Rate Matters, SPS (Details) (SPS, USD $) In Millions, unless otherwise specified	1 Months Ended
	Apr. 30, 2014 Public Utility Commission of Texas (PUCT) 2014 Electric Rate Case	Jan. 31, 2014 Public Utility Commission of Texas (PUCT) 2014 Electric Rate Case	Nov. 30, 2013 Public Utility Commission of Texas (PUCT) Transmission Cost Recovery Factor (TCRF) Rider	Mar. 31, 2014 New Mexico Public Regulation Commission (NMPRC) 2014 Electric Rate Case	Sep. 30, 2013 New Mexico Public Regulation Commission (NMPRC) 2014 Electric Rate Case	Dec. 31, 2012 New Mexico Public Regulation Commission (NMPRC) 2014 Electric Rate Case	Jun. 30, 2014 Federal Energy Regulatory Commission (FERC) Wholesale Electric Rate Complaints	Jul. 31, 2013 Federal Energy Regulatory Commission (FERC) Wholesale Electric Rate Complaints	Apr. 30, 2012 Federal Energy Regulatory Commission (FERC) Wholesale Electric Rate Complaints	Jul. 31, 2014 Subsequent Event Public Utility Commission of Texas (PUCT) Transmission Cost Recovery Factor (TCRF) Rider
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Net Amount		$ 52.7
Public Utilities, Requested Rate Increase (Decrease), Percentage		5.80%
Public Utilities, Requested Rate Increase (Decrease), Amended, Net Amount	48.1
Public Utilities, Requested Return on Equity, Percentage		10.40%				10.65%
Public Utilities, Requested Rate Base, Amount		1,270.0				479.8
Public Utilities, Requested Equity Capital Structure, Percentage		53.89%				53.89%
Public Utilities, Portion of requested rate increase (decrease) related to depreciation expense		16
Public Utilities, Requested increase (decrease) to rider revenue			13.0
Public Utilities, Approved increase (decrease) to rider revenue										4.0
Public Utilities, Requested Rate Increase (Decrease), Amount						45.9
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount					32.5
Public Utilities, Requested Return on Equity, Amended, Percentage					10.25%
Public Utilities, portion of revised rate increase (decrease) related to base and fuel revenue.					20.9
Public Utilities, portion of revised rate increase (decrease) related to rider revenue					12.1
Public Utilities, portion of revised rate increase (decrease) related to other costs					(0.5)
Public Utilities, Approved Rate Increase (Decrease), Amount				33.1
Public Utilities, portion of approved rate increase (decrease) to be recovered in base revenue				12.7
Public Utilities, portion of approved rate increase (decrease) to be recovered through rider revenue				18.1
Public Utilities, Approved Return on Equity, Percentage				9.96%
Public Utilities, Approved Equity Capital Structure, Percentage				53.89%
Public Utilities, portion of approved rate increase (decrease) to be recovered through fuel clause adjustment				2.3
Public Utilities, revised rate increase (decrease), fuel adjusted				34.8
Public Utilities, Approved adjustment to rate increase (decrease) due to return on equity				(1.2)
Public Utilities, Approved adjustment to rate increase (decrease) due to rider adjustment for renewable energy costs				6.0
Public Utilities, Approved adjustment to rate increase (decrease) due to base rate adjustment for renewable energy costs				(6.0)
Public Utilities, Approved adjustment to rate increase (decrease) due to other				$ (0.5)
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%
Public Utilities, Base return on equity requested by customers to be charged through transmission formula rates, Percentage								9.65%
Public Utilities, Number of steps to be used in discounted cash flow analysis							2