Regulatory and Rate Matters - TNMP Narrative (Details) - Texas-New Mexico Power Company			1 Months Ended	9 Months Ended
	May 25, 2017 USD ($)	Mar. 13, 2017 USD ($)	Jul. 30, 2011 USD ($)	Sep. 30, 2017 USD ($) customer	Oct. 20, 2017 customer	Dec. 31, 2016 advanced_meter
Public Utilities, Commitments And Contingencies [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)
Energy efficiency cost recovery factor, total amount requested	$ 6,000,000
Energy efficiency cost recovery factor, performance bonus requested	$ 1,100,000
Advanced Meter System Deployment and Surcharge Request
Public Utilities, Commitments And Contingencies [Line Items]
Approved deployment costs			$ 113,400,000
Collection of deployment costs through surcharge period			12 years
Number of advanced meters installed (more than) | 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
Minimum | Advanced Meter System Deployment and Surcharge Request
Public Utilities, Commitments And Contingencies [Line Items]
Approved non-standard metering service cost, initial fee range				$ 63.97
Maximum | Advanced Meter System Deployment and Surcharge Request
Public Utilities, Commitments And Contingencies [Line Items]
Approved non-standard metering service cost, initial fee range				$ 168.61
Subsequent event | Advanced Meter System Deployment and Surcharge Request
Public Utilities, Commitments And Contingencies [Line Items]
Current number of customers that have elected non-standard meter service (in customers) | customer					99