Commitments and Contingencies, Capital Commitments (Details) - Capital Commitments	12 Months Ended
	Dec. 31, 2017 kV MW
TUCO to Yoakum to Hobbs Plant Transmission Line
Capital Commitments [Abstract]
Voltage capacity for transmission line (in kV)	345
Hobbs Plant to China Draw Transmission Line
Capital Commitments [Abstract]
Voltage capacity for transmission line (in kV)	345
New Mexico and Texas Wind Projects [Member]
Capital Commitments [Abstract]
Public Utilities, Facility Generating Capacity | MW	1,000