Fair Value of Derivative and Other Financial Instruments - Overview and Commodity Derivatives (Details)	Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 MW
Public Service Company of New Mexico
Derivatives, Fair Value [Line Items]
Expected exposure to market risk (in megawatts) | MW	65		65
Power to be sold to third party (in megawatts) | MW	36		36
PNMR and PNM
Derivatives, Fair Value [Line Items]
Amounts recognized for the legal right to reclaim cash collateral	$ 0	$ 0
Amounts posted as cash collateral under margin arrangements	500,000	1,000,000.0
Obligations to return cash collateral	900,000	1,000,000.0
PNMR and PNM | Designated as hedging instrument | Commodity Contract
Derivatives, Fair Value [Line Items]
Other current assets	1,089,000	1,083,000
Other deferred charges	1,507,000	2,511,000
Total assets	2,596,000	3,594,000
Other current liabilities	(1,089,000)	(1,177,000)
Other deferred credits	(1,507,000)	(2,511,000)
Total Liabilities	(2,596,000)	(3,688,000)
Net	0	(94,000)
Other credit derivatives | Public Service Company of New Mexico | Designated as hedging instrument | Commodity Contract
Derivatives, Fair Value [Line Items]
Total assets	$ 2,600,000	$ 3,600,000
Tri-State | Public Service Company of New Mexico
Derivatives, Fair Value [Line Items]
Power to be sold to third party (in megawatts) | MW	100