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Commitments and Contingencies - The Clean Air Act (Details) | 9 Months Ended |  |  |  |  |  |
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Sep. 30, 2020 option T | Feb. 25, 2019 parts_per_billion | Mar. 31, 2016 parts_per_billion | Feb. 09, 2016 state | Oct. 01, 2015 parts_per_billion | May 14, 2015 lb_PER_MMBTU | Dec. 31, 1999 state
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Number of options for meeting BTA standards | option | 7 |  |  |  |  |  |
Clean Air Act related to regional haze |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Number of states to address regional haze (in states) |  |  |  |  |  |  | 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 |  |  |  | 29 |  |  |
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 |  | 75 | 75 |  | 70 |  |
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_PER_MMBTU |  |  |  |  |  | 0.10 |

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