Rate Matters (Details) (USD $)	1 Months Ended							3 Months Ended
	Oct. 31, 2014	Jul. 31, 2013	Apr. 30, 2012	Mar. 31, 2015	Dec. 31, 2014	Jan. 31, 2015	Aug. 31, 2013 Factor	Mar. 31, 2015
Public Utilities, General Disclosures [Line Items]
Other Liabilities, Current				$ 88,724,000	$ 80,211,000			$ 88,724,000
FERC Proceeding - 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 by customers to be charged through production formula rates, Percentage	8.61%	9.15%
Public Utilities, Base return on equity requested by customers to be charged through transmission formula rates, Percentage	9.11%	9.65%
Public Utilities, Incremental ROE basis point increase (decrease) recommended by third parties	50
Texas 2015 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					64,800,000
Public Utilities, Requested Rate Increase (Decrease), Percentage					6.70%
Public Utilities, Requested Return on Equity, Percentage					10.25%
Public Utilities, Requested Rate Base, Amount					1,600,000,000
Public Utilities, Requested Equity Capital Structure, Percentage					53.97%
Public Utilities, Revised requested rate increase (decrease)				58,900,000
Public Utilities, Additional capital investment					392,000,000
FERC Proceeding - FERC Orders
Public Utilities, General Disclosures [Line Items]
Number of components included in regulatory proceeding							2
Other Liabilities, Current					50,400,000
Current year increase (decrease) to pre-tax earnings resulting from regulatory proceedings								1,800,000
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 [Member] | FERC Proceeding - Wholesale Electric Rate Complaints
Public Utilities, General Disclosures [Line Items]
Public Utilities, Incremental ROE basis point increase (decrease) recommended by third parties								50
SPS [Member] | Wholesale Electric Rate Complaint, Refunds as of April 20, 2012 [Member]
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								10.39%	[1]
SPS [Member] | Texas 2015 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revised requested rate increase (decrease)				58,900,000
Public Utilities, Portion of requested rate increase (decrease) related to post-test year investments in capital expenditures				23,700,000
Public Utilities, Portion of requested rate increase (decrease) related to depreciation expense				13,900,000
Public Utilities, Portion of requested rate increase (decrease) related to wholesale load reductions				12,000,000
Public Utilities, Portion of requested rate increase (decrease) related to purchased power capacity costs				3,200,000
Public Utilities, Portion of requested rate increase (decrease) related to other costs				6,100,000
SPS [Member] | Wholesale Electric Rate Complaint, Refunds as of July 19, 2013 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Second refund period								2
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								11.20%	[1]
SPS [Member] | FERC Proceeding - FERC Orders
Public Utilities, General Disclosures [Line Items]
Number of coincident peaks used as demand allocator, original						12
SPS [Member] | Golden Spread [Member] | Wholesale Electric Rate Complaint, Refunds as of April 20, 2012 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, First refund period								1
Public Utilities, Base return on equity requested by customers to be charged through production formula rates, Percentage								8.78%
Public Utilities, Base return on equity requested by customers to be charged through transmission formula rates, Percentage								9.28%	[1]
SPS [Member] | Golden Spread [Member] | Wholesale Electric Rate Complaint, Refunds as of July 19, 2013 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Second refund period								2
Public Utilities, Base return on equity requested by customers to be charged through production formula rates, Percentage								8.51%
Public Utilities, Base return on equity requested by customers to be charged through transmission formula rates, Percentage								9.01%	[1]
SPS [Member] | FERC Staff [Member] | Wholesale Electric Rate Complaint, Refunds as of April 20, 2012 [Member]
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								9.47%	[1]
SPS [Member] | FERC Staff [Member] | Wholesale Electric Rate Complaint, Refunds as of July 19, 2013 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Second refund period								2
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								9.14%	[1]
Minimum [Member] | FERC Proceeding - 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								(4,000,000)
Maximum [Member] | FERC Proceeding - 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								$ (6,000,000)

[1] (a) Includes a Southwest Power Pool, Inc. (SPP) Regional Transmission Organization (RTO) membership adder up to 50 basis points.