Fair Value of Derivative and Other Financial Instruments - Overview and Commodity Derivatives (Details)	Sep. 30, 2019 USD ($) MW	Dec. 31, 2018 USD ($)
Derivatives, Fair Value [Line Items]
Obligations to return cash	$ 900,000	$ 1,000,000.0
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
Amounts recognized for right to reclaim cash	$ 0	0
Cash collateral under margin arrangements	500,000	1,000,000.0
PNM \| Designated as Hedging Instrument \| Commodity derivatives
Derivatives, Fair Value [Line Items]
Other current assets	1,157,000	1,083,000
Other deferred charges	1,770,000	2,511,000
Derivative asset	2,927,000	3,594,000
Other current liabilities	(1,097,000)	(1,177,000)
Other deferred credits	(1,770,000)	(2,511,000)
Derivative liability	(2,867,000)	(3,688,000)
Net	60,000	(94,000)
PNM \| Designated as Hedging Instrument \| Commodity derivatives \| Fuel and purchased power costs
Derivatives, Fair Value [Line Items]
Other current assets	100,000	0
PNM \| Designated as Hedging Instrument \| Commodity derivatives \| Hazard sharing arrangement
Derivatives, Fair Value [Line Items]
Derivative asset	$ 2,900,000	$ 3,600,000
Tri-State \| PNM
Derivatives, Fair Value [Line Items]
Power to be sold to third party (in megawatts) \| MW	100

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

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