Rate Matters (Details) - USD ($) $ in Thousands	1 Months Ended					6 Months Ended	12 Months Ended
	May 31, 2016	Apr. 30, 2016	Feb. 29, 2016	Oct. 31, 2015	Jun. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
PUCT Proceeding - Appeal of the Texas 2015 Electric Rate Case Decision [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 42,100			$ 64,800
PUCT Proceeding -Texas 2015 Electric Rate Net Refund Case [Member]
Public Utilities, General Disclosures [Line Items]
Customer Refund Liability, Current						$ 1,250
Length of refund period						6 months
PUCT Proceeding - Texas 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 71,900
Public Utilities, Requested Rate Increase (Decrease), Percentage			14.40%
Public Utilities, Requested Return on Equity, Percentage			10.25%
Public Utilities, Requested Rate Base, Amount			$ 1,700,000
Public Utilities, Requested Equity Capital Structure, Percentage			53.97%
Public Utilities, Revised requested rate increase		$ 68,600
Public utilities, Requested Increase Related to Capital Investment		38,900
Public Utilities, Increase to Jurisdictional Allocation Factors		9,800
Public Utilities, ROE and Capital Structure		11,600
Public Utilities, Increase to Rate Case Expenses		4,500
Public Utilities, Increase Related To Other, Net		$ 3,800
NMPRC Proceeding - New Mexico 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 45,400
Public Utilities, Requested Return on Equity, Percentage				10.25%
Public Utilities, Requested Rate Base, Amount				$ 734,000
Public Utilities, Requested Equity Capital Structure, Percentage				53.97%
Public Utilities, Decrease In Base Fuel Revenues				$ 21,100
Public Utilities, Non-fuel Base Rate Increase Under the Stipulation	$ 23,500
Public Utilities, Decrease In Base Fuel Revenues Under the Stipulation	$ 21,100
Public Utility Commission of Texas (PUCT) | PUCT Proceeding - Appeal of the Texas 2015 Electric Rate Case Decision [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Decrease, Net of Rate Case Expenses							$ 4,000
Minimum | SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Estimated Charges For Transmission Service Upgrades						$ 5,000
Maximum | SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Estimated Charges For Transmission Service Upgrades						$ 10,000