Fair Value of Derivative and Other Financial Instruments - Margin, Notional Amounts and Credit Rating (Details) MWh in Thousands, MMBTU in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) MWh MMBTU	Dec. 31, 2018 USD ($) MWh MMBTU
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	565	100
PNM | Commodity derivatives | Fair value hedging | Sell
Derivative [Line Items]
Economic Hedges (in mmbtu and mwh) | MWh	42	0