Franchises and Significant Customers (Details)	12 Months Ended
	Dec. 31, 2018 MW
El Paso
Franchises and Significant Customers [Line Items]
Franchise Agreement Extension Term	30 years
Retail Electric Service Contract, Expiration Date	Jul. 31, 2060
Las Cruces
Franchises and Significant Customers [Line Items]
Retail Electric Service Contract, Expiration Date	Apr. 30, 2009
October 1, 2018 through Present | El Paso
Franchises and Significant Customers [Line Items]
Franchise Fee, Percent	5.00%
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	2.60%
Holloman Air Force Base Solar Facility
Franchises and Significant Customers [Line Items]
Electric capacity	5
2005 Amendment | El Paso
Franchises and Significant Customers [Line Items]
Preceded Franchise Fee Percent	3.25%
2010 Amendment | El Paso
Franchises and Significant Customers [Line Items]
Percentage of Revenue Additional Franchise Fees, Restricted	0.75%
2018 Amendment | El Paso
Franchises and Significant Customers [Line Items]
Percentage of Revenue Additional Franchise Fees, Restricted	1.00%