Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets and Liabilities [Line Items]
Regulatory assets		$ 96,036	$ 118,861
Regulatory liabilities		296,685	18,435
Regulatory tax assets or liabilities
Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities	[1]	289,013	10,648
Accumulated deferred investment tax credit
Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities	[2]	4,816	3,328
Texas energy efficiency
Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities	[3]	895	1,288
New Mexico energy efficiency
Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities	[3]	1,394	2,159
Texas military base discount and recovery factor
Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities	[4]	$ 0	184
New Mexico gain on sale of assets
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Amortization Period		3 years
Regulatory liabilities	[5]	$ 567	828
Regulatory tax assets or liabilities
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[6]	40,512	66,670
Loss on reacquired debt
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[7]	14,926	15,780
Final coal reclamation
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[8],[9]	4,726	8,181
Four Corners decommissioning
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[10]	6,604	1,400
Nuclear fuel postload daily financing charge
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[8]	3,536	3,831
Unrecovered issuance costs due to reissuance of PCBs
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[7]	761	794
Texas 2015 rate case costs
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[11]	1,144	2,670
Texas 2017 rate case costs
Regulatory Assets and Liabilities [Line Items]
Regulatory assets		3,642	246
Texas relate back surcharge
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[12]	8,591	6,455
Texas demand response program
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[13]	133	0
Texas military base discount and recovery factor
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[4]	213	0
New Mexico renewable energy credits and related costs
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[14]	5,823	6,937
New Mexico 2010 FPPCAC audit
Regulatory Assets and Liabilities [Line Items]
Regulatory assets		326	398
New Mexico Palo Verde deferred depreciation
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[15]	4,263	4,415
New Mexico 2015 rate case costs
Regulatory Assets and Liabilities [Line Items]
Regulatory assets		644	1,074
New Mexico 2017 rate case costs
Regulatory Assets and Liabilities [Line Items]
Regulatory assets		0	10
New Mexico demand response program
Regulatory Assets and Liabilities [Line Items]
Regulatory assets	[16]	$ 192	$ 0

[1] This item primarily relates to the reduction in the federal corporate income tax rate from 35% to 21% as enacted by the TCJA. The amortization period for the recovery on this item will be addressed in the next base rate filings in all jurisdictions. See Part II, Item 8, Financial Statements and Supplementary Data, Note J for further details.

[2] The amortization period is based upon the life of the associated assets.

[3] This item is recovered or credited through a recovery factor that is set annually.

[4] This item represents the net asset/net liability related to the military discount which is recovered from non-military customers through a recovery factor that is set annually.

[5] This item relates to the gains on the sales of assets the Company shares with its New Mexico customers over a three year period.

[6] This item relates to (i) the regulatory treatment of the equity portion of AFUDC which is recovered in rate base by an offset with the related accumulated deferred income tax liability, and (ii) excess deferred state income taxes which are recovered through amortization to tax expense in cost of service. The amortization period for the excess deferred state income taxes is 15 years as established in the 2016 PUCT Final Order and the NMPRC Final Order.

[7] This item is recovered as a component of the weighted cost of debt and amortized over the life of the related debt issuance.

[8] This item is recovered through fuel recovery mechanisms established by tariffs.

[9] This item relates to coal reclamation costs associated with Four Corners. The Texas portion was approved for recovery in the 2016 Texas Fuel Reconciliation and will be recovered over seven years through June 2023. The New Mexico amortization period is anticipated to be established in the next general rate case.

[10] This item relates to the decommissioning of Four Corners. The Texas portion was approved for recovery in the 2017 PUCT Final Order and will be recovered over seven years through July 2024. The New Mexico amortization period is anticipated to be established in the next general rate case.

[11] The 2017 PUCT Final Order approved a new recovery period for these costs, beginning January 10, 2018.

[12] This item relates to the recovery of revenues through two separate surcharges; one for the 2015 Texas Retail Rate Case relate back revenues beginning October 1, 2016 and ending September 30, 2017, and a second surcharge for the 2017 Texas Retail Rate Case relate back revenues beginning January 10, 2018 and ending January 9, 2019. See Part II, Item 8, Financial Statements and Supplementary Data, Note C.

[13] Recovery of this item will be addressed in the next EECRF filing.

[14] This item relates to renewable energy credits and procurement plan costs, of which a component has been approved for recovery in the NMPRC Final Order. The remaining balance will be requested for recovery in the next general rate case.

[15] The amortization period for this item is based upon the Nuclear Regulatory Commission license life for each unit at Palo Verde.

[16] Amortization period is anticipated to be established in next general rate case.