Fair Value of Derivative and Other Financial Instruments - Overview and Commodity Derivatives (Details)	Jun. 30, 2020 USD ($) MW	Dec. 31, 2019 USD ($)
Derivatives, Fair Value [Line Items]
Obligations to return cash		$ 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
Amounts recognized for right to reclaim cash	$ 0	0
Cash collateral under margin arrangements		500,000
PNM | Designated as Hedging Instrument | Commodity derivatives
Derivatives, Fair Value [Line Items]
Other current assets	1,096,000	1,089,000
Other deferred charges	1,004,000	1,507,000
Derivative asset	2,100,000	2,596,000
Other current liabilities	(1,096,000)	(1,089,000)
Other deferred credits	(1,004,000)	(1,507,000)
Derivative liability	(2,100,000)	(2,596,000)
Net	0	0
PNM | Designated as Hedging Instrument | Commodity derivatives | Fuel and purchased power costs
Derivatives, Fair Value [Line Items]
Other current assets	$ 0	$ 0
Tri-State | PNM
Derivatives, Fair Value [Line Items]
Power to be sold to third party (in megawatts) | MW	100