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Regulatory and Rate Matters - Integrated Resource Plan, Four Corners Right of First Refusal and Application for Certificate (Details) - Public Service Company of New Mexico [Member]
$ in Millions
 1 Months Ended 3 Months Ended 4 Months Ended
Jul. 31, 2011
Sep. 30, 2015
USD ($)
MW
Jun. 17, 2015
Feb. 17, 2015
Four Corners [Member]        
Public Utilities, General Disclosures [Line Items]        
Jointly owned utility plant proportionate ownership share, other entities       7.00%
Period of time to file a waiver of rights of first refusal     120 days  
San Juan Generating Station Units 2 and 3 [Member] | Clean Air Act, SNCR [Member]        
Public Utilities, General Disclosures [Line Items]        
Public utilities, newly identified replacement gas-fired generation (in megawatts) | MW   187    
Public utilities, cost of replacement gas-generation   $ 133.2    
Integrated Resource Plan, 2011 [Member]        
Public Utilities, General Disclosures [Line Items]        
Public utilities, frequency of IRP filings   3 years    
Public utilities, planning period covered of IRP 20 years