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Fair Value of Derivative and Other Financial Instruments - Overview and Commodity Derivatives (Details)
Sep. 30, 2018
USD ($)
MW
Dec. 31, 2017
USD ($)
Derivatives, Fair Value [Line Items]    
Current assets $ 1,083,000 $ 1,088,000
Deferred charges 2,741,000 3,556,000
Current liabilities (1,092,000) (1,182,000)
Long-term liabilities (2,741,000) (3,556,000)
Obligations to return cash $ 1,000,000 900,000
PNM    
Derivatives, Fair Value [Line Items]    
Expected exposure to market risk (in megawatts) | MW 65  
Power to be sold to third party (in megawatts) | MW 36  
Current assets $ 1,083,000 1,088,000
Deferred charges 2,741,000 3,556,000
Current liabilities (1,092,000) (1,182,000)
Long-term liabilities (2,741,000) (3,556,000)
Amounts recognized for right to reclaim cash 0 0
Cash collateral under margin arrangements 500,000 800,000
PNM | Designated as Hedging Instrument | Commodity derivatives    
Derivatives, Fair Value [Line Items]    
Current assets 1,083,000 1,088,000
Deferred charges 2,741,000 3,556,000
Derivative Asset 3,824,000 4,644,000
Current liabilities (1,092,000) (1,182,000)
Long-term liabilities (2,741,000) (3,556,000)
Derivative Liability (3,833,000) (4,738,000)
Net (9,000) (94,000)
PNM | Designated as Hedging Instrument | Commodity derivatives | Fuel and purchased power costs    
Derivatives, Fair Value [Line Items]    
Current assets   0
PNM | Designated as Hedging Instrument | Commodity derivatives | Hazard sharing arrangement    
Derivatives, Fair Value [Line Items]    
Derivative Asset $ 3,800,000 $ 4,600,000