Rate Matters (Details) (USD $)	3 Months Ended		9 Months Ended			1 Months Ended			3 Months Ended	1 Months Ended								9 Months Ended		1 Months Ended		9 Months Ended	1 Months Ended				0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 SPS PUCT Proceeding - Texas 2014 Electric Rate Case	Apr. 30, 2014 SPS PUCT Proceeding - Texas 2014 Electric Rate Case	Jan. 31, 2014 SPS PUCT Proceeding - Texas 2014 Electric Rate Case	Sep. 30, 2014 SPS PUCT Proceeding - Texas 2014 Electric Rate Case	Sep. 30, 2014 SPS PUCT Proceeding - Transmission Cost Recovery Factor (TCRF) Rider	Nov. 30, 2013 SPS PUCT Proceeding - Transmission Cost Recovery Factor (TCRF) Rider	Sep. 30, 2013 SPS NMPRC Proceeding - New Mexico 2014 Electric Rate Case	Dec. 31, 2012 SPS NMPRC Proceeding - New Mexico 2014 Electric Rate Case	Jul. 31, 2013 SPS FERC Proceeding - Wholesale Electric Rate Complaints	Apr. 30, 2012 SPS FERC Proceeding - Wholesale Electric Rate Complaints	Oct. 31, 2013 SPS FERC Proceeding - FERC Orders	Aug. 31, 2013 SPS FERC Proceeding - FERC Orders Factor	Sep. 30, 2014 SPS FERC Proceeding - FERC Orders	Dec. 31, 2013 SPS FERC Proceeding - FERC Orders	Aug. 31, 2013 SPS FERC Proceeding - FERC Orders, Settlement Impact Through May 31, 2015	Aug. 31, 2013 SPS FERC Proceeding - FERC Orders, Settlement Impact Effective June 1, 2015	Sep. 30, 2014 SPS FERC Proceeding - Request for Waiver of SPP Tariff	Jul. 31, 2014 SPS Public Utility Commission of Texas (PUCT) PUCT Proceeding - Transmission Cost Recovery Factor (TCRF) Rider	Mar. 31, 2014 SPS New Mexico Public Regulation Commission (NMPRC) NMPRC Proceeding - New Mexico 2014 Electric Rate Case	Jun. 30, 2014 SPS Federal Energy Regulatory Commission (FERC) FERC Proceeding - Wholesale Electric Rate Complaints	Aug. 31, 2013 SPS Federal Energy Regulatory Commission (FERC) FERC Proceeding - FERC Orders	Oct. 20, 2014 Subsequent Event SPS FERC Proceeding - Wholesale Electric Rate Complaints
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Net Amount								$ 52,700,000
Public Utilities, Requested Rate Increase (Decrease), Percentage								5.80%
Public Utilities, Requested Rate Increase (Decrease), Amended, Net Amount							48,100,000
Public Utilities, Requested Return on Equity, Percentage								10.40%					10.65%
Public Utilities, Requested Rate Base, Amount								1,270,000,000					479,800,000
Public Utilities, Requested Equity Capital Structure, Percentage								53.89%					53.89%
Public Utilities, Portion of requested rate increase (decrease) related to depreciation expense								16,000,000
Public Utilities, Settlement Agreement, Requested Rate Increase (Decrease), Amount						37,000,000
Public Utilities, Settlement Agreement, Requested Rate Increase (Decrease), Percentage						3.50%
Public Utilities, Revenue recognized through surcharge									13,300,000
Public Utilities, Settlement Agreement, Requested Return on Equity, Percentage						9.70%
Public Utilities, Requested Rate Increase (Decrease), Amount								81,500,000					45,900,000
Public Utilities, Adjustment to requested rate increase (decrease) due to updated information							(4,600,000)
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount							76,900,000					32,500,000
Public Utilities, Adjustment to revised rate increase (decrease) request due to depreciation						(16,000,000)
Public Utilities, Adjustment to revised rate increase (decrease) request due to allocators for wholesale load reduction						(12,000,000)
Public Utilities, Adjustment to revised rate increase (decrease) request due to revised amortizations						(9,000,000)
Public Utilities, Adjustment to revised rate increase (decrease) request due to non-specified settlement adjustments						(2,900,000)
Public Utilities, Requested increase (decrease) to rider revenue											13,000,000
Public Utilities, Approved increase (decrease) to rider revenue																							4,000,000
Public Utilities, Interim Rate Refund, Amount										3,700,000
Public Utilities, Requested Return on Equity, Amended, Percentage												10.25%
Public Utilities, portion of revised rate increase (decrease) related to base and fuel revenue.												20,900,000
Public Utilities, portion of revised rate increase (decrease) related to rider revenue												12,100,000
Public Utilities, portion of revised rate increase (decrease) related to other costs												(500,000)
Public Utilities, Approved Rate Increase (Decrease), Amount																								33,100,000
Public Utilities, portion of approved rate increase (decrease) to be recovered in base revenue																								12,700,000
Public Utilities, portion of approved rate increase (decrease) to be recovered through rider revenue																								18,100,000
Public Utilities, Approved Return on Equity, Percentage																								9.96%
Public Utilities, Approved Equity Capital Structure, Percentage																								53.89%
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														9.15%													8.61%
Public Utilities, Base return on equity requested by customers to be charged through transmission formula rates, Percentage														9.65%													9.11%
Public Utilities, Number of steps to be used in discounted cash flow analysis																									2
Public Utilities, Number of prior complaints in a regulatory proceeding																											2
Number of components included in regulatory proceeding																	2
Number of coincident peaks used as demand allocator, revised																3										3
Number of coincident peaks used as demand allocator, original																										12
Other Liabilities, Current	81,868,000		81,868,000		75,355,000														44,500,000
Current year increase (decrease) to pre-tax earnings resulting from regulatory proceedings																		(4,000,000)
Public Utilities, Annual increase (decrease) in revenues resulting from regulatory proceeding																				(6,000,000)	(4,000,000)
Electric fuel and purchased power	$ 315,524,000	$ 293,831,000	$ 918,874,000	$ 814,077,000																		$ 2,900,000