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Regulatory and Rate Matters - Formula Transmission Rate Case (Details) - Formula Transmission Rate Case [Member] - Public Service Company of New Mexico [Member] $ in Millions |  |  | 6 Months Ended |
--- | --- | --- | --- | ---
May. 02, 2014 USD ($) | Jun. 03, 2013 USD ($) | Jun. 30, 2015 USD ($) | Mar. 20, 2015 party
Public Utilities, General Disclosures [Line Items] |  |  |  |
Amount of Regulatory Costs Not yet Approved |  |  | $ 3.2 |
Public Utilities, Return on Equity |  |  | 10.81% | 10.00%
Percentage ownership of EIP transmission line |  |  | 60.00% |
Public Utilities, Requested Rate Increase (Decrease), Amount | $ 0.5 | $ 1.3 |  |
Number of other parties to settlement | party |  |  |  | 5
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  | $ 1.3 |

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