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Fair Value of Derivative and Other Financial Instruments - Overview and Commodity Derivatives (Details) | Mar. 31, 2019 USD ($) MW | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
--- | --- | --- | ---
Derivatives, Fair Value [Line Items] |  |  |
Obligations to return cash | $ 900,000 | $ 1,000,000 |
PNM |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Expected exposure to market risk (in megawatts) | MW | 65 |  |
Power to be sold to third party (in megawatts) | MW | 36 |  |
Amounts recognized for right to reclaim cash | $ 0 | 0 |
Cash collateral under margin arrangements | 1,000,000 | 1,000,000 |
PNM | Designated as Hedging Instrument | Commodity derivatives |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Current assets | 1,086,000 | 1,083,000 |
Deferred charges | 2,265,000 | 2,511,000 |
Derivative asset | 3,351,000 | 3,594,000 |
Current liabilities | (1,150,000) | (1,177,000) |
Long-term liabilities | (2,265,000) | (2,511,000) |
Derivative liability | (3,415,000) | (3,688,000) |
Net | (64,000) | (94,000) |
PNM | Designated as Hedging Instrument | Commodity derivatives | Fuel and purchased power costs |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Current assets | 0 |  | $ 0
PNM | Designated as Hedging Instrument | Commodity derivatives | Hazard sharing arrangement |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative asset | $ 3,300,000 | $ 3,600,000 |
Tri-State | PNM |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Power to be sold to third party (in megawatts) | MW | 100 |  |

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