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Fair Value of Derivative and Other Financial Instruments - Available for Sale Securities (Details) - USD ($) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2017 | Mar. 31, 2016 | Dec. 31, 2016
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 | $ 1,100,000 | $ 1,600,000 |
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 |  |
PNMR and PNM |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 286,099,000 |  | $ 272,977,000
Unrealized Gains | 12,406,000 |  | 7,449,000
Proceeds from sales | 266,388,000 | 124,900,000 |
Gross realized gains | 8,645,000 | 10,716,000 |
Gross realized (losses) | (3,085,000) | $ (6,116,000) |
PNMR and PNM | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 286,099,000 |  | 272,977,000
PNMR and PNM | Nuclear Decommissioning Trust | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 266,100,000 |  | 253,900,000
PNMR and PNM | Mine Reclamation Trust | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 20,000,000 |  | 19,100,000
PNMR and PNM | Cash and cash equivalents |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 8,047,000 |  | 23,683,000
Unrealized Gains | 0 |  | 0
PNMR and PNM | Cash and cash equivalents | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 8,047,000 |  | 23,683,000
PNMR and PNM | Domestic value |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 70,043,000 |  | 34,796,000
Unrealized Gains | 3,044,000 |  | 1,135,000
PNMR and PNM | Domestic value | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 70,043,000 |  | 34,796,000
PNMR and PNM | Domestic growth |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 65,127,000 |  | 47,595,000
Unrealized Gains | 4,438,000 |  | 3,032,000
PNMR and PNM | Domestic growth | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 65,127,000 |  | 47,595,000
PNMR and PNM | International and other |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 36,442,000 |  | 27,481,000
Unrealized Gains | 3,555,000 |  | 2,029,000
PNMR and PNM | International and other | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 36,442,000 |  | 27,481,000
PNMR and PNM | U.S. Government |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 31,410,000 |  | 40,962,000
Unrealized Gains | 178,000 |  | 115,000
PNMR and PNM | U.S. Government | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 31,410,000 |  | 40,962,000
PNMR and PNM | Municipals |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 33,218,000 |  | 43,789,000
Unrealized Gains | 617,000 |  | 585,000
PNMR and PNM | Municipals | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 33,218,000 |  | 43,789,000
PNMR and PNM | Corporate and other |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 41,812,000 |  | 54,671,000
Unrealized Gains | 574,000 |  | 553,000
PNMR and PNM | Corporate and other | Fair Value, Measurements, Recurring |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | $ 41,812,000 |  | $ 54,671,000

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