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Commitments and Contingencies - Coal Supply (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Other current assets | $ 76,161 | $ 58,471 |  |
Public Service Company of New Mexico [Member] | Surface [Member] |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Regulatory assets | 100,000 |  |  |
Public Service Company of New Mexico [Member] | Loss on Long-term Purchase Commitment [Member] |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Public utilities, annual funding |  | 1,000 | $ 300 | $ 3,500
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member] |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Reclamation Trust Funding, current fiscal year | 4,600 |  |  |
Reclamation Trust Funding, next fiscal year | 4,300 |  |  |
Public Service Company of New Mexico [Member] | Loss on Long-term Purchase Commitment [Member] | Surface [Member] |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Loss contingency accrual | 24,700 | 25,700 |  |
Final reclamation, capped amount to be collected | 100,000 |  |  |
Public Service Company of New Mexico [Member] | Loss on Long-term Purchase Commitment [Member] | Underground [Member] |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Loss contingency accrual | 9,300 | 8,600 |  |
San Juan Generating Station [Member] | Public Service Company of New Mexico [Member] | Loss on Long-term Purchase Commitment [Member] | Surface [Member] |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Loss contingency, estimate of possible loss | 55,500 |  |  |
San Juan Generating Station [Member] | Public Service Company of New Mexico [Member] | Loss on Long-term Purchase Commitment [Member] | Underground [Member] |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Loss contingency, estimate of possible loss | 93,300 |  |  |
San Juan Generating Station [Member] | Coal Supply [Member] | Public Service Company of New Mexico [Member] |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Other current assets | $ 44,900 | $ 37,300 |  |
Public utilities, estimated increase in coal cost | 30.00% |  |  |

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