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Regulatory and Rate Matters - TNMP Narrative (Details) $ in Thousands |  |  |  |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Jan. 25, 2019 USD ($) | Nov. 02, 2018 USD ($) | May 30, 2018 USD ($) | Mar. 13, 2017 USD ($) | Jul. 31, 2011 USD ($) | Dec. 31, 2018 USD ($) advanced_meter | Dec. 31, 2017 USD ($)
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Unrecovered investment revenue |  |  | $ 21,800 |  |  |  |
Net increase in regulatory liabilities |  |  |  |  |  | $ 7,175 | $ 548,952
Texas-New Mexico Power Company |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Regulatory liabilities |  |  |  |  |  | 180,929 | 180,662
Net increase in regulatory liabilities |  |  |  |  |  | (4,069) | 146,451
Refund of regulatory liability to customers |  |  |  |  |  |  | 37,800
Net regulatory assets |  |  |  |  |  | $ 1,600 |
CTC Funding amount interest minimum |  |  |  |  |  | 15.00% |
Adjustment for collection of amortization |  |  |  | $ 1,100 |  |  |
Reduction to revenue |  |  |  |  |  | $ 5,400 |
Texas-New Mexico Power Company | 2018 TNMP Rate Case |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Requested rate increase (decrease) |  | $ 10,000 | $ 25,900 |  |  |  |
Requested return on equity |  | 9.65% | 10.50% |  |  |  |
Requested cost of debt, percentage |  | 6.44% | 7.20% |  |  |  |
Requested debt capital structure, percentage |  | 55.00% | 50.00% |  |  |  |
Requested equity capital structure, percentage |  | 45.00% | 50.00% |  |  |  |
New rate rider recovery, amount |  |  | $ 7,700 |  |  |  |
Regulatory liabilities |  |  |  |  |  |  | 146,500
Net increase in regulatory liabilities |  |  |  |  |  |  | $ 14,400
Refund of federal income tax rates period |  |  |  |  |  |  | 5 years
Investments excluded from rate, amount |  | $ 10,600 |  |  |  |  |
Texas-New Mexico Power Company | Advanced Meter System Deployment and Surcharge Request |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Revenue from AMS rider |  |  |  |  |  |  | $ 21,800
Approved deployment costs |  |  |  |  | $ 113,400 |  |
Period of time to collect deployment costs through surcharge period |  |  |  |  | 12 years |  |
Number of advanced meters installed | advanced_meter |  |  |  |  |  | 242,000 |
Texas-New Mexico Power Company | Transmission Cost of Service Rates | Subsequent Event |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Requested rate increase (decrease) | $ 111,800 |  |  |  |  |  |
Requested increase in annual transmission service revenue | $ 14,300 |  |  |  |  |  |

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