Rate Matters, SPS (Details) $ in Millions			1 Months Ended			3 Months Ended	12 Months Ended
		Apr. 13, 2018 USD ($)	Feb. 28, 2018 USD ($)	Oct. 31, 2017 USD ($) MW	Nov. 30, 2016 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
SPS | PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revised Incremental Revenue Request							$ 69.0
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 55.0
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							$ 55.0
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 | NMPRC Proceeding - New Mexico 2016 Electric Rate Case
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%	53.97%
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%
Public Utilities, Requested Rate Base, Amount				$ 885.0
Public Utilities, Decline in MW Sales from Wholesale Customers | MW				380
SPS | Public Utility Commission of Texas (PUCT) | PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Tax Cuts and Jobs Act of 2017, Estimated Electric Rate Reduction			$ 32.0
Public Utilities, Revised Requested Equity Capital Structure, Percentage			58.00%
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
Subsequent Event | NMPRC Staff [Member] | 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, Requested Rate Increase (Decrease), Amount		$ 43.0
Public Utilities, Recommended ROE, Percentage		9.00%
Public Utilities, Recommended Equity Capital Structure, Percentage		52.00%
Public Utilities, Requested Rate Increase (Decrease) Including TCJA Impact, Amount		$ 32.0
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
Subsequent Event | New Mexico Attorney General (NMAG) [Member] | 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, Requested Rate Increase (Decrease), Amount		$ 43.0
Public Utilities, Recommended ROE, Percentage		9.21%
Public Utilities, Recommended Equity Capital Structure, Percentage		53.97%
Public Utilities, Requested Rate Increase (Decrease) Including TCJA Impact, Amount		$ 32.0
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		(5.0)
Public Utilities, Total Recommended Rate Increase (Decrease)		$ 7.0
Transmission Cost Recovery Factory (TCRF) Rider | SPS | PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[1]						$ 14.0
Minimum | Subsequent Event | 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		51.00%
Maximum | Subsequent Event | 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%

[1] The roll-in of the TCRF rider revenue into base rates will not have an impact on customer bills or revenue as these costs are already being recovered through the rider. SPS can request another TCRF rider after the conclusion of this rate case to recover transmission investments subsequent to June 30, 2017.