unrealized Losses 12 Months or Longer	[1]			0
Marketable Securities Fair Value Total	[1]			962
Marketable Securities Unrealized Losses Total	[1]			$ (210)
[1]	Includes 146 securities.
[2]	Includes 147 securities.

Financial Instruments And Inv53

Financial Instruments And Investments (Unrealized Holding Losses Included In Pre-Tax Income) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Financial Instruments And Investments [Abstract]
Unrealized losses included in pre-tax income	$ 0	$ 0	$ 0	$ (196)

Financial Instruments And Inv54

Financial Instruments And Investments (Marketable Securities Fair Value and Unrecognized Gross Unrealized Gains) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financial Instruments and Investments [Line Items]
Available-for-sale Securities, Gross Unrealized Gain	$ 873	$ 53,130
Marketable Securities Fair Value, Total	24,191	194,867
Debt Securities
Financial Instruments and Investments [Line Items]
Available-for-sale Securities, Unrealized Gain Position, Fair Value	24,191	39,134
Available-for-sale Securities, Gross Unrealized Gain	873	1,635
Federal Agency Mortgage Backed Securities
Financial Instruments and Investments [Line Items]
Available-for-sale Securities, Unrealized Gain Position, Fair Value	9,469	5,933
Available-for-sale Securities, Gross Unrealized Gain	143	203
U.S. Government Bonds
Financial Instruments and Investments [Line Items]
Available-for-sale Securities, Unrealized Gain Position, Fair Value	2,205	11,129
Available-for-sale Securities, Gross Unrealized Gain	92	256
Municipal Obligations
Financial Instruments and Investments [Line Items]
Available-for-sale Securities, Unrealized Gain Position, Fair Value	1,073	2,558
Available-for-sale Securities, Gross Unrealized Gain	92	109
Corporate Obligations
Financial Instruments and Investments [Line Items]
Available-for-sale Securities, Unrealized Gain Position, Fair Value	11,444	19,514
Available-for-sale Securities, Gross Unrealized Gain	546	1,067
Equity Securities | Domestic Equity Securities
Financial Instruments and Investments [Line Items]
Available-for-sale Securities, Unrealized Gain Position, Fair Value	0	120,065
Available-for-sale Securities, Gross Unrealized Gain	0	45,587
Equity Securities | International Equity Securities
Financial Instruments and Investments [Line Items]
Available-for-sale Securities, Unrealized Gain Position, Fair Value	0	28,804
Available-for-sale Securities, Gross Unrealized Gain	0	5,908
Cash | Held in trust Decommissioning Fund Investment
Financial Instruments and Investments [Line Items]
Available-for-sale Securities, Gross Unrealized Gain	0	0
Cash	$ 0	$ 6,864

Financial Instruments And Inv55

Financial Instruments And Investments (Contractual Year For Maturity Of Available-For-Sale Securities ) (Details) $ in Thousands	Mar. 31, 2018USD ($)
Federal Agency Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Total	$ 25,840
2,018	0
2019 through 2022	16
2023 through 2027	261
2028 and Beyond	25,563
U.S. Government Bonds
Schedule of Available-for-sale Securities [Line Items]
Total	54,548
2,018	1,894
2019 through 2022	28,263
2023 through 2027	11,415
2028 and Beyond	12,976
Municipal Obligations
Schedule of Available-for-sale Securities [Line Items]
Total	12,331
2,018	76
2019 through 2022	5,483
2023 through 2027	5,735
2028 and Beyond	1,037
Corporate Obligations
Schedule of Available-for-sale Securities [Line Items]
Total	36,068
2,018	213
2019 through 2022	17,518
2023 through 2027	7,151
2028 and Beyond	$ 11,186

Financial Instruments And Inv56

Financial Instruments And Investments (Sale Of Securities And The Related Effects On Pre-Tax Income ) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Investment [Line Items]
Proceeds from sales or maturities of available-for-sale securities		$ 26,055	$ 82,739	$ 93,245
Proceeds from Sale of Available-for-sale Securities, Debt	$ 11,757
Gross realized gains included in pre-tax income	9	2,587	9,195	9,968
Gross realized losses included in pre-tax income	(527)	(396)	(1,278)	(1,329)
Gross unrealized losses included in pre-tax income	0	0	0	(196)
Net gains included in pre-tax income	$ (518)	$ 2,191	$ 7,917	$ 8,443

