Rate Matters (Details)		1 Months Ended						3 Months Ended
	May. 02, 2016 USD ($)	Apr. 30, 2016 USD ($)	Feb. 29, 2016 USD ($)	Dec. 31, 2015 USD ($) MW	Oct. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2016 USD ($)
NMPRC Proceeding - New Mexico 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 45,400,000
Public Utilities, Requested Return on Equity, Percentage					10.25%
Public Utilities, Requested Rate Base, Amount					$ 734,000,000
Public Utilities, Requested Equity Capital Structure, Percentage					53.97%
Public Utilities, Decrease In Base Fuel Revenues					$ 21,100,000
SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Accrual For Transmission Service Upgrades								$ 0
Southwest Power Pool (SPP) | SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Maximum Retroactive Charges Indicated by Filing								$ 720,000,000
Subsequent Event | NMPRC Proceeding - New Mexico 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Non-fuel Base Rate Increase Under the Stipulation	$ 23,500,000
Public Utilities, Decrease In Base Fuel Revenues Under the Stipulation	$ 21,100,000
SPS | PUCT Proceeding - Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 42,100,000		$ 42,100,000	$ 64,800,000
Public Utilities, Requested Rate Increase (Decrease), Percentage							6.70%
Public Utilities, Requested Return on Equity, Percentage							10.25%
Public Utilities, Requested Rate Base, Amount							$ 1,600,000,000
Public Utilities, Requested Equity Capital Structure, Percentage							53.97%
Public utilities, Additional capital investment							$ 392,000,000
SPS | PUCT Proceeding - Texas 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 71,900,000
Public Utilities, Requested Rate Increase (Decrease), Percentage			14.40%
Public Utilities, Requested Return on Equity, Percentage			10.25%
Public Utilities, Requested Rate Base, Amount			$ 1,700,000,000
Public Utilities, Requested Equity Capital Structure, Percentage			53.97%
SPS | Administrative Law Judge | PUCT Proceeding - Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Post Test Year Decrease Adjustments				(8,900,000)
Public Utilities, Decrease To Requested Return On Equity				(6,300,000)
Public Utilities, Decrease to Cost of Debt and Capital Structure				0
Public Utilities, Decrease to annual incentive compensation				(200,000)
Public Utilities, Decrease to O&M Expenses				(4,600,000)
Public Utilities, Decrease to Depreciation Expense				(2,700,000)
Public Utilities, Decrease To Property Taxes				(900,000)
Public Utilities, Decrease to Revenue Adjustments				(1,100,000)
Public Utilities, Decrease To Wholesale Load Reductions				0
Public Utilities, Decrease To Transmission Expansion plan				(4,200,000)
Public Utilities, Increase Related To Other, Net				1,400,000
Public Utilities, Recommended Rate Increase (Decrease) (Including Case Expenses)				14,600,000
Public Utilities, Decrease Adjustment To Move Rate Case Expenses To Separate Docket				(200,000)
Public Utilities, Recommended Rate Increase (Decrease), Amount				14,400,000
Public Utilities, Impact Of New Depreciation Rules				(11,200,000)
Public Utilities, Earnings Impact				$ 3,200,000
SPS | Public Utility Commission of Texas | PUCT Proceeding - Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Original Wholesale Load Reduction | MW				500
Public Utilities, Disallowance of Requested Wholesale Load Reduction | MW				300
Public Utilities, Approved Equity Capital Structure, Percentage				51.00%
Public Utilities, Approved Return on Equity, Percentage				9.70%
Public Utilities, Post Test Year Decrease Adjustments				$ (8,900,000)
Public Utilities, Decrease To Requested Return On Equity				(6,300,000)
Public Utilities, Decrease to Cost of Debt and Capital Structure				(3,700,000)
Public Utilities, Decrease to annual incentive compensation				(300,000)
Public Utilities, Decrease to O&M Expenses				(4,600,000)
Public Utilities, Decrease to Depreciation Expense				(2,700,000)
Public Utilities, Decrease To Property Taxes				(900,000)
Public Utilities, Decrease to Revenue Adjustments				(1,600,000)
Public Utilities, Decrease To Wholesale Load Reductions				(11,500,000)
Public Utilities, Decrease To Transmission Expansion plan				(4,200,000)
Public Utilities, Decrease related to other, net				(1,200,000)
Public Utilities, Approved Rate Increase (Decrease) (Including Case Expenses)				(3,800,000)
Public Utilities, Decrease Adjustment To Move Rate Case Expenses To Separate Docket				(200,000)
Public Utilities, Approved Rate Increase (Decrease), Amount				(4,000,000)
Public Utilities, Impact Of New Depreciation Rules				(11,200,000)
Public Utilities, Earnings Impact				$ (15,200,000)
SPS | Subsequent Event | PUCT Proceeding - Texas 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public utilities, Additional capital investment		$ 38,900,000
Public Utilities, Increase Related To Other, Net		3,800,000
Public Utilities, Revised requested rate increase		68,600,000
Public Utilities, Increase (Decrease) to Jurisdictional Allocation Factors		9,800,000
Public Utilities, ROE and Capital Structure		11,600,000
Public Utilities, Increase (Decrease) to Rate Case Expenses		$ 4,500,000