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Fair Value of Derivative and Other Financial Instruments - Margin, Notional Amounts and Credit Rating (Details) - PNMR and PNM
3 Months Ended 12 Months Ended
Mar. 31, 2017
USD ($)
MMBTU
MWh
Dec. 31, 2016
USD ($)
MMBTU
MWh
Derivative [Line Items]    
Contractual liability $ 0 $ 0
Posted cash collateral 0 0
Net exposure $ 0 $ 0
Commodity derivatives | Fair value hedging | Buy    
Derivative [Line Items]    
Economic Hedges (in mmbtu and mwh) | MMBTU 100,000 254,100
Commodity derivatives | Fair value hedging | Sell    
Derivative [Line Items]    
Economic Hedges (in mmbtu and mwh) | MWh 1,834,125 2,471,600