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Regulatory and Rate Matters - Energy Efficiency and Load Management (Details) $ in Millions | Nov. 08, 2017 USD ($) | Nov. 07, 2017 | Jul. 26, 2017 USD ($) GWh | Apr. 14, 2017 USD ($) GWh | Jan. 11, 2017 USD ($) | Apr. 15, 2016 USD ($) GWh program | Oct. 06, 2014 USD ($) | Dec. 31, 2017 | Dec. 31, 2013
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Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Tax-adjusted WACC | 9.59% | 7.71% |  |  |  |  |  |  |
Public Service Company of New Mexico | 2014 Energy Efficiency and Load Management Program | Disincentives/Incentives Added |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Program costs related to energy efficiency |  |  |  |  |  |  | $ 25.8 |  |
Public Service Company of New Mexico | 2017 Energy Efficiency and Load Management Program | Disincentives/Incentives Added |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Program costs related to energy efficiency |  |  |  |  | $ 26.0 | $ 28.0 |  |  |
Number of energy efficiency and load management programs (in programs) | program |  |  |  |  |  | 10 |  |  |
Application of incentive based on target savings |  |  |  |  |  | $ 2.4 |  |  |
Energy efficiency targeted savings (in gwh) | GWh |  |  |  |  |  | 75 |  |  |
Minimum profit incentive |  |  |  |  | $ 1.8 |  |  |  |
Public Service Company of New Mexico | Energy Efficiency and Load Management Program, Proposed 2018 Portfolio | Disincentives/Incentives Added |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Program costs related to energy efficiency | $ 23.6 |  |  | $ 25.1 |  |  |  |  |
Application of incentive based on target savings |  |  |  | $ 1.9 |  |  |  |  |
Energy efficiency targeted savings (in gwh) | GWh |  |  |  | 53 |  |  |  |  |
Projected earned incentive |  |  | $ 1.9 | $ 2.1 |  |  |  |  |
Targeted energy savings | GWh |  |  | 69 | 70 |  |  |  |  |
Public Service Company of New Mexico | Energy Efficiency and Load Management Program, Proposed 2019 Portfolio | Disincentives/Incentives Added |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Program costs related to energy efficiency | $ 24.9 |  |  | $ 28.2 |  |  |  |  |
Application of incentive based on target savings |  |  | $ 1.7 | $ 2.1 |  |  |  |  |
Public Service Company of New Mexico | Maximum | Renewable Portfolio Standard |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Reasonable cost threshold |  |  |  |  | 3.00% |  |  |  | 3.00%
Sliding scale profit incentive |  |  |  |  | 9.00% |  |  |  |
Public Service Company of New Mexico | Minimum | Renewable Portfolio Standard |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Reasonable cost threshold |  |  |  |  |  |  |  | 3.00% |
Sliding scale profit incentive |  |  |  |  | 7.10% |  |  |  |

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