Regulatory and Rate Matters - TNMP Narrative (Details) - Texas-New Mexico Power Company [Member] advanced_meter in Thousands		1 Months Ended	12 Months Ended
	Jan. 20, 2017 USD ($)	Jul. 31, 2011 USD ($)	Dec. 31, 2016 USD ($) advanced_meter	Feb. 21, 2017 customer	Oct. 02, 2015 USD ($)	Jun. 20, 2014 USD ($)	Aug. 15, 2013 USD ($) customer
Advanced Meter System Deployment and Surcharge Request [Member]
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
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 [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Current number of customers that have elected non-standard meter service (in customers) | customer				104
Advanced Meter System Deployment and Surcharge Request [Member] | Minimum [Member]
Public Utilities, General Disclosures [Line Items]
Non-standard metering service cost initial fee range							$ 63.97
Advanced Meter System Deployment and Surcharge Request [Member] | Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Non-standard metering service cost initial fee range							$ 168.61
Transmission Cost of Service Rates [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Requested increase in annual transmission service revenue	$ 4,800,000
Requested rate increase (decrease)	$ 30,200,000