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Regulatory and Rate Matters - Firm-Requirements Wholesale Customers (Details) - Public Service Company of New Mexico [Member]
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Dec. 06, 2012
USD ($)
Sep. 30, 2015
USD ($)
Jun. 30, 2014
USD ($)
Dec. 31, 2014
USD ($)
MW
Firm Requirements Wholesale Power Rate Case, Navopache [Member] [Member]        
Public Utilities, General Disclosures [Line Items]        
Public utilities, approved rate increase (decrease) $ 5.3      
Public utilities, average monthly usage (in megawatts) | MW       55
Public utilities, revenue for power sold under specific contract       $ 28.4
Firm Requirements Wholesale Power Rate Case [Member]        
Public Utilities, General Disclosures [Line Items]        
Public utilities, contract extension 10 years      
City of Gallup, New Mexico Contract [Member]        
Public Utilities, General Disclosures [Line Items]        
Public utilities, revenue for power sold under specific contract     $ 6.1  
Public utilities, gain on sale of substation   $ 1.1