Rate Matters (Details) - USD ($) $ in Millions		1 Months Ended						9 Months Ended	12 Months Ended
	Sep. 05, 2018	Aug. 31, 2018	Jun. 30, 2018	May 31, 2018	Feb. 28, 2018	Oct. 31, 2017	Nov. 30, 2016	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
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, Total Recommended Rate Increase (Decrease)			$ 12.0
SPS | PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
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 Capital Structure, Percentage				58.00%
Public Utilities, Revised requested rate increase after TCRF rider adjustment				$ 17.0
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
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						53.97%
Public Utiilities, Federal Income Tax Rate Used in Rate Request						35.00%
Public Utilities, Requested Rate Increase (Decrease) Including TCJA Impact, Amount				$ 27.0
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%
SPS | Public Utility Commission of Texas (PUCT) | PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Intended Equity Capital Structure, Percentage			57.00%
Public Utilities, Requested Return on Equity, Percentage to Calculate AFUDC			9.50%
SPS | New Mexico Public Regulation Commission [Member] | New Mexico 2017 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 8.0
Public Utilities, Approved Rate Increase (Decrease), Percentage	2.10%
Public Utilities, Approved Return on Equity, Percentage	9.10%
Public Utilities, Approved Equity Capital Structure, Percentage	51.00%
Tax Cuts and Jobs Act of 2017, Approved Refund of 2018 TCJA Benefits	$ 10.0
Tax Cuts and Jobs Act of 2017, Regulatory Liability for Customer Refund	$ 10.0
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
Public Utilities, Annual Transmission Revenue Requirement Increase		$ 9.5
Public Utilities, Annual Transmission Revenue Requirement Increase (SPS)		6.0
Public Utilities, Annual Transmission Revenue Requirement (Other Utilities)		$ 3.5