Rate Matters (Details) $ in Thousands					1 Months Ended														9 Months Ended
		Oct. 29, 2015 USD ($)	Oct. 28, 2015 USD ($)	Oct. 12, 2015 USD ($)	Oct. 31, 2015 USD ($)	Aug. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)		May. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Jan. 31, 2015	Dec. 31, 2014 USD ($)	Oct. 31, 2014	Dec. 31, 2013 USD ($)	Aug. 31, 2013 Factor	Jul. 31, 2013	Mar. 31, 2013	Apr. 30, 2012	Sep. 30, 2015 USD ($)
Public Utilities, General Disclosures [Line Items]
Other Liabilities, Current												$ 80,211							$ 96,378
Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 42,100	[1]				$ 64,800
Public Utilities, Requested Rate Increase (Decrease), Percentage							4.40%					6.70%
Public Utilities, Requested Return on Equity, Percentage												10.25%
Public Utilities, Requested Rate Base, Amount												$ 1,600,000
Public Utilities, Requested Equity Capital Structure, Percentage												53.97%
Public Utilities, Revised Requested Rate Increase										$ 58,900
Public Utilities, Additional Capital Investment												$ 392,000
Public Utilities, Post Test Year Decrease Adjustments							$ 0
Public Utilities, Decrease To Requested Return On Equity							0
Public Utilities, Decrease To Rate Base							0
Public Utilities, Decrease To O&M Expenses							(1,600)
Public Utilities, Decrease To Depreciation Expense							0
Public Utilities, Decrease To Property Taxes							(1,800)
Public Utilities, Decrease To Revenue Adjustments							0
Public Utilities, Decrease To Wholesale Load Reductions							0
Public Utilities, Decrease To Transmission Expansion plan							(7,300)
Public Utilities, Decrease Related To Other, Net							(1,800)
Public Utilities, Requested Rate Increase (Including Case Expenses), Amount							46,400
Public Utilities, Decrease Adjustment To Move Rate Case Expenses To Separate Docket							$ (4,300)
Global Settlement Agreement [Member]
Public Utilities, General Disclosures [Line Items]
Number Of Customers For Which A Public Utility Proposes To Revise Formula Rates						4
Public Utilities, Disclosure Of Regulatory Matters Pending						9
Public Utilities, Number Of Coincident Peaks Used As Demand Allocator, Revised						3
Public Utilities, Base Return On Equity Charged To Customers Through Transmission Formula Rates, Percentage						10.50%
Public Utilities, Incremental ROE Basis Point Increase						50
Public Utilities, Base Return On Equity Charged To Customers Through Production Formula Rates, Percentage						10.00%
Number Of Coincident Peaks Utilized As Demand Allocator						12
Number Of Coincident Peaks Used As Demand Allocator, Original															12
Number Of Days Until Settlement Agreement Effective After Approval						30 days
Federal Energy Regulatory Commission (FERC) Orders [Member]
Public Utilities, General Disclosures [Line Items]
Number Of Customers For Which A Public Utility Proposes To Revise Formula Rates											4
Number Of Coincident Peaks Used As Demand Allocator, Original											12
Number Of Components Included In Regulatory Proceeding | Factor															2
Current Year Pre-tax Earnings Impact Of Regulatory Proceedings																			$ 1,900
Wholesale Electric Rate Complaints [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Base Return On Equity Charged To Customers Through Transmission Formula Rates, Percentage																		11.27%
Public Utilities, Base Return On Equity Charged To Customers Through Production Formula Rates, Percentage																		10.25%
Public Utilities, Base Return On Equity Requested Charged To Customers Through Production Formula Rates, Percentage													8.61%			9.15%		9.15%
Public Utilities, Base Return On Equity Requested By Customers To Be Charged Through Transmission Formula Rates, Percentage													9.11%			9.65%		9.65%
Golden Spread [Member] | Global Settlement Agreement [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Regulatory Settlement To Be Paid						$ 44,900
Public Service Company of New Mexico [Member] | Global Settlement Agreement [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Regulatory Settlement To Be Paid						$ 4,200
Alliance of Xcel Municipalities [Member] | Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revised Requested Rate Increase										58,900
Public Utilities, Recommended Rate Increase (Decrease), Amount									$ (13,600)
Public Utilities, Post Test Year Decrease Adjustments									(11,300)
Public Utilities, Decrease To Requested Return On Equity									(10,900)
Public Utilities, Decrease To Rate Base									(6,200)
Public Utilities, Decrease To O&M Expenses									(13,700)
Public Utilities, Decrease To Depreciation Expense									(13,300)
Public Utilities, Decrease To Property Taxes									0
Public Utilities, Decrease To Revenue Adjustments									(2,200)
Public Utilities, Decrease To Wholesale Load Reductions									(13,200)
Public Utilities, Decrease To Transmission Expansion plan									0
Public Utilities, Decrease Related To Other, Net									(1,700)
Public Utilities, Recommended Rate Increase (Decrease) (Including Case Expenses)									(13,600)
Public Utilities, Decrease Adjustment To Move Rate Case Expenses To Separate Docket									0
