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Commitments and Contingencies - The Clean Air Act (Details) |  | 1 Months Ended | 35 Months Ended |  |  |  |  |  |  |
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Jul. 17, 2015 | Dec. 31, 2015 USD ($) MWh MW | Jan. 31, 2016 USD ($) | Dec. 31, 2019 USD ($) lb / MMBTU joint_owner T MW | Jul. 01, 2019 USD ($) | Dec. 31, 2018 USD ($) MW | Dec. 31, 2017 USD ($) joint_owner MW | Feb. 09, 2016 state | Aug. 06, 2012 compliance_alternative | Dec. 31, 1999 state
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Plant in service, held for future use, and to be abandoned |  |  |  | $ 7,918,601,000 |  | $ 7,548,581,000 |  |  |  |
Accumulated depreciation and amortization |  |  |  | 2,713,503,000 |  | 2,604,177,000 |  |  |  |
Regulatory assets |  |  |  | $ 556,930,000 |  | 598,930,000 |  |  |  |
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, held for future use, and to be abandoned |  |  |  | $ 5,753,267,000 |  | 5,623,520,000 |  |  |  |
Accumulated depreciation and amortization |  |  |  | 2,076,291,000 |  | 2,006,266,000 |  |  |  |
Regulatory assets |  |  |  | $ 435,467,000 |  | $ 460,903,000 |  |  |  |
Power to be sold to third party (in megawatts) | MW |  |  |  | 36 |  |  | 36 |  |  |
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, held for future use, and to be abandoned |  |  |  |  |  |  | $ 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 |  |  |  |  |  |  | 3,000,000.0 |  |  |
Reversal of recorded loss for other unrecoverable costs |  |  |  |  |  |  | 1,000,000.0 |  |  |
Total reversed losses |  |  |  |  |  |  | $ 4,000,000.0 |  |  |
Public Service Company of New Mexico | San Juan Generating Station |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Forecasted undepreciated investment |  |  |  |  | $ 283,000,000 |  |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station | Surface | Loss on long-term purchase commitment |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  | $ 92,600,000 |  |  |  |  |  |
Public Service Company of New Mexico | Four Corners Units 4 and 5 (Coal) |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Plant in service, held for future use, and to be abandoned |  |  |  | 283,939,000 |  |  |  |  |  |
Accumulated depreciation and amortization |  |  |  | $ 100,137,000 |  |  |  |  |  |
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 | San Juan Generating Station Units 2 and 3 |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Current ownership (in megawatts) | MW |  | 418 |  |  |  |  |  |  |  |
Requested time period to recover retired units NBV |  | 20 years |  |  |  |  |  |  |  |
Recovery percentage of estimated undepreciated value |  | 50.00% |  |  |  |  |  |  |  |
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 (in megawatts) | MW |  |  |  |  |  |  | 134 |  |  |
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Additional ownership to be obtained (in megawatts) | MW |  | 132 |  |  |  | 132 |  |  |  |
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 |  | 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 |  |  |  |  |  |  |  |
Potential acquisition of ownership (in megawatts) | MW |  | 65 |  |  |  | 65 |  |  |  |
Pre-tax impairment of investments |  |  |  |  |  | $ 35,000,000.0 |  |  |  |
Undepreciated investment in ownership to be obtained |  |  |  |  |  | 11,900,000 |  |  |  |
Forecasted undepreciated investment |  |  |  |  |  | $ 23,100,000 |  |  |  |
Percentage of ownership held by exiting owners |  |  |  |  |  |  | 38.80% |  |  |
Ownership percentage |  |  |  |  |  |  | 38.50% |  |  |
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 | 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 (in megawatts) | MW |  | 134 |  |  |  |  |  |  |  |
Clean Air Act, SNCR | PNMR Development | San Juan Generating Station Unit 4 |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Additional ownership to be obtained (in megawatts) | MW |  |  |  |  |  |  | 132 |  |  |
Potential acquisition of ownership (in megawatts) | MW |  | 65 |  |  |  | 65 | 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 |  |  |  |
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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