Rate Matters (Details) $ in Thousands					1 Months Ended										3 Months Ended	6 Months Ended
		Jun. 08, 2015 USD ($)	Jun. 01, 2015 MW	May. 12, 2015	Jul. 31, 2015	Jun. 30, 2015 USD ($)	May. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Jan. 31, 2015	Dec. 31, 2014 USD ($)	Oct. 31, 2014	Aug. 31, 2013 Factor	Jul. 31, 2013	Apr. 30, 2012	Jun. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)
Public Utilities, General Disclosures [Line Items]
Other Liabilities, Current						$ 90,838				$ 80,211					$ 90,838	$ 90,838
Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount										$ 64,800
Public Utilities, Requested Rate Increase (Decrease), Percentage										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 (decrease)								$ 58,900
Public Utilities, Additional capital investment										$ 392,000
FERC Proceeding - FERC Orders
Public Utilities, General Disclosures [Line Items]
Number of components included in regulatory proceeding | Factor												2
Other Liabilities, Current										$ 50,400
Current year increase (decrease) to pre-tax earnings resulting from regulatory proceedings																$ 1,500
Number of customers for which a public utility proposes to revise formula rates									6
Federal Energy Regulatory Commission (FERC) | FERC Proceeding - FERC Orders
Public Utilities, General Disclosures [Line Items]
Number of coincident peaks used as demand allocator, revised												3
Number of coincident peaks used as demand allocator, original												12
SPS | Wholesale Electric Rate Complaint, April 20, 2012 through July 18, 2013 Impact
Public Utilities, General Disclosures [Line Items]
Public Utilities, First refund period															1
Public Utilities, Base return on equity requested charged to customers through production formula rates, Percentage															10.25%
Public Utilities, Base return on equity requested charged to customers through transmission formula rates, Percentage	[1]														10.39%
SPS | Wholesale Electric Rate Complaints
Public Utilities, General Disclosures [Line Items]
Public Utilities, Base return on equity charged to customers through production formula rates, Percentage														10.25%
Public Utilities, Base return on equity charged to customers through transmission formula rates, Percentage														10.77%
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%
Public Utilities, Incremental ROE basis point increase (decrease) recommended by third parties															50
SPS | Wholesale Electric Rate Complaints, Refunds as of April 20, 2012 and July 19, 2013
Public Utilities, General Disclosures [Line Items]
FERC denied the rehearing request as it pertained to the first two rate complaints				2
Public Utilities, rehearing sought in FERC decision regarding number of consecutive month periods															15 months
Number of hearings scheduled that were canceled and parties agreed to file briefs based on pre-filed testimony																2
Number of complaints where decision is expected by ALJ in the future																2
SPS | Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount						$ 42,000
Public Utilities, Requested Rate Increase (Decrease), Percentage						4.40%
Public Utilities, Revised requested rate increase (decrease)								58,900
Public Utilities, Rate increase (decrease) recommended by rebuttal testimony						$ 46,400
Public Utilities, Post test year adjustment related to capital expenditures						0
Public Utilities, ROE						0
Rate base adjustments (largely the removal of the prepaid pension asset)						0
Public Utilities, Operating and maintenance expenses						(1,600)
Public Utilities, Depreciation expense						0
Public Utilities, Property taxes						(1,800)
Public Utilities, Revenue adjustments						0
Public Utilities, Wholesale load reductions						0
Public Utilities, SPP transmission expansion plan						(7,300)
Public Utilities, Other net						(1,800)
Public Utilities, Adjustment to move rate case expenses to separate docket						(4,300)
Public Utilities, Rate increase (decrease) recommended by rebuttal testimony, excluding rate case expenses						$ 42,100
SPS | New Mexico 2014 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Return on Equity, Percentage		10.25%
Public Utilities, Jurisdictional electric rate base		$ 777,900
Public Utilities, Requested Equity Capital Structure, Percentage		53.97%
Public Utilities, Non-fuel proposed base rate increase (decrease)		$ 31,500
Public Utilities, Proposed base fuel rate increase (decrease)		$ (30,100)
