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Regulatory and Rate Matters - Renewable Energy Rider (Details) - Public Service Company of New Mexico - Renewable energy rider - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Public Utilities, General Disclosures [Line Items] |  |  |
Revenue from renewable energy rider | $ 45.2 | $ 42.0 | $ 41.9
Maximum |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Annual revenue to be collected | $ 43.5 |  |
NMPRC-approved return on equity | 0.50% |  | 10.50%

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