Fair Value of Derivative and Other Financial Instruments - Overview and Commodity Derivatives (Details)	Mar. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Derivatives, Fair Value [Line Items]
Obligations to return cash	$ 900,000	$ 1,000,000
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	1,000,000	1,000,000
PNM | Designated as Hedging Instrument | Commodity derivatives
Derivatives, Fair Value [Line Items]
Current assets	1,086,000	1,083,000
Deferred charges	2,265,000	2,511,000
Derivative asset	3,351,000	3,594,000
Current liabilities	(1,150,000)	(1,177,000)
Long-term liabilities	(2,265,000)	(2,511,000)
Derivative liability	(3,415,000)	(3,688,000)
Net	(64,000)	(94,000)
PNM | Designated as Hedging Instrument | Commodity derivatives | Fuel and purchased power costs
Derivatives, Fair Value [Line Items]
Current assets	0		$ 0
PNM | Designated as Hedging Instrument | Commodity derivatives | Hazard sharing arrangement
Derivatives, Fair Value [Line Items]
Derivative asset	$ 3,300,000	$ 3,600,000
Tri-State | PNM
Derivatives, Fair Value [Line Items]
Power to be sold to third party (in megawatts) | MW	100