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Commitments and Contingencies - The Clean Air Act (Details) |  |  |  | 1 Months Ended | 12 Months Ended | 35 Months Ended |  |  |  |  |
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Dec. 16, 2015 USD ($) MWh MW | Jul. 17, 2015 | Dec. 20, 2013 USD ($) $ / kw MW | Aug. 31, 2011 generating_unit | Dec. 31, 2018 USD ($) lb / MMBTU generating_unit joint_owner MW | Dec. 31, 2017 USD ($) MW | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Jan. 31, 2016 USD ($) | Jan. 26, 2016 state | Jul. 31, 2015 MW | Aug. 06, 2012 compliance_alternative | Jul. 31, 2005 T | Dec. 31, 1999 state
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant in service and held for future use |  |  |  |  | $ 7,548,581,000 | $ 7,238,285,000 |  |  |  |  |  |  |  |
Accumulated depreciation and amortization |  |  |  |  | 2,604,177,000 | 2,592,692,000 |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  | 65,598,000 | 27,036,000 | $ 15,011,000 |  |  |  |  |  |  |
Regulatory assets |  |  |  |  | $ 598,930,000 | 600,672,000 |  |  |  |  |  |  |  |
San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of other entities (in joint owner) | joint_owner |  |  |  |  | 5 |  |  |  |  |  |  |  |  |
Four Corners |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved lease extension term |  | 25 years |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant in service and held for future use |  |  |  |  | $ 5,623,520,000 | 5,501,070,000 |  |  |  |  |  |  |  |
Accumulated depreciation and amortization |  |  |  |  | 2,006,266,000 | 2,029,534,000 |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  | 66,339,000 | 27,036,000 | $ 15,011,000 |  |  |  |  |  |  |
Regulatory assets |  |  |  |  | $ 460,903,000 | $ 459,239,000 |  |  |  |  |  |  |  |
Power to be sold to third party (in megawatts) | MW |  |  |  |  | 36 | 36 |  |  |  |  |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station | Surface | Loss on long-term purchase commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  |  |  |  | $ 16,500,000 |  |  |  |  |  |
Estimate of possible loss |  |  |  |  | $ 103,200,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station Units 2 and 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recovery percentage of estimated undepreciated value |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |
Plant in service and held for future use |  |  |  |  | $ 439,400,000 |  |  |  |  |  |  |  |  |
Accumulated depreciation and amortization |  |  |  |  | 188,300,000 |  |  |  |  |  |  |  |  |
Net book value |  |  |  |  | 251,100,000 |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  | 125,500,000 |  |  |  |  |  |  |  |  |
Reversal of plant write-off |  |  |  |  | 3,000,000 |  |  |  |  |  |  |  |  |
Reversal of recorded loss for other unrecoverable costs |  |  |  |  | 1,000,000 |  |  |  |  |  |  |  |  |
Total reversed losses |  |  |  |  | 4,000,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station Unit 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax impairment of investments |  |  |  |  | $ 121,800,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Four Corners |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of units (in generating units) | generating_unit |  |  |  |  | 2 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Four Corners Units 4 and 5 (Coal) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant in service and held for future use |  |  |  |  | $ 276,960,000 |  |  |  |  |  |  |  |  |
Accumulated depreciation and amortization |  |  |  |  | 98,085,000 |  |  |  |  |  |  |  |  |
PNMR Development | Energy Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost of installing equipment |  |  |  |  | $ 7,600,000 |  |  |  |  |  |  |  |  |
Environmental Protection Agency | Public Service Company of New Mexico | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of units (in generating units) | generating_unit |  |  |  | 4 |  |  |  |  |  |  |  |  |  |
Clean Air Act related to Regional Haze |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of states to address regional haze | state |  |  |  |  |  |  |  |  |  |  |  |  |  | 50
Potential to emit tons per year of visibility impairing pollution, maximum (in tons) | T |  |  |  |  |  |  |  |  |  |  |  |  | 250 |
Clean Air Act, SNCR | San Juan Generating Station Unit 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of other entities (in joint owner) | joint_owner |  |  |  |  | 8 |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | San Juan Generating Station Unit 4 | CALIFORNIA |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of other entities (in joint owner) | joint_owner |  |  |  |  | 3 |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated costs to remain unrecovered | $ 20,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  |  |  |  | $ 21,600,000 |  |  |  |  |  |
Net expense |  |  |  |  |  | $ 3,700,000 |  |  |  |  |  |  |  |
Revision of net book value |  |  |  |  |  | 900,000 |  |  |  |  |  |  |  |
Reclamation expense |  |  |  |  |  | 4,500,000 |  |  |  |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | Restructuring costs |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net expense |  |  |  |  |  | $ 1,700,000 |  |  |  |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Overall reduction of ownership (in megawatts) | MW |  |  | 418 |  |  |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Units 2 and 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested time period to recover retired units NBV | 20 years |  | 20 years |  |  |  |  |  |  |  |  |  |  |
Newly identified replacement solar generation (in megawatts) | MW |  |  | 40 |  |  |  |  |  |  |  |  |  |  |
