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Fair Value of Derivative and Other Financial Instruments - Non-Derivative Financial Instruments (Details) - USD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2018 | Jan. 01, 2017
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Cumulative effect adjustment |  |  |  | $ 0 | $ 10,382,000
Available for sale securities realized impairment losses | $ 3,000,000.0 | $ 9,400,000 | $ 3,300,000 |  |
Debt securities, unrealized loss position | 0 |  |  |  |
Transfer to fair value hierarchy Level 2 |  | 0 |  |  |
PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt securities, available-for-sale | 249,818,000 |  |  |  |
Proceeds from sales | 494,528,000 | 984,533,000 | 637,492,000 |  |
Gross realized gains | 25,760,000 | 19,358,000 | 36,896,000 |  |
Gross realized (losses) | (17,453,000) | (16,624,000) | $ (12,993,000) |  |
Impairment losses other than temporary included in AOCI | 0 |  |  |  |
Nuclear Decommissioning Trust | Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt securities, available-for-sale | 336,000,000.0 | 287,100,000 |  |  |
Mine Reclamation Trust | Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt securities, available-for-sale | $ 52,800,000 | $ 41,100,000 |  |  |
Retained Earnings |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Cumulative effect adjustment |  |  |  | 11,208,000 | $ 10,382,000
Accounting Standards Update 2016-01 | Retained Earnings |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Cumulative effect adjustment |  |  |  | $ 11,200,000 |
Minimum |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Investment allocation targets distribution period | 10 years |  |  |  |
Maximum |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Investment allocation targets distribution period | 15 years |  |  |  |
Pension Plan | Equity Securities |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Investment allocation targets | 30.00% |  |  |  |
Pension Plan | Alternative Investments |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Investment allocation targets | 20.00% |  |  |  |
Pension Plan | Fixed income |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Investment allocation targets | 50.00% |  |  |  |

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