Rate Matters (Details) - USD ($) $ in Millions		1 Months Ended			9 Months Ended	12 Months Ended
		Oct. 31, 2017	Aug. 31, 2017	Nov. 30, 2016	Sep. 30, 2017	Dec. 31, 2015	Dec. 31, 2014
PUCT Proceeding - Appeal of the Texas 2015 Electric Rate Case Decision
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 42.1
PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 66.4
Public Utilities, Requested Rate Increase (Decrease), Percentage			7.10%
Public Utilities, Number of Months in Test Year			12 months
Public Utilities, Number of Months in Test Year which are Estimated			3 months
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%
NMPRC Proceeding - New Mexico 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 41.4
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 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
Subsequent Event | PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revised Requested Rate Increase		$ 54.6
Public Utilities, Revised Requested Rate Increase, Percentage		5.80%
Public Utilities, Rate Case Expenses Addressed in Separate Proceeding		$ 4.4
Public Utilities, Revised Incremental revenue request		69.2
Transmission Cost Recovery Factory (TCRF) Rider | Subsequent Event | PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates	[1]	$ 14.6
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				$ 12.8
Public Utilities, Length of Payment Period Requested, In Years					5 years
Public Utilities, Monthly Billed Charges For Transmission Service Upgrades					$ 0.5

[1] (a) The roll-in of the TCRF rider revenue into base rates will not have an impact on customer bills or total 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.