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Commitments and Contingencies - National Ambient Air Quality Standards (Details) - Public Service Company of New Mexico [Member] | Mar. 02, 2015 lb / MMBTU T | Oct. 01, 2015 opp | Sep. 30, 2015 opp | Dec. 17, 2014 opp | Nov. 25, 2014 opp | Nov. 08, 2013 lb / MMBTU | Jan. 31, 2010 opp
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National Ambient Air Quality Standards, 2015 EPA Legal Settlement [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Period of time to act on settlement | 16 months |  |  |  |  |  |
Public utilities, emissions of SO2 per year (in tons) | T | 16,000 |  |  |  |  |  |
San Juan Generating Station [Member] | National Ambient Air Quality Standards [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Public utilities, revised SO2 emissions, agreed upon (in pounds per MMBTU) | lb / MMBTU |  |  |  |  |  | 0.10 |
Minimum [Member] | National Ambient Air Quality Standards, 2015 EPA Legal Settlement [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Public utilities, emissions of SO2 per year (in tons) | T | 2,600 |  |  |  |  |  |
Public utilities, 1-hour SO2 emissions rate (in pounds per MMBTU) | lb / MMBTU | 0.45 |  |  |  |  |  |
Minimum [Member] | San Juan Generating Station And Four Corners [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Public utilities, proposed government standard emission limit (in ozone parts per million) |  |  |  | 65 | 65 |  | 60
Maximum [Member] | San Juan Generating Station And Four Corners [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Public utilities, proposed government standard emission limit (in ozone parts per million) |  |  |  | 70 |  |  | 70
Public utilities, government standard emission limit (in ozone parts per million) |  |  | 75 |  |  |  |
Subsequent Event [Member] | Maximum [Member] | San Juan Generating Station And Four Corners [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Public utilities, government standard emission limit (in ozone parts per million) |  | 70 |  |  |  |  |

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