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Fair Value of Derivative and Other Financial Instruments - Available for Sale Securities (Details) - USD ($) | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 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 | (2,600,000) | $ (100,000) | (3,800,000) | $ 1,000,000 |  |
Proceeds from sales | 167,359,000 | 91,657,000 | 794,088,000 | 358,045,000 |  |
Gross realized gains | 7,549,000 | 7,971,000 | 13,570,000 | 16,617,000 |  |
Gross realized (losses) | (6,192,000) | $ (2,236,000) | (10,869,000) | $ (5,321,000) |  |
Nuclear Decommissioning Trust | Recurring | PNM |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Available-for-sale securities | 292,800,000 |  | 292,800,000 |  |  | $ 293,700,000
Mine Reclamation Trust | Recurring | PNM |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Available-for-sale securities | $ 30,300,000 |  | $ 30,300,000 |  |  | 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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