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Fair Value of Derivative and Other Financial Instruments - Statement of Earnings Information (Details) - PNM - Designated as Hedging Instrument - Commodity derivatives - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]        
Total gain $ 0 $ (694) $ 2 $ 2,657
Electric operating revenues        
Derivative Instruments, Gain (Loss) [Line Items]        
Total gain 8 4,592 (2) 7,933
Cost of energy        
Derivative Instruments, Gain (Loss) [Line Items]        
Total gain $ (8) $ (5,286) $ 4 $ (5,276)