Fair Value of Derivative and Other Financial Instruments - Overview and Commodity Derivatives (Details)	Sep. 30, 2017 USD ($) MW	Dec. 31, 2016 USD ($)
Derivatives, Fair Value [Line Items]
Current assets	$ 3,093,000	$ 5,224,000
Deferred charges	3,846,000	0
Current liabilities	(1,279,000)	(2,339,000)
Long-term liabilities	$ (3,846,000)	0
PNM
Derivatives, Fair Value [Line Items]
Expected exposure to market risk (in megawatts) | MW	65
Power to be sold to third party (in megawatts) | MW	36
Current assets	$ 3,093,000	5,224,000
Deferred charges	3,846,000	0
Current liabilities	(1,279,000)	(2,339,000)
Long-term liabilities	(3,846,000)	0
Amounts recognized for the right to reclaim cash collateral	0	0
Amounts posted as cash collateral under margin arrangements	1,200,000	2,600,000
PNM | Designated as Hedging Instrument | Commodity derivatives
Derivatives, Fair Value [Line Items]
Current assets	3,093,000	5,224,000
Deferred charges	3,846,000	0
Derivative Asset	6,939,000	5,224,000
Current liabilities	(1,279,000)	(2,339,000)
Long-term liabilities	(3,846,000)	0
Derivative Liability	(5,125,000)	(2,339,000)
Net	1,814,000	2,885,000
PNM | Designated as Hedging Instrument | Commodity derivatives | Fuel and purchased power costs
Derivatives, Fair Value [Line Items]
Current assets	100,000	200,000
Current liabilities	(200,000)	(100,000)
PNM | Designated as Hedging Instrument | Commodity derivatives | Palo Verde Nuclear Generating Station
Derivatives, Fair Value [Line Items]
Current assets	700,000	2,700,000
PNM | Designated as Hedging Instrument | Commodity derivatives | Hazard sharing arrangement
Derivatives, Fair Value [Line Items]
Derivative Asset	4,900,000	500,000
Derivative Liability	$ (5,200,000)	$ (500,000)