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Regulatory and Rate Matters - Formula Transmission Rate Case (Details) - Public Service Company of New Mexico [Member] $ in Millions |  |  | 12 Months Ended
--- | --- | --- | ---
Mar. 20, 2015 USD ($) party | Mar. 01, 2013 | Dec. 31, 2012 USD ($)
Wholesale Electric Transmission Rate Case [Member] [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Approved rate increase (decrease) |  |  | $ 3.2
Initial approved return on equity |  |  | 10.81%
Lease ownership in EIP |  | 60.00% |
Approved additional rate increase | $ 1.3 |  |
Formula Transmission Rate Case [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Number of additional parties entered into settlement agreement (in parties) | party | 5 |  |
Return on equity | 10.00% |  |

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