Rate Matters (Details) - USD ($) $ in Millions	1 Months Ended					6 Months Ended	12 Months Ended
	Jun. 30, 2018	May 31, 2018	Feb. 28, 2018	Oct. 31, 2017	Nov. 30, 2016	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Hearing Examiner [Member] | New Mexico 2017 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended ROE, Percentage	9.40%
Public Utilities, Recommended Equity Capital Structure, Percentage	53.97%
Public Utilities, Increase (Decrease) of Recommended ROE	$ (5.0)
Public Utilities, Increase (Decrease) of Recommended Capital Structure and Cost of Debt	(3.0)
Public Utilities, Increase (Decrease) from Recommended Accelerated Depreciation for Tolk Plant	(3.0)
Public Utilities, Increase (Decrease) Rate Case Expense Disallowance	0.0
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment	(1.0)
Public Utilities, Increase (Decrease) from Post Test Year Plant Adjustment	0.0
Public Utilities, Increase (Decrease) Other, net	(6.0)
Public Utilities, Total Recommended Rate Increase (Decrease)	12.0
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment Aligning Costs With Plant In Rate Base	(1.0)
Public Utilities, Requested Increase (Decrease) from Excess Generation Adjustment	$ (1.0)
NMPRC Staff [Member] | New Mexico 2017 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended ROE, Percentage						9.00%
Public Utilities, Recommended Equity Capital Structure, Percentage						52.00%
Public Utilities, Increase (Decrease) of Recommended ROE						$ (4.0)
Public Utilities, Increase (Decrease) of Recommended Capital Structure and Cost of Debt						(7.0)
Public Utilities, Increase (Decrease) from Recommended Accelerated Depreciation for Tolk Plant						(3.0)
Public Utilities, Increase (Decrease) Rate Case Expense Disallowance						(2.0)
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment						0.0
Public Utilities, Increase (Decrease) from Post Test Year Plant Adjustment						(1.0)
Public Utilities, Increase (Decrease) Other, net						(4.0)
Public Utilities, Total Recommended Rate Increase (Decrease)						11.0
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment Aligning Costs With Plant In Rate Base						0.0
Public Utilities, Requested Increase (Decrease) from Excess Generation Adjustment						$ 0.0
New Mexico Attorney General (NMAG) [Member] | New Mexico 2017 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended ROE, Percentage						9.21%
Public Utilities, Recommended Equity Capital Structure, Percentage						53.97%
Public Utilities, Increase (Decrease) of Recommended ROE						$ (6.0)
Public Utilities, Increase (Decrease) of Recommended Capital Structure and Cost of Debt						(3.0)
Public Utilities, Increase (Decrease) from Recommended Accelerated Depreciation for Tolk Plant						(3.0)
Public Utilities, Increase (Decrease) Rate Case Expense Disallowance						(3.0)
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment						(3.0)
Public Utilities, Increase (Decrease) from Post Test Year Plant Adjustment						(2.0)
Public Utilities, Increase (Decrease) Other, net						(4.0)
Public Utilities, Total Recommended Rate Increase (Decrease)						7.0
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment Aligning Costs With Plant In Rate Base						0.0
Public Utilities, Requested Increase (Decrease) from Excess Generation Adjustment						(1.0)
SPS | PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Incremental revenue request							$ 69.0
Public Utilities, Intended Equity Capital Structure, Percentage	57.00%
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 54.0
Public Utilities, Requested Rate Increase (Decrease), Percentage							5.80%
Public Utilities, Requested Return on Equity, Percentage							10.25%
Public Utilities, Requested Rate Base, Amount							$ 1,900.0
Public Utilities, Requested Equity Capital Structure, Percentage							53.97%
Public Utilities, Revised Requested Rate Increase		$ 32.0
Public Utilities, Revised Requested Rate Increase, Percentage		5.90%
Public Utilities, Revised requested rate increase after equity ratio adjustment		$ 32.0
Public Utilities, Revised Requested Capital Structure, Percentage		58.00%
Public Utilities, Revised requested rate increase after TCRF rider adjustment		$ 17.0
SPS | PUCT Proceeding - Appeal of the Texas 2015 Electric Rate Case Decision
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount										$ 42.0
SPS | New Mexico 2017 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Tax Cuts and Jobs Act of 2017, Estimated Electric Rate Reduction		$ 11.0	$ 11.0	$ 11.0
Public Utilities, Revised Requested Equity Capital Structure, Percentage			58.00%
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 43.0
Public Utilities, Requested Return on Equity, Percentage		10.25%		10.25%
Public Utilities, Requested Rate Base, Amount				$ 885.0
Public Utilities, Requested Equity Capital Structure, Percentage		58.00%		53.97%
Public Utilities, Requested Rate Increase (Decrease) Including TCJA Impact, Amount		$ 27.0		$ 32.0
Public Utilities, Increase (Decrease) of Recommended ROE		0.0
Public Utilities, Increase (Decrease) of Recommended Capital Structure and Cost of Debt		0.0
Public Utilities, Increase (Decrease) from Recommended Accelerated Depreciation for Tolk Plant		0.0
Public Utilities, Increase (Decrease) Rate Case Expense Disallowance		(1.0)
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment		0.0
Public Utilities, Increase (Decrease) from Post Test Year Plant Adjustment		(3.0)
Public Utilities, Increase (Decrease) Other, net		(1.0)
Public Utilities, Total Recommended Rate Increase (Decrease)		27.0
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment Aligning Costs With Plant In Rate Base		0.0
Public Utilities, Requested Increase (Decrease) from Excess Generation Adjustment		$ 0.0
SPS | NMPRC Proceeding - New Mexico 2016 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 41.0
Public Utilities, Requested Rate Increase (Decrease), Percentage					10.90%
Public Utilities, Requested Return on Equity, Percentage					10.10%
Public Utilities, Requested Rate Base, Amount					$ 832.0
Public Utilities, Requested Equity Capital Structure, Percentage					53.97%
Public Utiilities, Federal Income Tax Rate Used in Rate Request				35.00%
SPS | Settlement Group [Member] | PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Incremental revenue request	$ 17.0
Public Utilities, Requested Return on Equity, Percentage to Calculate AFUDC	9.50%
Public Utilities, Increase (Decrease) from Recommended Accelerated Depreciation for Tolk Plant	$ (4.0)
Public Utilities, Increase (Decrease) Related to Tax Cuts and Jobs Act and Other Items	(37.0)
Public Utilities, Increase (Decrease) Related to Unspecified Settlement Adjustments	(13.0)
Public Utilities, Rate Increase Under the Settlement	$ 0.0
Public Utilities, Decrease in Requested Accelerated Depreciation for Tolk Plant, Percent	50.00%
SPS | Public Utility Commission of Texas (PUCT) | PUCT Proceeding - Appeal of the Texas 2015 Electric Rate Case Decision
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Decrease, Net of Rate Case Expenses									$ 4.0
SPS | Southwest Power Pool (SPP) | SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Billed Charges For Transmission Service Upgrades								$ 13.0
Public Utilities, Monthly Billed Charges For Transmission Service Upgrades						$ 0.5
SPS | Transmission Cost Recovery Factory (TCRF) Rider | PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates							$ (15.0)