SPS | FERC Proceeding - FERC Orders
Public Utilities, General Disclosures [Line Items]
Number of coincident peaks used as demand allocator, original									12
Golden Spread contract demand quantity subject to the formula rate change prior to June 1, 2015 | MW			500
Change in Golden Spread contract demand quantity subject to the formula rate change | MW			300
SPS | Wholesale Electric Rate Complaint, July 19, 2013 through Oct. 19, 2014 Impact
Public Utilities, General Disclosures [Line Items]
Public Utilities, Base return on equity requested charged to customers through production formula rates, Percentage															10.25%
Public Utilities, Base return on equity requested charged to customers through transmission formula rates, Percentage	[1]														11.20%
Public Utilities, Second refund period															2
SPS | Wholesale Electric Rate Complaint, Oct. 20, 2014 Impact
Public Utilities, General Disclosures [Line Items]
Public Utilities, Base return on equity requested charged to customers through production formula rates, Percentage	[2]														10.40%
Public Utilities, Base return on equity requested charged to customers through transmission formula rates, Percentage	[1],[2]														11.20%
Public Utilities, Third refund period															3
SPS | Golden Spread [Member] | Wholesale Electric Rate Complaint, April 20, 2012 through July 18, 2013 Impact
Public Utilities, General Disclosures [Line Items]
Public Utilities, First refund period	[3]														1
Public Utilities, Base return on equity requested by customers to be charged through production formula rates, Percentage	[3]														8.78%
Public Utilities, Base return on equity requested by customers to be charged through transmission formula rates, Percentage	[1],[3]														9.28%
SPS | Golden Spread [Member] | Wholesale Electric Rate Complaint, July 19, 2013 through Oct. 19, 2014 Impact
Public Utilities, General Disclosures [Line Items]
Public Utilities, Base return on equity requested by customers to be charged through production formula rates, Percentage	[3]														8.51%
Public Utilities, Base return on equity requested by customers to be charged through transmission formula rates, Percentage	[1],[3]														9.01%
Public Utilities, Second refund period	[3]														2
SPS | Golden Spread [Member] | Wholesale Electric Rate Complaint, Oct. 20, 2014 Impact
Public Utilities, General Disclosures [Line Items]
Public Utilities, Base return on equity requested by customers to be charged through production formula rates, Percentage	[3]														8.45%
Public Utilities, Base return on equity requested by customers to be charged through transmission formula rates, Percentage	[1],[3]														8.95%
Public Utilities, Third refund period	[3]														3
SPS | Alliance of Xcel Municipalities [Member] | Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revised requested rate increase (decrease)								58,900
Public Utilities, Rate increase (decrease) recommended by third parties							$ (13,600)
Public utilities, ROE recommended by third parties							9.40%
Public Utilities, Equity capital structure recommended by third parties							53.97%
Public Utilities, Post test year adjustment related to capital expenditures							$ (11,300)
Public Utilities, ROE							(10,900)
Rate base adjustments (largely the removal of the prepaid pension asset)							(6,200)
Public Utilities, Operating and maintenance expenses							(13,700)
Public Utilities, Depreciation expense							(13,300)
Public Utilities, Property taxes							0
Public Utilities, Revenue adjustments							(2,200)
Public Utilities, Wholesale load reductions							(13,200)
Public Utilities, SPP transmission expansion plan							0
Public Utilities, Other net							(1,700)
Public Utilities, Adjustment to move rate case expenses to separate docket							0
Public Utilities, Rate increase (decrease) recommended by third parties excluding rate case expenses							(13,600)
SPS | Office of Public Utility Counsel [Member] | Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revised requested rate increase (decrease)								58,900
Public Utilities, Rate increase (decrease) recommended by third parties							$ 1,800
Public utilities, ROE recommended by third parties							9.20%
Public Utilities, Equity capital structure recommended by third parties							52.38%
Public Utilities, Post test year adjustment related to capital expenditures							$ (23,800)
Public Utilities, ROE							(13,500)
