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Fair Value of Derivative and Other Financial Instruments - Available for Sale Securities (Details) - USD ($) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items] |  |  |
Available-for-sale securities for which carrying value exceeds fair value | $ 0 |  |
Held-to-maturity securities for which carrying value exceeds fair value | 0 |  |
Impairments considered to be other than temporary | 0 |  |
PNM |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
(Increase)/decrease in other than temporary losses of available-for-sale securities, net portion recognized in earnings | (1,200,000) | $ 1,100,000 |
Proceeds from sales | 626,729,000 | 266,388,000 |
Gross realized gains | 6,021,000 | 8,645,000 |
Gross realized (losses) | 4,677,000 | 3,085,000 |
PNM | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Gross realized gains | 324,003,000 | 323,524,000 |
PNM | Nuclear Decommissioning Trust | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 294,000,000 |  | $ 293,700,000
PNM | Mine Reclamation Trust | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 30,000,000 |  | $ 29,800,000
PNM | Cash and cash equivalents | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Gross realized gains | 22,810,000 | 52,636,000 |
PNM | Domestic value | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Gross realized gains |  | 40,032,000 |
PNM | Domestic growth | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Gross realized gains |  | 35,456,000 |
PNM | International and other | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Gross realized gains |  | 45,867,000 |
PNM | U.S. Government | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Gross realized gains | 23,829,000 | 34,317,000 |
PNM | Municipals | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Gross realized gains | 35,782,000 | 48,076,000 |
PNM | Corporate and other | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Gross realized gains | $ 114,813,000 | $ 67,140,000 |

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