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Commitments and Contingencies - The Clean Air Act (Details) $ in Millions | Dec. 31, 2020 USD ($) lb_PER_MMBTU T | Feb. 21, 2020 USD ($) | Dec. 31, 2018 USD ($) MW | Dec. 31, 2015 MW | Aug. 06, 2012 compliance_alternative | Dec. 31, 1999 state
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Public Service Company of New Mexico | San Juan Generating Station |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |
Forecasted undepreciated investment | $ 283.0 | $ 361.0 |  |  |  |
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 | 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 |  |
Pre-tax impairment of investments |  |  | $ 35.0 |  |  |
Undepreciated investment in ownership to be obtained |  |  | 11.9 |  |  |
Forecasted undepreciated investment |  |  | $ 23.1 |  |  |
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 | PNMR Development | San Juan Generating Station Unit 4 |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |
Potential acquisition of ownership (in megawatts) | MW |  |  | 65 | 65 |  |
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_PER_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 |  |  | $ 88.7 |  |  |
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% |  |  |  |  |

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