Fair Value of Derivative and Other Financial Instruments - Volume Positions and Requirements to Provide Collateral (Details) - Commodity Contract - Economic Hedges - PNMR and PNM	12 Months Ended
Dec. 31, 2017 MMBTU MWh	Dec. 31, 2016 MMBTU MWh
Long
Derivative [Line Items]
Power-related contracts \| MMBTU	100,000	254,100
Short
Derivative [Line Items]
Power-related contracts \| MWh	0	2,471,600

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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