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Fair Value of Derivative and Other Financial Instruments - Available for Sale Securities (Details) - USD ($) | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Jan. 01, 2018
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale, unrealized loss position | $ 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 | 3,400,000 | $ (1,200,000) |  |
Proceeds from sales | 74,460,000 | 626,729,000 |  |
Gross realized gains | 4,189,000 | 6,021,000 |  |
Gross realized (losses) | (3,170,000) | $ (4,677,000) |  |
Recurring | PNM |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Available-for-sale debt securities | 351,563,000 |  | $ 328,242,000 |
Nuclear Decommissioning Trust | Recurring | PNM |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Available-for-sale debt securities | 308,000,000 |  | 287,100,000 |
Mine Reclamation Trust | Recurring | PNM |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Available-for-sale debt securities | $ 43,600,000 |  | $ 41,100,000 |
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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