Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets and Liabilities [Line Items]
Regulatory assets, non-current		$ 74,848	$ 96,036
Regulatory Assets		81,820	96,036
Regulatory Assets, Current		6,972	0
Regulatory liabilities, non-current		298,570	296,685
Regulatory Liabilities		313,256	302,910
Regulatory Liability, Current		$ 14,686	$ 6,225
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	35.00%	35.00%
Regulatory tax assets or liabilities
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[1]	$ 291,557	$ 289,013
Accumulated deferred investment tax credit
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[2]	7,872	4,816
Texas energy efficiency
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[3]	0	895
New Mexico energy efficiency
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[3]	1,694	1,394
Fuel revenue over-recovery
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[4]	11,047	6,225
Other Regulatory Assets (Liabilities)
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities		$ 1,086	567
Regulatory tax assets or liabilities
Regulatory Assets and Liabilities [Line Items]
Regulatory Asset, Amortization Period		15 years
Regulatory Assets	[5]	$ 39,519	40,512
Loss on reacquired debt
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[6]	14,074	14,926
Final coal reclamation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[7]	4,065	4,726
Four Corners decommissioning
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[8]	5,813	6,604
Nuclear fuel postload daily financing charge
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[9]	3,717	3,536
Unrecovered issuance costs due to reissuance of PCBs
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[6]	728	761
Texas 2015 rate case costs
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[10]	747	1,144
Texas 2017 rate case costs
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets		2,634	3,642
Texas relate back surcharge
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[11]	0	8,591
New Mexico renewable energy credits and related costs
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[12]	4,709	5,823
New Mexico Palo Verde deferred depreciation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[13]	4,111	4,263
Other Regulatory Assets (Liabilities)
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets		$ 1,703	$ 1,508

[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 K of Notes to Financial Statements for further discussion.

[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 over-recovery of fuel and purchased power expense which is refunded to customers through fuel rates.

[5] 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.

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

[7] This item relates to coal reclamation costs associated with Four Corners. The Texas portion was approved for recovery in PUCT Docket No. 46308 and will be recovered over seven years through June 2023. The New Mexico amortization period will be established in the next general rate case.

[8] This item relates to the decommissioning of Four Corners. The Texas portion was approved for recovery in PUCT Docket No. 46308 and will be recovered over seven years through July 2024. The New Mexico amortization period will be established in the next general rate case.

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

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

[11] 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. Amounts over-recovered through these surcharges will be addressed in the next Texas fuel reconciliation. See Part II, Item 8, Financial Statements and Supplementary Data, Note D of Notes to Financial Statements for further discussion.

[12] 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.

[13] The amortization period for this item is based upon the U.S. Nuclear Regulatory Commission ("NRC") license life for each unit at Palo Verde.