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Commitments and Contingencies - Litigation and Rulemaking (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2015
USD ($)
Sep. 30, 2015
USD ($)
Dec. 21, 2011
mw
Public Utilities, Commitments And Contingencies [Line Items]      
Minimum megawatt capacity from coal and oil-fired electric generating units under jurisdiction of the Mercury and Air Toxics Standards (in megawatts) | mw     25
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member]      
Public Utilities, Commitments And Contingencies [Line Items]      
Mercury removal rate 99.00% 99.00%  
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Mercury Control [Member]      
Public Utilities, Commitments And Contingencies [Line Items]      
Current annual mercury control costs   $ 0.7  
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Maximum [Member] | Mercury Control [Member]      
Public Utilities, Commitments And Contingencies [Line Items]      
Contingent estimated annual mercury control cost   $ 6.6  
Public Service Company of New Mexico [Member] | Four Corners Units 4 and 5 (Coal) [Member] | Clean Air Act Lawsuit [Member]      
Public Utilities, Commitments And Contingencies [Line Items]      
Litigation settlement, amount $ 1.5    
Litigation settlement, expected capital spend, environmental $ 6.2    
Public utilities, ownership percentage 13.00% 13.00%