Regulatory and Rate Matters - TNMP Narrative (Details) - Texas-New Mexico Power Company		1 Months Ended	3 Months Ended
	Mar. 13, 2017 USD ($)	Jul. 30, 2011 USD ($)	Mar. 31, 2017 USD ($) customer	Apr. 24, 2017 customer	Dec. 31, 2016 advanced_meter
Public Utilities, General Disclosures [Line Items]
Adjustment of the most recent annual CTC funding amount (greater than or equal to)			15.00%
Requested increase (decrease) adjustment to reduce 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
Collection of deployment costs through surcharge period		12 years
Number of advanced meters installed | advanced_meter					242,000
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
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				103
Advanced Meter System Deployment and Surcharge Request | Minimum
Public Utilities, General Disclosures [Line Items]
Approved non-standard metering service cost, initial fee range			$ 63.97
Advanced Meter System Deployment and Surcharge Request | Maximum
Public Utilities, General Disclosures [Line Items]
Approved non-standard metering service cost, initial fee range			$ 168.61