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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
Jun. 30, 2015
USD ($)
Jun. 30, 2015
USD ($)
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), Amount   $ 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