Fair Value of Derivative and Other Financial Instruments - Overview and Commodity Derivatives (Details)	Jun. 30, 2018 USD ($) MW	Dec. 31, 2017 USD ($)
Derivatives, Fair Value [Line Items]
Current assets	$ 1,094,000	$ 1,088,000
Deferred charges	3,014,000	3,556,000
Current liabilities	(1,416,000)	(1,182,000)
Long-term liabilities	(3,014,000)	(3,556,000)
Obligations to return cash	$ 1,900,000	900,000
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	$ 1,094,000	1,088,000
Deferred charges	3,014,000	3,556,000
Current liabilities	(1,416,000)	(1,182,000)
Long-term liabilities	(3,014,000)	(3,556,000)
Amounts recognized for right to reclaim cash	0	0
Cash collateral under margin arrangements	500,000	800,000
PNM \| Designated as Hedging Instrument \| Commodity derivatives
Derivatives, Fair Value [Line Items]
Current assets	1,094,000	1,088,000
Deferred charges	3,014,000	3,556,000
Derivative Asset	4,108,000	4,644,000
Current liabilities	(1,416,000)	(1,182,000)
Long-term liabilities	(3,014,000)	(3,556,000)
Derivative Liability	(4,430,000)	(4,738,000)
Net	(322,000)	(94,000)
PNM \| Designated as Hedging Instrument \| Commodity derivatives \| Fuel and purchased power costs
Derivatives, Fair Value [Line Items]
Current assets	100,000	0
Current liabilities	(300,000)
PNM \| Designated as Hedging Instrument \| Commodity derivatives \| Hazard sharing arrangement
Derivatives, Fair Value [Line Items]
Derivative Asset	$ 4,100,000	$ 4,600,000

Public Utilities, Concentration Risk, Amount To Sell Due To Agreement

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Public Utilities, Concentration Risk, Potential Market Risk

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