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Fair Value of Derivative and Other Financial Instruments - Non-Derivative Financial Instruments (Details) - USD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jan. 01, 2018 | Jan. 01, 2017
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Cumulative effect adjustment |  |  |  | $ 0 | $ 10,382,000
Transfers between levels | $ 0 | $ 0 |  |  |
Available for sale securities realized impairment losses | (9,400,000) | 3,300,000 | $ (1,200,000) |  |
Debt securities, unrealized loss position | 0 |  |  |  |
PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Fair value | 205,738,000 |  |  |  |
Proceeds from sale of debt securities AFS | 984,533,000 | 637,492,000 | 522,601,000 |  |
Available for sale securities realized gain | 19,358,000 | 36,896,000 | 46,116,000 |  |
Available for sale securities realized loss | (16,624,000) | (12,993,000) | $ (25,430,000) |  |
Impairment losses other than temporary included in AOCI | 0 |  |  |  |
Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Fair value | 328,242,000 | 323,524,000 |  |  |
Nuclear Decommissioning Trust | Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Fair value | 287,100,000 | 293,700,000 |  |  |
Mine Reclamation Trust | Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Fair value | 41,100,000 | 29,800,000 |  |  |
Cash and cash equivalents | Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Fair value | 11,472,000 | 52,636,000 |  |  |
Domestic value | Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Fair value | 32,997,000 | 40,032,000 |  |  |
Domestic growth | Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Fair value | 7,258,000 | 35,456,000 |  |  |
International and other | Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Fair value | 70,777,000 | 45,867,000 |  |  |
U.S. Government | Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Fair value | 29,503,000 | 34,317,000 |  |  |
Municipals | Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Fair value | 53,642,000 | 48,076,000 |  |  |
Corporate and other | Measured on a recurring basis | PNMR and PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Fair value | $ 114,158,000 | 67,140,000 |  |  |
Fixed income investments |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Investment allocation targets | 65.00% |  |  |  |
Scenario, adjustment | Fixed income investments |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Investment allocation targets | 54.00% |  |  |  |
Retained Earnings |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Cumulative effect adjustment |  | $ 11,200,000 |  | $ 11,208,000 | $ 10,382,000

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