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Commitments and Contingencies - National Ambient Air Quality Standards (Details) - Public Service Company of New Mexico [Member] | Feb. 25, 2016 | Mar. 02, 2015 lb / MMBTU T | Oct. 01, 2015 parts_per_billion | Sep. 30, 2015 parts_per_billion | May 14, 2015 lb / MMBTU | Jan. 31, 2010 parts_per_billion
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Minimum [Member] | San Juan Generating Station And Four Corners [Member] |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |
Proposed government standard emission limit (in parts per billion) |  |  |  |  |  | 60
Maximum [Member] | San Juan Generating Station And Four Corners [Member] |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |
Proposed government standard emission limit (in parts per billion) |  |  |  |  |  | 70
Government standard emissions limit (in parts per billion) |  |  | 70 | 75 |  |
National Ambient Air Quality Standards, 2015 EPA Legal Settlement [Member] |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |
Period of time to act on settlement |  | 16 months |  |  |  |
Emissions tons of SO2 per year | T |  | 16,000 |  |  |  |
National Ambient Air Quality Standards, 2015 EPA Legal Settlement [Member] | Minimum [Member] |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |
Emissions tons of SO2 per year | T |  | 2,600 |  |  |  |
SO2 emissions rate (in pounds per MMBTU) | lb / MMBTU |  | 0.45 |  |  |  |
National Ambient Air Quality Standards [Member] | San Juan Generating Station [Member] |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |
Revised SO2 emissions agreed upon (in pounds per MMBTU) | lb / MMBTU |  |  |  |  | 0.10 |
National Ambient Air Quality Standards, Proposed 2016 SO2 Rule [Member] |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |
Period of time from state designation to provide implementation plans | 36 months |  |  |  |  |

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