Regulatory and Rate Matters - TNMP Narrative (Details) - Texas-New Mexico Power Company [Member]			1 Months Ended	9 Months Ended
	Jul. 27, 2016 USD ($)	May 27, 2016 USD ($)	Jul. 30, 2011 USD ($)	Sep. 30, 2016 USD ($) advanced_meter	Oct. 21, 2016 customer	Oct. 02, 2015 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
Collection of deployment costs through surcharge period			12 years
Number of advanced meters installed | advanced_meter				242,246
Completion period of advanced meter deployment			5 years
Recovery in cost through initial fees							$ 200,000
Ongoing annual expenses							500,000
Approved 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 savings						$ 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					100
Advanced Meter System Deployment and Surcharge Request [Member] | Minimum [Member]
Public Utilities, General Disclosures [Line Items]
Approved 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]
Approved non-standard metering service cost, initial fee range							$ 168.61
Energy Efficiency [Member]
Public Utilities, General Disclosures [Line Items]
Requested adjustment to EECRF		$ 6,100,000
Requested adjustment to EECRF, performance bonus		$ 800,000
Approved program cost recovery	$ 6,000,000
Approved performance bonus	$ 800,000