Rate Matters (Details) (USD $)	12 Months Ended			1 Months Ended																		0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2014	Sep. 30, 2014	Apr. 30, 2014	Jan. 31, 2014	Nov. 30, 2014	Nov. 30, 2013	Sep. 30, 2013	Dec. 31, 2012	Oct. 31, 2014	Jul. 31, 2013	Apr. 30, 2012	Aug. 31, 2013	Jul. 31, 2014	Mar. 31, 2014	Jun. 30, 2014	Jan. 31, 2015	Jan. 30, 2015
Rate Matters [Abstract]
Other current liabilities	$ 80,211,000	$ 75,355,000		$ 75,355,000		$ 80,211,000
Proceeds from Sale of Property, Plant, and Equipment	0	37,118,000	0
Cost of Domestic Regulated Electric	1,192,176,000	1,059,330,000	889,567,000
SPS | Sale of Texas Transmission Assets
Rate Matters [Abstract]
Number of substations included in purchase and sale agreement					2
Proceeds from Sale of Property, Plant, and Equipment				37,100,000
SPS | Sale of Texas Transmission Assets | Customers
Rate Matters [Abstract]
Public Utilities, Regulatory liabilities recognized for jurisdictional gain sharing				7,200,000
SPS | Sale of Texas Transmission Assets | Shareholders
Rate Matters [Abstract]
Pre-tax earnings impact of regulatory proceedings				13,600,000
SPS | PUCT Proceeding - Texas 2015 Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount						64,750,000
Public Utilities, Requested Rate Increase (Decrease), Percentage						6.70%
Public Utilities, Requested Return on Equity, Percentage						10.25%
Public Utilities, Requested Rate Base, Amount						1,560,000,000
Public Utilities, Requested Equity Capital Structure, Percentage						53.97%
Public utilities, Additional capital investment proposed for inclusion in rate case						442,000,000
Public Utilities, Portion of requested rate increase (decrease) related to post-test year investments in capital expenditures						29,600,000
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						6,050,000
SPS | PUCT Proceeding - Texas 2014 Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount									81,500,000
Public Utilities, Requested Rate Increase (Decrease), Percentage									5.80%
Public Utilities, Requested Return on Equity, Percentage									10.40%
Public Utilities, Requested Rate Base, Amount									1,270,000,000
Public Utilities, Requested Equity Capital Structure, Percentage									53.89%
Public Utilities, Portion of requested rate increase (decrease) related to depreciation expense									16,000,000
Public Utilities, Requested Rate Increase (Decrease), Net Amount									52,700,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Net Amount								48,100,000
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	15,400,000
Public Utilities, Settlement Agreement, Requested Return on Equity, Percentage							9.70%
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
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)
SPS | PUCT Proceeding - Transmission Cost Recovery Factor (TCRF) Rider
Rate Matters [Abstract]
Public Utilities, Requested increase (decrease) to rider revenue											13,000,000
Public Utilities, Interim Rate Refund, Amount										3,700,000
SPS | NMPRC Proceeding - New Mexico 2014 Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount													45,900,000
Public Utilities, Requested Return on Equity, Percentage													10.65%
Public Utilities, Requested Rate Base, Amount													479,800,000
Public Utilities, Requested Equity Capital Structure, Percentage													53.89%
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount												32,500,000
Public Utilities, Requested Return on Equity, Revised, 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)
SPS | FERC Proceeding - Wholesale Electric Rate Complaints
Rate Matters [Abstract]
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%
SPS | FERC Proceeding - FERC Orders
Rate Matters [Abstract]
Number of components included in regulatory proceeding																	2
Other current liabilities		44,500,000		44,500,000
Pre-tax earnings impact of regulatory proceedings	(5,900,000)
SPS | FERC Proceeding - Request for Waiver of SPP Tariff
Rate Matters [Abstract]
Cost of Domestic Regulated Electric	2,900,000
SPS | Public Utility Commission of Texas (PUCT) | PUCT Proceeding - Transmission Cost Recovery Factor (TCRF) Rider
Rate Matters [Abstract]
Public Utilities, Approved increase (decrease) to rider revenue																		4,000,000
SPS | New Mexico Public Regulation Commission (NMPRC) | NMPRC Proceeding - New Mexico 2014 Electric Rate Case
Rate Matters [Abstract]
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%
SPS | Federal Energy Regulatory Commission (FERC) | FERC Proceeding - Wholesale Electric Rate Complaints
Rate Matters [Abstract]
Public Utilities, Number of steps used in discounted cash flow analysis																				2
SPS | Federal Energy Regulatory Commission (FERC) | FERC Proceeding - FERC Orders
Rate Matters [Abstract]
Number of coincident peaks used as demand allocator, revised																	3
Number of coincident peaks used as demand allocator, original																	12
Subsequent Event | SPS | PUCT Proceeding - Texas 2014 Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Revenue requested to be collected through surcharge																					$ 15,600,000
Subsequent Event | SPS | FERC Proceeding - Wholesale Electric Rate Complaints, April 20, 2012 through July 18, 2013 Impact
Rate Matters [Abstract]
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%
Subsequent Event | SPS | FERC Proceeding - Wholesale Electric Rate Complaints, July 19, 2013 through Oct. 19, 2014 Impact
Rate Matters [Abstract]
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%
Subsequent Event | SPS | FERC Proceeding - FERC Orders
Rate Matters [Abstract]
Number of customers for which a public utility proposes to revise formula rates																						6
Number of coincident peaks requested to be used as demand allocator																						12