Rate base adjustments (largely the removal of the prepaid pension asset)							(6,800)
Public Utilities, Operating and maintenance expenses							(11,000)
Public Utilities, Depreciation expense							0
Public Utilities, Property taxes							(1,200)
Public Utilities, Revenue adjustments							(200)
Public Utilities, Wholesale load reductions							0
Public Utilities, SPP transmission expansion plan							0
Public Utilities, Other net							(600)
Public Utilities, Adjustment to move rate case expenses to separate docket							0
Public Utilities, Rate increase (decrease) recommended by third parties excluding rate case expenses							1,800
SPS | Public Utility Commission of Texas Staff [Member] | Texas 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revised requested rate increase (decrease)								$ 58,900
Public Utilities, Rate increase (decrease) recommended by third parties							$ (2,600)
Public utilities, ROE recommended by third parties							9.30%
Public Utilities, Equity capital structure recommended by third parties							53.97%
Public Utilities, Post test year adjustment related to capital expenditures							$ (23,800)
Public Utilities, ROE							(12,100)
Rate base adjustments (largely the removal of the prepaid pension asset)							0
Public Utilities, Operating and maintenance expenses							(7,900)
Public Utilities, Depreciation expense							0
Public Utilities, Property taxes							(4,400)
Public Utilities, Revenue adjustments							0
Public Utilities, Wholesale load reductions							(11,100)
Public Utilities, SPP transmission expansion plan							0
Public Utilities, Other net							(2,200)
Public Utilities, Adjustment to move rate case expenses to separate docket							0
Public Utilities, Rate increase (decrease) recommended by third parties excluding rate case expenses							$ (2,600)
SPS | FERC Staff [Member] | Wholesale Electric Rate Complaint, April 20, 2012 through July 18, 2013 Impact
Public Utilities, General Disclosures [Line Items]
Public Utilities, First refund period															1
Public Utilities, Base return on equity requested by the FERC through production formula rates, Percentage															8.97%
Public Utilities, Base return on equity requested by the FERC through transmission formula rates, Percentage	[1]														9.47%
SPS | FERC Staff [Member] | Wholesale Electric Rate Complaint, July 19, 2013 through Oct. 19, 2014 Impact
Public Utilities, General Disclosures [Line Items]
Public Utilities, Base return on equity requested by the FERC through production formula rates, Percentage															8.64%
Public Utilities, Base return on equity requested by the FERC through transmission formula rates, Percentage	[1]														9.14%
Public Utilities, Second refund period															2
SPS | FERC Staff [Member] | Wholesale Electric Rate Complaint, Oct. 20, 2014 Impact
Public Utilities, General Disclosures [Line Items]
Public Utilities, Base return on equity requested by the FERC through production formula rates, Percentage															8.53%
Public Utilities, Base return on equity requested by the FERC through transmission formula rates, Percentage	[1]														9.03%
Public Utilities, Third refund period															3
Minimum | SPS | Wholesale Electric Rate Complaints
Public Utilities, General Disclosures [Line Items]
Public Utilities, Increase (Decrease) of transmission and production revenues, net of expense due to potential ROE adjustments																$ (4,000)
Maximum | SPS | Wholesale Electric Rate Complaints
Public Utilities, General Disclosures [Line Items]
Public Utilities, Increase (Decrease) of transmission and production revenues, net of expense due to potential ROE adjustments																$ (6,000)
Subsequent Event | SPS | Wholesale Electric Rate Complaints, Refunds as of April 20, 2012 and July 19, 2013
Public Utilities, General Disclosures [Line Items]
Public Utilities, Number of rate complaints where appeal was filed with the D.C. Circuit Court of Appeals due to the FERC orders to allow back-to-back complaints for consecutive month refund periods					2
Public Utilities, appeal of the number of months refund period					15 months

[1] Includes a SPP RTO membership adder up to 50 basis points.

[2] In addition to the recommended ROEs, SPS also filed testimony recommending the ROEs remain unchanged.

[3] For the third refund period, the recommended production and transmission ROEs are supported by Golden Spread, certain New Mexico cooperatives and the West Texas Municipal Power Agency (transmission ROE only).