Rate Matters, SPS (Details) (SPS, USD $) In Millions, unless otherwise specified	1 Months Ended
	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	Apr. 30, 2014 Subsequent Event Public Utility Commission of Texas (PUCT) 2014 Electric Rate Case
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 Return on Equity, Percentage	10.40%			10.25%	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 Rate Increase (Decrease), Amended, Net Amount						48.1
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage						5.30%
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount				32.5		76.9
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to resetting TCRF to zero						(12.9)
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to customer credit for gain on sale						(4.9)
Public Utilities, Requested base revenue increase (decrease) excluding fuel clause offsets, Amended						59.1
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to fuel offsets						(11.0)
Public Utilities, Requested increase (decrease) to rider revenue		13.0
Public Utilities, Requested Rate Increase (Decrease), Amount					45.9
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, Approved Return on Equity, Percentage			9.96%
Public Utilities, Approved Equity Capital Structure, Percentage			53.89%
Public Utilities, Approved Rate Increase (Decrease) due to pension, Amount			2.4
Public Utilities, portion of approved rate increase (decrease) to be recovered through rider revenue			18.1
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 Rate Increase (Decrease) due to return on equity, Amount			(1.2)
Public Utilities, approved rate increase (decrease) due to rider adjustment for renewable energy costs			6.0
Public Utilities, approved rate increase (decrease) due to base rate adjustment for renewable energy costs			(6.0)
Public Utilities, Approved Rate Increase (Decrease) due to other, Amount			(0.5)
Public Utilities, portion of approved rate increase (decrease) to be recovered in base revenue			$ 12.7