Fair Value of Derivative and Other Financial Instruments - Statement of Earnings Information (Details) - PNM - Designated as Hedging Instrument - Commodity derivatives - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Total gain	$ 0	$ (2,251)	$ 2	$ 408
Electric operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Total gain	(93)	(2,237)	(95)	5,697
Cost of energy
Derivative Instruments, Gain (Loss) [Line Items]
Total gain	$ 93	$ (14)	$ 97	$ (5,289)