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Fair Value of Derivative and Other Financial Instruments - Available for Sale Securities (Details) - USD ($) | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Jan. 01, 2018 | Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Available-for-sale debt securities, fair value | $ 0 |  | $ 0 |  |  |
Other than temporary impairments |  |  | 0 |  |  |
PNM |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
(Increase)/decrease in other than temporary losses of available-for-sale securities, net portion recognized in earnings | (800,000) | $ 100,000 | (4,600,000) | $ 1,100,000 |  |
Proceeds from sales | 117,801,000 | 98,532,000 | 911,899,000 | 456,577,000 |  |
Gross realized gains | 3,460,000 | 8,128,000 | 17,030,000 | 24,745,000 |  |
Gross realized (losses) | (3,149,000) | $ (2,829,000) | (14,018,000) | $ (8,150,000) |  |
Nuclear Decommissioning Trust | Recurring | PNM |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Available-for-sale securities | 298,400,000 |  | 298,400,000 |  |  | $ 293,700,000
Mine Reclamation Trust | Recurring | PNM |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Available-for-sale securities | $ 33,300,000 |  | $ 33,300,000 |  |  | 29,800,000
Retained Earnings |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Cumulative effect adjustment (Note 7) |  |  |  |  |  | 11,208,000
Retained Earnings | PNM |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Cumulative effect adjustment (Note 7) |  |  |  |  |  | $ 11,208,000
Accounting Standards Update 2016-01 | Retained Earnings |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Cumulative effect adjustment (Note 7) |  |  |  |  | $ 11,200,000 |

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