Franchises and Significant Customers (Details)	12 Months Ended
	Dec. 31, 2015
El Paso
Franchises and Significant Customers [Line Items]
Preceded Franchise Fee Percent	3.25%
Percentage of Revenue Additional Franchise Fees, Restricted	0.75%
Retail Electric Service Contract, Expiration Date	Jul. 31, 2030
Las Cruces
Franchises and Significant Customers [Line Items]
Retail Electric Service Contract, Expiration Date	Apr. 30, 2009
Fort Bliss
Franchises and Significant Customers [Line Items]
Contract Term to Provide Retail Electric Service	3 years
Holloman
Franchises and Significant Customers [Line Items]
Contract Term to Provide Retail Electric Service	10 years
Retail Electric Service Contract, Expiration Date	Jan. 31, 2016
August 1, 2010 through Present | El Paso
Franchises and Significant Customers [Line Items]
Franchise Fee, Percent	4.00%
February 1, 2000 through Present | Las Cruces
Franchises and Significant Customers [Line Items]
Franchise Fee, Percent	2.00%
Sales Revenue, Services, Net | Military Bases
Franchises and Significant Customers [Line Items]
Concentration Risk, Percentage	4.00%