Regulatory and Rate Matters - TNMP Narrative (Details) - Texas-New Mexico Power Company			1 Months Ended	12 Months Ended
	Jan. 30, 2018 USD ($)	Mar. 13, 2017 USD ($)	Jul. 31, 2011 USD ($)	Dec. 31, 2017 USD ($) advanced_meter	Feb. 20, 2018 customer	Oct. 02, 2015 USD ($)	Jun. 20, 2014 USD ($)	Aug. 15, 2013 USD ($) customer $ / fee
Public Utilities, General Disclosures [Line Items]
CTC Funding amount interest minimum				15.00%
Adjustment for collection of amortization		$ (1,100,000)
Advanced Meter System Deployment and Surcharge Request
Public Utilities, General Disclosures [Line Items]
Approved deployment costs			$ 113,400,000
Period of time to collect deployment costs through surcharge period			12 years
Number of advanced meters installed | advanced_meter				242,000
Non-standard metering service cost total to be borne by opt-out customers								$ 200,000
Non-standard metering ongoing annual expenses to be borne by opt-out customers								$ 500,000
Non-standard metering ongoing expenses monthly charge							$ 36.78
Presumed number of customers that will elect non-standard meter service (in customers) | customer								1,081
Cost and savings reconciliation filed with regulators, capital cost savings						$ 71,000,000
Cost and savings reconciliation filed with regulators, operations and maintenance expense						$ 18,000,000
Program costs incurred to date				$ 1,500,000
Advanced Meter System Deployment and Surcharge Request | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Current number of customers that have elected non-standard meter service (in customers) | customer					99
Advanced Meter System Deployment and Surcharge Request | Minimum
Public Utilities, General Disclosures [Line Items]
Non-standard metering service cost initial fee range | $ / fee								63.97
Advanced Meter System Deployment and Surcharge Request | Maximum
Public Utilities, General Disclosures [Line Items]
Non-standard metering service cost initial fee range | $ / fee								168.61
Transmission Cost of Service Rates | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Requested increase in annual transmission service revenue	$ 600,000
Requested rate increase (decrease)	$ 3,200,000