Rate Matters (Details) $ in Millions		1 Months Ended			12 Months Ended
	Feb. 16, 2018	Feb. 28, 2018	Oct. 31, 2017 USD ($) MW	Nov. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Appeal of the Texas 2015 Electric Rate Case Decision
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount								$ 42.0
Texas 2017 Electric Rate Case [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 55.0
Public Utilities, Requested Rate Increase (Decrease), Percentage					5.80%
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%
Public Utilities, Incremental revenue request					$ 69.0
Appeal of the New Mexico 2016 Electric Rate Case Decision
Rate Matters [Abstract]
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%
New Mexico 2017 Electric Rate Case [Member]
Rate Matters [Abstract]
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, Requested Equity Capital Structure, Percentage			53.97%
Public Utilities, Decline in MW Sales from Wholesale Customers | MW			380
Public Utility Commission of Texas (PUCT) | Appeal of the Texas 2015 Electric Rate Case Decision
Rate Matters [Abstract]
Public Utilities, Approved Rate Decrease, Net of Rate Case Expenses							$ 4.0
Southwest Power Pool (SPP)
Rate Matters [Abstract]
Public Utilities, Billed Charges For Transmission Service Upgrades						$ 13.0
Southwest Power Pool (SPP) | SPP Open Access Transmission Tariff Upgrade Costs
Rate Matters [Abstract]
Public Utilities, Monthly Billed Charges For Transmission Service Upgrades					0.5
Transmission Cost Recovery Factory (TCRF) Rider [Member] | Texas 2017 Electric Rate Case [Member]
Rate Matters [Abstract]
Public Utilities, Rider Conversion to Base Rates					$ (14.0)
Subsequent Event | New Mexico 2017 Electric Rate Case [Member]
Rate Matters [Abstract]
Public Utilities, Revised Requested Equity Capital Structure, Percentage		58.00%
SPS | Subsequent Event | Texas 2017 Electric Rate Case [Member]
Rate Matters [Abstract]
Public Utilities, Revised Requested Equity Capital Structure, Percentage	58.00%