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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 | Mar. 20, 2015 USD ($) party | May. 02, 2014 USD ($) | Jun. 03, 2013 USD ($) | Mar. 01, 2013 | Dec. 31, 2012 USD ($)
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Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  | $ 3.2
Public utilities, return on equity | 10.00% |  |  |  | 10.81%
Percentage ownership of EIP transmission line |  |  |  | 60.00% |
Public utilities, requested rate increase (decrease) |  | $ 0.5 | $ 1.3 |  |
Number of other parties to settlement | party | 5 |  |  |  |
Public utilities, approved rate increase (decrease) | $ 1.3 |  |  |  |

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