Rate Matters, SPS (Details) - SPS $ in Millions		1 Months Ended					12 Months Ended
		Feb. 28, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 30, 2016 USD ($) MW	Sep. 30, 2016 USD ($)	Feb. 29, 2016 USD ($)	Dec. 31, 2016	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Appeal of the Texas 2015 Electric Rate Case Decision
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount									$ 64.8
Public Utilities, Revised Requested Rate Increase									$ 42.1
PUCT Proceeding - Texas 2016 Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount						$ 71.9
Public Utilities, Revised Requested Rate Increase					$ 61.5
Public Utilities, Requested Rate Increase (Decrease), Percentage						14.40%
Public Utilities, Requested Return on Equity, Percentage						10.25%
Public Utilities, Requested Rate Base, Amount						$ 1,700.0
Public Utilities, Requested Equity Capital Structure, Percentage						53.97%
Public Utilities, Revised Requested Rate Increase, Including Rate Case Expenses					$ 65.5
Public Utilities, Base Rate Increase Under the Stipulation			$ 35.2
Public Utilities, Mechanism Ensuring Recover in Revenues from Power Factor Charge	[1]		12.6
Public Utilities, Rate Case Expenses Addressed in Separate Proceeding			4.0
Public Utilities, Total Estimated Impact of Stipulation			51.8
Public Utilities, Additional Revenue Cap in Future TCRF Application Under the Stipulation			$ 19.0
Public Utilities, Number of Days for PUCT Approval of the Next Transmission Cost Recovery Factor Application Under the Stipulation			100 days
NMPRC Proceeding - New Mexico 2016 Electric Rate Case
Rate Matters [Abstract]
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 Utilities, Future Decline in MW Sales from Certain Wholesale Customers included in Rate Case | MW				380
Public Utilities, Capital Expenditures				$ 20.1
Public Utilities, Changes in Allocator, Including Wholesale Load Reductions				11.5
Public Utilities, Transmission Expenses				4.7
Public Utilities, Depreciation, Including Adjustment of Tolk Service Life				3.6
Public Utilities, Rate Case Expenses				1.1
Public Utilities, Other Net				0.4
SPP Open Access Transmission Tariff Upgrade Costs
Rate Matters [Abstract]
Public Utilities, Billed Charges For Transmission Service Upgrades				$ 12.8
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) | SPP Open Access Transmission Tariff Upgrade Costs
Rate Matters [Abstract]
Public Utilities, Length of Payment Period Requested, In Years							5 years
Subsequent Event | Texas 2016 TCRF Application [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 16.1
Public Utilities, Requested Rate Increase (Decrease), Percentage		1.79%

[1] (a) SPS’ request assumed customers would adjust their power factors, which would reduce revenue. To the extent power factor revenues are less than $12.6 million, a mechanism will be established to ensure SPS recovers this amount and effectively offset lower anticipated power factor charges.