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Commitments and Contingencies - The Clean Air Act (Details) $ in Thousands | Dec. 16, 2015 MW | Mar. 02, 2015 lb / MMBTU T | Mar. 31, 2018 USD ($) lbsofnox / mmbtu joint_owner MW | Dec. 31, 2017 USD ($) | Jan. 26, 2016 state | Oct. 01, 2015 parts_per_billion | Sep. 30, 2015 parts_per_billion | May 14, 2015 lb / MMBTU | Dec. 30, 2013 | Apr. 30, 2013 well | Feb. 28, 2013 lawsuit state mine | Aug. 06, 2012 compliance_alternative | Jul. 31, 2005 T | Dec. 31, 1999 state
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Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant in service and held for future use |  |  | $ 7,284,727 | $ 7,238,285 |  |  |  |  |  |  |  |  |  |
Accumulated depreciation and amortization |  |  | $ 2,627,329 | 2,592,692 |  |  |  |  |  |  |  |  |  |
Clean Air Act related to regional haze |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of states to address regional haze (in states) | state |  |  |  |  |  |  |  |  |  |  |  |  |  | 50
Potential to emit tons per year of visibility impairing pollution (in tons, more than) | T |  |  |  |  |  |  |  |  |  |  |  |  | 250 |
Clean Power Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of states that filed a petition against the Clean Power Plan | state |  |  |  |  | 29 |  |  |  |  |  |  |  |  |
San Juan Generating Station Unit 4 | Clean Air Act, SNCR |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of other joint owners | joint_owner |  |  | 8 |  |  |  |  |  |  |  |  |  |  |
PNM |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant in service and held for future use |  |  | $ 5,537,638 | 5,501,070 |  |  |  |  |  |  |  |  |  |
Accumulated depreciation and amortization |  |  | $ 2,052,380 | $ 2,029,534 |  |  |  |  |  |  |  |  |  |
Power to be sold to third party (in megawatts) | MW |  |  | 36 |  |  |  |  |  |  |  |  |  |  |
PNM | National Ambient Air Quality Standards, 2015 EPA Legal Settlement |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period of time to act on settlement |  | 16 months |  |  |  |  |  |  |  |  |  |  |  |
Emissions tons of SO2 per year (more than) | T |  | 16,000 |  |  |  |  |  |  |  |  |  |  |  |
PNM | WEG v OSM lawsuit |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of mines impacted | mine |  |  |  |  |  |  |  |  |  |  | 7 |  |  |
Number of claims for relief, filed | lawsuit |  |  |  |  |  |  |  |  |  |  | 15 |  |  |
Number of states impacted | state |  |  |  |  |  |  |  |  |  |  | 4 |  |  |
PNM | Santa Fe generating station |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of wells with elevated levels of nitrates | well |  |  |  |  |  |  |  |  |  | 3 |  |  |  |
PNM | San Juan Generating Station | WEG v OSM lawsuit |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of claims for relief, filed | lawsuit |  |  |  |  |  |  |  |  |  |  | 2 |  |  |
PNM | San Juan Generating Station Unit 4 | Clean Air Act, SNCR |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional ownership to be obtained (in megawatts) | MW | 65 |  |  |  |  |  |  |  |  |  |  |  |  |
PNM | San Juan Generating Station Unit 4 | Clean Air Act, Post-2022 coal supply future rate case |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period of time agreed to by all parties to settle rate case | 6 months |  |  |  |  |  |  |  |  |  |  |  |  |
PNM | San Juan Generating Station Unit 4 | Clean Air Act, SNCR Hearing Examiner, Recommended Denial |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional ownership to be obtained (in megawatts) | MW | 132 |  |  |  |  |  |  |  |  |  |  |  |  |
PNM | Palo Verde Nuclear Generating Station Unit 3 | Clean Air Act, SNCR |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of megawatts nuclear generation (in megawatts) | MW | 134 |  |  |  |  |  |  |  |  |  |  |  |  |
PNM | Four Corners | Clean Air Act related to post combustion controls |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of compliance alternatives | compliance_alternative |  |  |  |  |  |  |  |  |  |  |  | 2 |  |
Plant requirement to meet NOx emissions limit (in pounds of NOx per MMBTU) | lbsofnox / 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 |  |  | $ 89,000 |  |  |  |  |  |  |  |  |  |  |
PNM | Four Corners Units 4 and 5 (Coal) | Clean Air Act related to post combustion controls |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  | 13.00% |  |  |  |  |  |  |  |  |  |  |
PNM | Four Corners Units 1, 2 and 3 (Coal) | Clean Air Act related to post combustion controls |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  |  |  | 0.00% |  |  |  |  |
Maximum | PNM | San Juan Generating Station and Four Corners |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Government standard emission limit (in ozone parts per million) | parts_per_billion |  |  |  |  |  | 70 | 75 |  |  |  |  |  |  |
Minimum | PNM | National Ambient Air Quality Standards, 2015 EPA Legal Settlement |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Emissions tons of SO2 per year (more than) | T |  | 2,600 |  |  |  |  |  |  |  |  |  |  |  |
One-hour SO2 emissions rate (in pounds per MMBTU) | lb / MMBTU |  | 0.45 |  |  |  |  |  |  |  |  |  |  |  |
Surface | Loss on long-term purchase commitment | PNM | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  | $ 99,700 |  |  |  |  |  |  |  |  |  |  |
San Juan Generating Station | PNM | National Ambient Air Quality Standards |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revised SO2 emissions (in pounds per MMBTU) | lb / MMBTU |  |  |  |  |  |  |  | 0.10 |  |  |  |  |  |

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