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Commitments and Contingencies - Nuclear Spent Fuel and Waste Disposal (Details) - Public Service Company of New Mexico - Palo Verde Nuclear Generating Station - USD ($)
 3 Months Ended
May 16, 2014
Mar. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Department of energy, spent nuclear fuel removal July 2011 - June 2014        
Loss Contingencies [Line Items]        
PNM's share of third party settlement claim   $ 5,600,000    
Litigation settlement, portion credited to customers   $ 3,600,000    
Nuclear spent fuel and waste disposal        
Loss Contingencies [Line Items]        
Estimate of possible loss     $ 57,700,000  
DOE adjusted one-mill fee $ 0      
Nuclear spent fuel and waste disposal | Other deferred credits        
Loss Contingencies [Line Items]        
Liability for interim storage costs     $ 12,300,000 $ 12,100,000