Fair Value of Derivative and Other Financial Instruments - Volume Positions and Requirements to Provide Collateral (Details) MMBTU in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) MWh MMBTU	Dec. 31, 2018 USD ($) MWh MMBTU
Derivative [Line Items]
Contract in a net liability position | $	$ 0	$ 0
Long | Commodity Contract | Economic Hedges | PNMR and PNM
Derivative [Line Items]
Power-related contracts | MMBTU	0	100
Short | Commodity Contract | Economic Hedges | PNMR and PNM
Derivative [Line Items]
Power-related contracts | MWh	0	0