Franchises and Significant Customers Franchises and Significant Customers (Details)	12 Months Ended
	Dec. 31, 2012
Franchises and Significant Customers [Line Items]
Percentage of Revenue Additional Franchise Fees, Restricted	0.75%
White Sands [Member]
Franchises and Significant Customers [Line Items]
Retail Electric Service Contract, Expiration Date	2009
Contract Term to Provide Retail Electric Service	10 years
Las Cruces
Franchises and Significant Customers [Line Items]
Retail Electric Service Contract, Expiration Date	April 30, 2009
Fort Bliss [Member]
Franchises and Significant Customers [Line Items]
Retail Electric Service Contract, Expiration Date	2010
Holloman [Member]
Franchises and Significant Customers [Line Items]
Retail Electric Service Contract, Expiration Date	January 2016
Contract Term to Provide Retail Electric Service	10 years
July 1, 2005 through August 1, 2010 | El Paso
Franchises and Significant Customers [Line Items]
Franchise Fee, Percent	3.25%	[1]
August 1, 2010 through Present | El Paso
Franchises and Significant Customers [Line Items]
Franchise Fee, Percent	4.00%	[1],[2]
February 1, 2000 through Present | Las Cruces
Franchises and Significant Customers [Line Items]
Franchise Fee, Percent	2.00%	[1]
Sales Revenue, Services, Net | Military Bases
Franchises and Significant Customers [Line Items]
Concentration Risk, Percentage	5.00%

[1] Based on a percentage of revenue.

[2] The additional fee of 0.75% is to be placed in a restricted fund to be used solely for economic development and renewable energy purposes.