Current ownership (in megawatts) | MW | 418 |  |  |  |  |  |  |  |  |  |  |  |  |
Recovery percentage of estimated undepreciated value | 50.00% |  |  |  |  |  |  | 50.00% |  |  |  |  |  |
Pre tax losses aggregated |  |  |  |  |  |  |  | $ 165,700,000 |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  |  |  |  | $ 127,600,000 |  |  |  |  |  |
Accumulated plant write-off, disallowance |  |  |  |  | $ 128,600,000 |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | Palo Verde Nuclear Generating Station Unit 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of megawatts | MW |  |  |  |  |  | 134 |  |  |  |  |  |  |  |
Estimated undepreciated value | $ 154,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Unit 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional ownership to be obtained (in megawatts) | MW | 65 |  |  |  | 65 |  |  |  |  |  | 132 |  |  |
Estimated rate base value | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |
Coal-fired generation (in megawatts) | MW | 197 |  |  |  |  |  |  |  |  |  |  |  |  |
Number of megawatt hours of renewable energy certificates to be acquired and retired (in megawatt hours) | MWh | 1 |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of ownership held by exiting owners |  |  |  |  | 38.80% |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 38.50% |  |  |  |  |  |  |  |  |
Potential acquisition of ownership (in megawatts) | MW |  |  |  |  | 65 |  |  |  |  |  |  |  |  |
Estimated increase in cost of acquired interest due to plant in service and accumulated depreciation |  |  |  |  | $ 261,800,000 |  |  |  |  |  |  |  |  |
Pre-tax impairment of investments |  |  |  |  | 35,000,000 |  |  |  |  |  |  |  |  |
Undepreciated investment in ownership to be obtained |  |  |  |  | 11,900,000 |  |  |  |  |  |  |  |  |
Forecasted undepreciated investment |  |  |  |  | 23,100,000 |  |  |  |  |  |  |  |  |
Remaining undepreciated investment |  |  |  |  | $ 373,600,000 |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Unit 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of ownership held by exiting owners |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | Maximum | San Juan Generating Station Unit 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual cost of renewable energy credits, maximum | $ 7,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | Installation costs including construction management, gross receipts taxes, AFUDC, and other PNM costs | San Juan Generating Station Units 1 and 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Portion of costs for SNCRs and BDT equipment |  |  |  |  |  |  |  |  | $ 77,700,000 |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | Installation costs including construction management, gross receipts taxes, AFUDC, and other PNM costs | Maximum | San Juan Generating Station Units 1 and 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated installation capital costs |  |  | $ 82,000,000 |  |  |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | Scenario, plan | San Juan Generating Station Units 1 and 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 66.30% |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | Scenario, plan | San Juan Generating Station Unit 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 77.30% |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | PNMR and PNM | Palo Verde Nuclear Generating Station Unit 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of megawatts | MW | 134 |  | 134 |  |  |  |  |  |  |  |  |  |  |
Proposed value per kilowatt | $ / kw |  |  | 2,500 |  |  |  |  |  |  |  |  |  |  |
Clean Air Act, SNCR | PNMR Development | San Juan Generating Station Unit 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reimbursement amount |  |  |  |  |  | $ 600,000 |  |  |  |  |  |  |  |
Potential acquisition of ownership (in megawatts) | MW |  |  |  |  |  |  |  |  |  |  | 65 |  |  |
Clean Air Act, SNCR Hearing Examiner Recommended Denial | Public Service Company of New Mexico | San Juan Generating Station Unit 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional ownership to be obtained (in megawatts) | MW | 132 |  |  |  | 132 |  |  |  |  |  |  |  |  |
Estimated undepreciated value | $ 20,700,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Clean Air Act Related to Post Combustion Controls | Public Service Company of New Mexico | Four Corners |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of compliance alternatives | compliance_alternative |  |  |  |  |  |  |  |  |  |  |  | 2 |  |
Government standard emissions limit (in pounds per MMBTU) | lb / MMBTU |  |  |  |  | 0.015 |  |  |  |  |  |  |  |  |
Plant requirement to meet opacity limit |  |  |  |  | 20.00% |  |  |  |  |  |  |  |  |
Rule imposes opacity limitation on certain fugitive dust emissions from coal and material handling operations |  |  |  |  | 20.00% |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  | $ 88,700,000 |  |  |  |  |  |  |  |  |
Clean Air Act Related to Post Combustion Controls | Public Service Company of New Mexico | Four Corners Units 4 and 5 (Coal) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  | 13.00% |  |  |  |  |  |  |  |  |
Clean Power Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of states that filed a petition against the Clean Power Plan | state |  |  |  |  |  |  |  |  |  | 29 |  |  |  |

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- Definition Public Utilities, Rule Imposes Opacity Limitation on Certain Fugitive Dust Emissions From Coal and Material Handling Operations + References No definition available. + Details
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