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Commitments and Contingencies - Four Corners (Details) $ in Millions | Sep. 30, 2015 USD ($) lbsofnox / mmbtu | Dec. 31, 2013 | Aug. 06, 2012 compliance_alternative
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Four Corners [Member] |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Public utilities, jointly owned utility plant, sale of ownership percentage |  | 48.00% |
Clean Air Act Related to Post Combustion Controls [Member] | Public Service Company of New Mexico [Member] | Four Corners [Member] |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Public utilities, number of compliance alternatives |  |  | 2
Public utilities, plant requirement to meet NOx emissions limit (in pounds of NOx per MMBTU) | lbsofnox / mmbtu | 0.015 |  |
Public utilities, plant requirement to meet opacity limit | 20.00% |  |
Public utilities, rule imposes opacity limitation on certain fugitive dust emissions from coal and material handling operations | 20.00% |  |
Loss contingency, estimate of possible loss | $ | $ 91.4 |  |
Clean Air Act Related to Post Combustion Controls [Member] | Public Service Company of New Mexico [Member] | Four Corners Units 4 and 5 (Coal) [Member] |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Public utilities, ownership percentage | 13.00% |  |

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