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Commitments and Contingencies - Four Corners (Details) $ in Millions | Jun. 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 | lbsofnox / mmbtu | 0.015 |  |
Public Utilities, Plant Requirement to Meet Opacity Limit, Percentage | 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 | $ | $ 91.8 |  |
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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