Financial Instruments And Inv57

Financial Instruments And Investments Unrealized gains and losses on equity securities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018USD ($)
Unrealized Gains and losses recognized [Line Items]
Net gains and (losses) recognized on equity securities	$ (1,991)
Less: Net (gains) and losses recognized on equity securities sold	1,790
Unrealized gains and (losses) recognized on equity securities still held at reporting date	$ (3,781)

Financial Instruments And Inv58

Financial Instruments And Investments (Company's Decommissioning Trust Funds And Investments In Debt Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	$ 284,082
Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	209,843
Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	74,239
Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Cash
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	7,409
Cash | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	7,409
Cash | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Cash | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		$ 286,866
Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		214,876
Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		71,990
Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Federal Agency Mortgage Backed Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		20,732
Federal Agency Mortgage Backed Securities | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Federal Agency Mortgage Backed Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		20,732
Federal Agency Mortgage Backed Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
U.S. Government Bonds | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		58,181
U.S. Government Bonds | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		58,181
U.S. Government Bonds | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
U.S. Government Bonds | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Municipal Obligations | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		16,584
Municipal Obligations | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Municipal Obligations | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		16,584
Municipal Obligations | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Corporate Obligations | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		34,674
Corporate Obligations | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Corporate Obligations | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		34,674
Corporate Obligations | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Debt Securities
Financial Instruments and Investments [Line Items]
Trading Securities	1,726	1,735
Debt Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Trading Securities	0	0
Debt Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Trading Securities	0	0
Debt Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Trading Securities	1,726	1,735
Debt Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		130,171
Debt Securities | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		58,181
Debt Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		71,990
Debt Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	147,886
Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	147,886
Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Equity Securities | Domestic Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	119,344
Equity Securities | Domestic Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	119,344
Equity Securities | Domestic Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Equity Securities | Domestic Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Equity Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		149,831
Equity Securities | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		149,831
Equity Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Equity Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Equity Securities | International Equity Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	28,542
Equity Securities | International Equity Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	28,542
Equity Securities | International Equity Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Equity Securities | International Equity Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Cash | Held in trust Decommissioning Fund Investment
Financial Instruments and Investments [Line Items]
Cash	0	6,864
Cash | Held in trust Decommissioning Fund Investment | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Cash		6,864
Cash | Held in trust Decommissioning Fund Investment | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Cash		0
Cash | Held in trust Decommissioning Fund Investment | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Cash		0
Available-for-sale Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	128,787
Available-for-sale Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	54,548
Available-for-sale Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	74,239
Available-for-sale Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Federal Agency Mortgage Backed Securities
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	25,840
Available-for-sale Securities | Federal Agency Mortgage Backed Securities | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Federal Agency Mortgage Backed Securities | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	25,840
Available-for-sale Securities | Federal Agency Mortgage Backed Securities | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | U.S. Government Bonds
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	54,548
Available-for-sale Securities | U.S. Government Bonds | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	54,548
Available-for-sale Securities | U.S. Government Bonds | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | U.S. Government Bonds | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Municipal Obligations
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	12,331
Available-for-sale Securities | Municipal Obligations | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Municipal Obligations | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	12,331
Available-for-sale Securities | Municipal Obligations | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Corporate Obligations
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	36,068
Available-for-sale Securities | Corporate Obligations | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	0
Available-for-sale Securities | Corporate Obligations | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	36,068
Available-for-sale Securities | Corporate Obligations | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Decommissioning Fund Investments, Fair Value	$ 0
Domestic Equity Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		121,027
Domestic Equity Securities | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		121,027
Domestic Equity Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
Domestic Equity Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
International Equity Securities | Decommisioning Fund Investments
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		28,804
International Equity Securities | Decommisioning Fund Investments | Quoted Prices In Active Markets For Identical Assets (Level 1)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		28,804
International Equity Securities | Decommisioning Fund Investments | Significant Other Observable Inputs (Level 2)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		0
International Equity Securities | Decommisioning Fund Investments | Significant Unobservable Inputs (Level 3)
Financial Instruments and Investments [Line Items]
Available-for-sale Securities		$ 0

Financial Instruments And Inv59

Financial Instruments And Investments Transfers Between Levels (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value, Assets, Level 1 to Level 2 Transfers, Description	no	no	no	no
Fair Value, Assets, Level 2 to Level 1 Transfers, Description	no	no	no	no
Fair Value, Asset, Level 3 Purchases, (Sales), Issuances, (Settlements)	$ 0	$ 0	$ 0	$ 0