Fair Value of Derivative and Other Financial Instruments - Margin, Notional Amounts and Credit Rating (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) MMBTU MWh	Dec. 31, 2017 USD ($) MMBTU MWh
Derivative [Line Items]
Contract in a liability position | $	$ 0	$ 0
PNM | Commodity derivatives | Fair value hedging | Buy
Derivative [Line Items]
Economic Hedges (in mmbtu and mwh) | MMBTU	100,000	100,000
PNM | Commodity derivatives | Fair value hedging | Sell
Derivative [Line Items]
Economic Hedges (in mmbtu and mwh) | MWh	4,800	0