Rate Matters (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended				1 Months Ended			0 Months Ended			1 Months Ended			0 Months Ended	1 Months Ended
	Mar. 29, 2013 Certain Texas Transmission Assets [Member] Facility	Sep. 21, 2013 Public Utility Commission of Texas [Member] Certain Texas Transmission Assets [Member]	Sep. 21, 2013 Public Utility Commission of Texas [Member] Certain Texas Transmission Assets [Member] Shareholders [Member]	Sep. 21, 2013 Public Utility Commission of Texas [Member] Certain Texas Transmission Assets [Member] Customers [Member]	Nov. 30, 2012 Public Utility Commission of Texas [Member] Electric Rate Case 2012 [Member]	Jun. 30, 2013 Public Utility Commission of Texas [Member] Electric Rate Case 2012, Settlement Rates Effective May 1, 2013 [Member]	Jun. 30, 2013 Public Utility Commission of Texas [Member] Electric Rate Case 2012, Settlement Rates Effective September 1, 2013 [Member]	Sep. 12, 2013 New Mexico Public Regulation Commission [Member] Certain Texas Transmission Assets [Member] Customers [Member]	Sep. 09, 2013 New Mexico Public Regulation Commission [Member] 2014 Electric Rate Case [Member]	Jun. 19, 2013 New Mexico Public Regulation Commission [Member] 2014 Electric Rate Case [Member]	Aug. 31, 2013 New Mexico Public Regulation Commission [Member] 2014 Electric Rate Case [Member]		Dec. 31, 2012 New Mexico Public Regulation Commission [Member] 2014 Electric Rate Case [Member]	Sep. 12, 2013 New Mexico Attorney General [Member] Certain Texas Transmission Assets [Member] Customers [Member]	Aug. 31, 2013 New Mexico Attorney General [Member] 2014 Electric Rate Case [Member]		Aug. 31, 2013 Federal Energy Regulatory Commission (FERC) [Member] Federal Energy Regulatory Commission (FERC) Orders [Member] Factor	Aug. 31, 2013 Federal Energy Regulatory Commission (FERC) [Member] FERC Orders, Settlement Impact Through May 31, 2015 [Member]	Aug. 31, 2013 Federal Energy Regulatory Commission (FERC) [Member] FERC Orders, Settlement Impact Effective June 1, 2015 [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, ROE recommended by third parties											9.80%				8.63%
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 90.2								$ 45.9
Public Utilities, Number of months included in test year for rate filing					12 months
Public Utilities, Requested Return on Equity, Percentage					10.65%				10.25%				10.65%
Public Utilities, Requested Rate Base, Amount					1,150.0								479.8
Public Utilities, Requested Equity Capital Structure, Percentage					52.00%								53.89%
Public Utilities, Approved Rate Increase (Decrease), Amount						37.0	13.8
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount									32.5	43.3
Number of substations included in purchase and sale agreement	2
Proceeds from Sale of Property, Plant, and Equipment	37
Public utilities, adjustment to requested rate increase (decrease) recommended by third parties related to the rate rider for renewable energy costs											(14.5)	[1]			(8.5)	[1]
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to revenue adjustments											(4.4)				(6.4)
Public Utilities, Adjustment recommended by third parties related to return on equity											(3.2)				(8.1)
Public utilities, adjustment to requested rate increase (decrease) related to change in capital structure											(1.5)				(1.1)
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to reduced recovery for employee benefits											(2.8)				(1.8)
Public utilities, adjustment to requested rate increase (decrease) recommended by third parties related to reduced recovery for payroll expense											(0.1)				(0.1)
Public utilities, adjustment to requested rate increase (decrease) recommended by third parties related to gain on sale of assets											0				(1.7)
Public utilities, adjustment to requested rate increase (decrease) recommended by third parties related to fuel clause revenue											6.0				0
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to other costs											(5.0)				(6.6)
Public Utilities, Rate increase (decrease) recommended by third parties											17.8				9.0
Public utilities, portion of rate increase (decrease) recommended by third parties to be recovered in base revenue											8.8				(6.0)
Public utilities, portion of rate increase (decrease) recommended by third parties to be recovered in rider revenue											7.3				13.3
Public utilities, portion of rate increase (decrease) recommended by third parties to be recovered in fuel cost adjustment revenue											1.7				1.7
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)
Number of components included in regulatory proceeding																	2
Number of coincident peaks used as demand allocator, original																	12
Number of coincident peaks used as demand allocator, revised																	3
Public utilities, estimated refund to customers resulting from regulatory proceedings																	42
Current year pre-tax earnings impact of regulatory proceedings																	(35)
Public utilities, annual increase (decrease) in revenues resulting from regualtory proceeding																		$ (6)	$ (4)
Amount of the net pre-tax gain allocable to the Texas retail jurisdiction		45.00%
Percentage of net pre-tax gain associated with the Texas retail jurisdiction to be allocated			40.00%	60.00%
Percentage of net pre-tax gain allocable to the New Mexico jurisdiction to be retained by customers								100.00%						100.00%

[1] Adjustments represent recommended deferrals, extended amortizations and moving costs from rider to fuel in base rates.