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Construction Program and Jointly-Owned Electric Generating Plants - Construction Program and Jointly-Owned Electric Generating Plants (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($) generating_unit | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Construction expenditures | $ 500,461 | $ 600,076 | $ 558,589
Plant in Service | 7,238,285 | 6,944,534 |
Accumulated depreciation | (2,592,692) | (2,334,938) |
Regulatory disallowance | 27,036 | 15,011 | 167,471
Regulatory assets | $ 600,672 | 501,223 |
Operating lease, option term extensions | 2 years |  |
Public Service Company of New Mexico |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Construction expenditures | $ 309,142 | 445,464 | 404,840
Plant in Service | 5,501,070 | 5,359,211 |
Accumulated depreciation | (2,029,534) | (1,809,528) |
Regulatory disallowance | 27,036 | 15,011 | 167,471
Regulatory assets | 459,239 | 365,413 |
Public Service Company of New Mexico | Clean Air Act, SNCR |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Regulatory disallowance |  |  | 21,600
Texas-New Mexico Power Company |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Construction expenditures | 145,495 | 122,518 | 124,584
Plant in Service | 1,504,778 | 1,380,584 |
Accumulated depreciation | (460,858) | (429,397) |
Regulatory assets | 141,433 | 135,810 |
Joint Projects | Public Service Company of New Mexico |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Construction expenditures | 309,100 |  |
Joint Projects | Texas-New Mexico Power Company |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Construction expenditures | 145,500 |  |
Joint Projects | PNMR |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Construction expenditures | $ 500,500 |  |
SJGS (Coal) | Unit 4 | Other Unrelated Entities 2 |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, ownership percentage | 12.80% |  |
SJGS (Coal) | Unit 4 | Other Unrelated Entities 3 |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, ownership percentage | 8.475% |  |
SJGS (Coal) | Unit 4 | Other Unrelated Entities 4 |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, ownership percentage | 7.20% |  |
SJGS (Coal) | Unit 4 | Other Unrelated Entities 5 |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Jointly owned utility plant, ownership percentage | 7.028% |  |
SJGS (Coal) | Public Service Company of New Mexico |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Plant in Service | $ 920,950 |  |
Accumulated depreciation | (522,750) |  |
Construction Work in Progress | $ 8,512 |  |
Composite Interest | 46.30% |  |
SJGS (Coal) | Public Service Company of New Mexico | Unit 4 |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Composite Interest | 77.297% |  |
Palo Verde Nuclear Generating Station | Public Service Company of New Mexico |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Plant in Service | $ 918,830 |  |
Accumulated depreciation | (353,054) |  |
Construction Work in Progress | $ 35,038 |  |
Composite Interest | 10.20% |  |
Period of time for the original full power operating licenses | 40 years |  |
Number of units (in generating units) | generating_unit | 3 |  |
Operating lease, option term extensions | 20 years |  |
Four Corners | Public Service Company of New Mexico |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Number of units (in generating units) | generating_unit | 2 |  |
Four Corners Units 4 and 5 (Coal) | Public Service Company of New Mexico |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Plant in Service | $ 204,432 |  |
Accumulated depreciation | (100,914) |  |
Construction Work in Progress | $ 61,755 |  |
Composite Interest | 13.00% |  |
Luna (Gas) | Public Service Company of New Mexico |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Plant in Service | $ 70,995 |  |
Accumulated depreciation | (27,023) |  |
Construction Work in Progress | $ (13) |  |
Composite Interest | 33.33% |  |
San Juan Generating Station Units 2 and 3 | Public Service Company of New Mexico |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Plant in Service | $ 439,400 | 471,800 | 468,200
Accumulated depreciation | (188,300) | (203,900) | (193,300)
Construction Work in Progress |  |  | 2,200
Net book value | 251,100 | $ 139,300 | 277,100
Regulatory assets | $ 125,500 |  |
San Juan Generating Station Units 2 and 3 | Public Service Company of New Mexico | Clean Air Act, SNCR |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Regulatory disallowance |  |  | $ 127,600
SJGS Units 1 and 2 | Other Unrelated Entities 1 |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Composite Interest | 50.00% |  |
SJGS Units 1 and 2 | Public Service Company of New Mexico |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |
Composite Interest | 50.00% |  |

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