Federal Energy Regulatory Commission (FERC) | Federal Energy Regulatory Commission (FERC) Orders [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Number Of Coincident Peaks Used As Demand Allocator, Revised															3
Number Of Coincident Peaks Used As Demand Allocator, Original															12
Other Liabilities, Current												$ 50,400
Office of Public Utility Counsel [Member] | Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revised Requested Rate Increase										58,900
Public Utilities, Recommended Rate Increase (Decrease), Amount									1,800
Public Utilities, Post Test Year Decrease Adjustments									(23,800)
Public Utilities, Decrease To Requested Return On Equity									(13,500)
Public Utilities, Decrease To Rate Base									(6,800)
Public Utilities, Decrease To O&M Expenses									(11,000)
Public Utilities, Decrease To Depreciation Expense									0
Public Utilities, Decrease To Property Taxes									(1,200)
Public Utilities, Decrease To Revenue Adjustments									(200)
Public Utilities, Decrease To Wholesale Load Reductions									0
Public Utilities, Decrease To Transmission Expansion plan									0
Public Utilities, Decrease Related To Other, Net									(600)
Public Utilities, Recommended Rate Increase (Decrease) (Including Case Expenses)									1,800
Public Utilities, Decrease Adjustment To Move Rate Case Expenses To Separate Docket									0
Public Utility Commission of Texas Staff [Member] | Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revised Requested Rate Increase										$ 58,900
Public Utilities, Recommended Rate Increase (Decrease), Amount									(2,600)
Public Utilities, Post Test Year Decrease Adjustments									(23,800)
Public Utilities, Decrease To Requested Return On Equity									(12,100)
Public Utilities, Decrease To Rate Base									0
Public Utilities, Decrease To O&M Expenses									(7,900)
Public Utilities, Decrease To Depreciation Expense									0
Public Utilities, Decrease To Property Taxes									(4,400)
Public Utilities, Decrease To Revenue Adjustments									0
Public Utilities, Decrease To Wholesale Load Reductions									(11,100)
Public Utilities, Decrease To Transmission Expansion plan									0
Public Utilities, Decrease Related To Other, Net									(2,200)
Public Utilities, Recommended Rate Increase (Decrease) (Including Case Expenses)									(2,600)
Public Utilities, Decrease Adjustment To Move Rate Case Expenses To Separate Docket									$ 0
Subsequent Event | New Mexico 2015 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage					9.96%
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, Net Requested Increase Base Rates, Amount					$ 24,300
Public Utilities, Decrease In Base Fuel Revenues					21,100
Public Utilities, Base Period Deficiency					19,700
Public Utilities, Post Test Year Adjustment Related to Capital Expenditures					12,300
Public Utilities, Depreciation Expense					3,700
Public Utilities, SPP Transmission Expansion Plan					2,000
Public Utilities, ROE					1,600
Public Utilities, Rider Costs To Be Recovered Through Base Rates, Amount					1,300
Public Utilities, Other Net					$ 4,800
Subsequent Event | Global Settlement Agreement [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Estimated Net Gain Expected To Be Recognized		$ 7,900
Subsequent Event | Administrative Law Judge [Member] | Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended Rate Increase (Decrease), Amount	[1]			$ 1,200
Public Utilities, Recommended Return On Equity, Percentage				9.70%
Public Utilities, Recommended Equity Capital Structure, Percentage				53.97%
Public Utilities, Post Test Year Decrease Adjustments	[1]			$ (16,700)
Public Utilities, Decrease To Requested Return On Equity	[1]			(6,300)
Public Utilities, Decrease To Rate Base	[1]			0
Public Utilities, Decrease To O&M Expenses	[1]			(5,300)
Public Utilities, Decrease To Depreciation Expense	[1]			(3,900)
Public Utilities, Decrease To Property Taxes	[1]			(3,700)
Public Utilities, Decrease To Revenue Adjustments	[1]			0
Public Utilities, Decrease To Wholesale Load Reductions	[1]			0
Public Utilities, Decrease To Transmission Expansion plan	[1]			(4,200)
Public Utilities, Decrease Related To Other, Net	[1]			(600)
Public Utilities, Recommended Rate Increase (Decrease) (Including Case Expenses)	[1]			1,400
Public Utilities, Decrease Adjustment To Move Rate Case Expenses To Separate Docket	[1]			$ (200)
Public Utilities, Revised Recommended Rate Increase (Decrease), Amount			$ 14,400
Certain Texas Transmission Assets [Member] | Federal Energy Regulatory Commission (FERC)
Public Utilities, General Disclosures [Line Items]
Current Year Pre-tax Earnings Impact Of Regulatory Proceedings														$ 13,600
Public Utilities, Number Of Substations Included In Purchase And Sale Agreement																	2
Proceeds From Sale Of Other Property, Plant, And Equipment														37,100
Public Utilities, Regulatory Liabilities Recognized For Jurisdictional Gain Sharing														$ 7,200

[1] The ALJs’ recommendation reflects proposed adjustments to SPS’ rebuttal testimony, as of Oct. 12, 2015, which supports a $42.1 million rate increase.