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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details)
$ in Millions
Jul. 13, 2015
a
Jun. 30, 2015
USD ($)
Jan. 22, 2015
Allotment_Parcel
Apr. 02, 2014
Allotment_Parcel
Apr. 01, 2014
USD ($)
Jan. 06, 2014
Allotment_Parcel
Sep. 30, 2012
Allotment_Parcel
Continuous Highwall Mining [Member] | San Juan Generating Station [Member]              
Public Utilities, Commitments And Contingencies [Line Items]              
Public Utilities, Proposed Retroactive Surface Mining Royalty Rate   12.50%          
Public Utilities, Current Surface Mining Royalty Rate applied between 2000 and 2003   8.00%          
Public Utilities, Estimated Underpaid Surface Mining Royalties under proposed rate change   $ 5.0          
Public Utilities, PNM Share Estimated Underpaid Surface Mining Royalties under proposed rate change   46.30%          
NMTRD Coal Severance Tax [Member] | Four Corners [Member]              
Public Utilities, Commitments And Contingencies [Line Items]              
Public Utilities, Assessed Coal Severance Surtax Penalty and Interest   $ 30.0          
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member]              
Public Utilities, Commitments And Contingencies [Line Items]              
Public Utilities, Ownership Percentage in Nuclear Reactor   10.20%          
Public Utilities, Maximum Potential Assessment Per Incident   $ 38.9          
Public Utilities, Annual Payment Limitation Related to Incident   5.7          
Public Utilities, Aggregate Amount of All Risk Insurance   2,750.0          
Public Utilities, Maximum Amount under Nuclear Electric Insurance Limited   5.4          
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member] | Maximum [Member]              
Public Utilities, Commitments And Contingencies [Line Items]              
Public Utilities, Liability Insurance Coverage   13,400.0          
Public Utilities, Liability Insurance Coverage Sublimit         $ 2,250.0    
Public Service Company of New Mexico [Member] | Commercial Providers [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member]              
Public Utilities, Commitments And Contingencies [Line Items]              
Public Utilities, Liability Insurance Coverage   375.0          
Public Service Company of New Mexico [Member] | Industry Wide Retrospective Assessment Program [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member]              
Public Utilities, Commitments And Contingencies [Line Items]              
Public Utilities, Liability Insurance Coverage   $ 13,000.0          
Public Service Company of New Mexico [Member] | NMTRD Coal Severance Tax [Member] | Four Corners [Member]              
Public Utilities, Commitments And Contingencies [Line Items]              
Public Utilities, PNM Share Assessed Coal Severance Surtax Penalty and Interest to pass through FFPAC   9.40%          
Public Service Company of New Mexico [Member] | Navajo Nation Allottee Matters [Member]              
Public Utilities, Commitments And Contingencies [Line Items]              
Number of landowners claiming to be Navajo allottees | Allotment_Parcel             43
Number of allotment parcels' appraisal requested for review | Allotment_Parcel           58  
Number of allotments where landowners are revoking rights of way renewal consents | Allotment_Parcel     10 6      
Public Service Company of New Mexico [Member] | Navajo Nation Allottee Matters [Member] | Subsequent Event [Member]              
Public Utilities, Commitments And Contingencies [Line Items]              
Area of Land | a 15.49