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Fair Value of Derivative and Other Financial Instruments - Overview and Commodity Derivatives (Details)
 3 Months Ended
Mar. 31, 2018
USD ($)
MW
Mar. 31, 2017
USD ($)
Jan. 01, 2018
USD ($)
Dec. 31, 2017
USD ($)
Derivatives, Fair Value [Line Items]        
Current assets $ 1,087,000     $ 1,088,000
Deferred charges 3,277,000     3,556,000
Current liabilities (1,328,000)     (1,182,000)
Long-term liabilities (3,277,000)     (3,556,000)
Obligations to return cash 900,000     900,000
Net liability contracts $ 100,000     0
Cumulative effect adjustment       (11,208,000)
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 $ 1,087,000     1,088,000
Deferred charges 3,277,000     3,556,000
Current liabilities (1,328,000)     (1,182,000)
Long-term liabilities (3,277,000)     (3,556,000)
Amounts recognized for right to reclaim cash 0     0
Cash collateral under margin arrangements 500,000     800,000
Cumulative effect adjustment       (11,208,000)
(Increase)/decrease in other than temporary losses of available-for-sale securities, net portion recognized in earnings (1,200,000) $ 1,100,000    
PNM | Designated as Hedging Instrument | Commodity derivatives        
Derivatives, Fair Value [Line Items]        
Current assets 1,087,000     1,088,000
Deferred charges 3,277,000     3,556,000
Derivative Asset 4,364,000     4,644,000
Current liabilities (1,328,000)     (1,182,000)
Long-term liabilities (3,277,000)     (3,556,000)
Derivative Liability (4,605,000)     (4,738,000)
Net (241,000)     (94,000)
PNM | Designated as Hedging Instrument | Commodity derivatives | Fuel and purchased power costs        
Derivatives, Fair Value [Line Items]        
Current assets       0
Current liabilities (200,000)      
PNM | Designated as Hedging Instrument | Commodity derivatives | Hazard sharing arrangement        
Derivatives, Fair Value [Line Items]        
Derivative Asset $ 4,400,000     4,600,000
Retained Earnings        
Derivatives, Fair Value [Line Items]        
Cumulative effect adjustment       11,208,000
Retained Earnings | PNM        
Derivatives, Fair Value [Line Items]        
Cumulative effect adjustment       $ 11,208,000
Accounting Standards Update 2016-01 | Retained Earnings        
Derivatives, Fair Value [Line Items]        
Cumulative effect adjustment     $ 11,200,000