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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, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Available-for-sale securities for which carrying value exceeds fair value | $ 0 |  | $ 0 |  |
Held-to-maturity securities for which carrying value exceeds fair value | 0 |  | 0 |  |
Impairments considered to be other than temporary |  |  | 0 |  |
PNM |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 306,444,000 |  | 306,444,000 |  | $ 272,977,000
Unrealized Gains | 18,631,000 |  | 18,631,000 |  | 7,449,000
(Increase)/decrease in other than temporary losses of available-for-sale securities, net portion recognized in earnings | 100,000 | $ 100,000 | 1,100,000 | $ 1,000,000 |
Proceeds from sales | 98,532,000 | 86,975,000 | 456,577,000 | 280,989,000 |
Gross realized gains | 8,128,000 | 7,026,000 | 24,745,000 | 27,273,000 |
Gross realized (losses) | (2,829,000) | $ (2,565,000) | (8,150,000) | $ (12,913,000) |
PNM | Fair Value, Measurements, Recurring |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 306,444,000 |  | 306,444,000 |  | 272,977,000
PNM | Nuclear Decommissioning Trust | Fair Value, Measurements, Recurring |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 283,000,000 |  | 283,000,000 |  | 253,900,000
PNM | Mine Reclamation Trust | Fair Value, Measurements, Recurring |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 23,400,000 |  | 23,400,000 |  | 19,100,000
PNM | Cash and cash equivalents |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 8,151,000 |  | 8,151,000 |  | 23,683,000
Unrealized Gains | 0 |  | 0 |  | 0
PNM | Cash and cash equivalents | Fair Value, Measurements, Recurring |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 8,151,000 |  | 8,151,000 |  | 23,683,000
PNM | Domestic value |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 72,162,000 |  | 72,162,000 |  | 34,796,000
Unrealized Gains | 5,252,000 |  | 5,252,000 |  | 1,135,000
PNM | Domestic value | Fair Value, Measurements, Recurring |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 72,162,000 |  | 72,162,000 |  | 34,796,000
PNM | Domestic growth |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 73,345,000 |  | 73,345,000 |  | 47,595,000
Unrealized Gains | 5,775,000 |  | 5,775,000 |  | 3,032,000
PNM | Domestic growth | Fair Value, Measurements, Recurring |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 73,345,000 |  | 73,345,000 |  | 47,595,000
PNM | International and other |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 43,167,000 |  | 43,167,000 |  | 27,481,000
Unrealized Gains | 4,865,000 |  | 4,865,000 |  | 2,029,000
PNM | International and other | Fair Value, Measurements, Recurring |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 43,167,000 |  | 43,167,000 |  | 27,481,000
PNM | U.S. Government |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 28,960,000 |  | 28,960,000 |  | 40,962,000
Unrealized Gains | 307,000 |  | 307,000 |  | 115,000
PNM | U.S. Government | Fair Value, Measurements, Recurring |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 28,960,000 |  | 28,960,000 |  | 40,962,000
PNM | Municipals |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 41,131,000 |  | 41,131,000 |  | 43,789,000
Unrealized Gains | 998,000 |  | 998,000 |  | 585,000
PNM | Municipals | Fair Value, Measurements, Recurring |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 41,131,000 |  | 41,131,000 |  | 43,789,000
PNM | Corporate and other |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | 39,528,000 |  | 39,528,000 |  | 54,671,000
Unrealized Gains | 1,434,000 |  | 1,434,000 |  | 553,000
PNM | Corporate and other | Fair Value, Measurements, Recurring |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Fair Value | $ 39,528,000 |  | $ 39,528,000 |  | $ 54,671,000

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