Rate Matters (Details) - USD ($) $ in Millions	1 Months Ended							12 Months Ended
	Oct. 31, 2016	Sep. 30, 2016	Aug. 31, 2016	Apr. 30, 2016	Feb. 29, 2016	Oct. 31, 2015	Jun. 30, 2015	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									$ 64.8
Public Utilities, Revised Requested Rate Increase							$ 42.1
PUCT Proceeding - Texas 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 71.9
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				$ 68.6
NMPRC Proceeding - New Mexico 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount						$ 45.4
Public Utilities, Requested Return on Equity, Percentage						10.25%
Public Utilities, Requested Rate Base, Amount						$ 734.0
Public Utilities, Requested Equity Capital Structure, Percentage						53.97%
Public Utilities, Decrease In Base Fuel Revenues						$ 21.1
Public Utilities, Non-fuel Base Rate Increase Under the Stipulation			$ 23.5
Public Utilities, Decrease In Base Fuel Revenues Under the Stipulation			21.1
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
Public Utility Commission of Texas (PUCT) Staff | PUCT Proceeding - Texas 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Total Rate Increase Recommended By Third Party, Amount			$ 32.9
Public Utilities, Recommended ROE By Third Party			9.30%
Public Utilities, Recommended Equity Capital Structure By Third Party, Percentage			51.00%
Southwest Power Pool (SPP) | SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Length of Payment Period Requested, In Years		5 years
Alliance of Xcel Municipalities (AXM) | PUCT Proceeding - Texas 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Total Rate Increase Recommended By Third Party, Amount			$ 25.2
Public Utilities, Recommended ROE By Third Party			9.40%
Public Utilities, Recommended Equity Capital Structure By Third Party, Percentage			51.00%
Office of Public Utility Counsel (OPUC) | PUCT Proceeding - Texas 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended ROE By Third Party			9.20%
Texas Industrial Energy Consumers (TIEC) | PUCT Proceeding - Texas 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended ROE By Third Party			9.15%
Subsequent Event [Member] | Minimum | SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Estimated Charges For Transmission Service Upgrades	$ 12.0
Subsequent Event [Member] | Maximum | SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Estimated Charges For Transmission Service Upgrades	$ 14.0