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Fair Value of Derivative and Other Financial Instruments - Non-Derivative Financial Instruments (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items] |  |  |
Investment percentage in fixed income (debt) securities | 85.00% |  |
Realized impairment loss | $ 3,300,000 | $ (1,200,000) | $ (4,300,000)
Fixed income investments |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Investment allocation targets | 65.00% |  |
Fixed income investments | Scenario, adjustment |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Investment allocation targets | 50.00% |  |
PNMR and PNM |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | $ 323,524,000 | 272,977,000 |
Unrealized Gains | 18,028,000 | 7,449,000 |
Proceeds from sales | 637,492,000 | 522,601,000 | 252,174,000
Gross realized gains | 36,896,000 | 46,116,000 | 29,663,000
Gross realized (losses) | (12,993,000) | (25,430,000) | $ (9,259,000)
Value of other than temporary impairments | 0 |  |
PNMR and PNM | Cash and cash equivalents |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 52,636,000 | 23,683,000 |
Unrealized Gains | 0 | 0 |
PNMR and PNM | Domestic value |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 40,032,000 | 34,796,000 |
Unrealized Gains | 4,011,000 | 1,135,000 |
PNMR and PNM | Domestic growth |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 35,456,000 | 47,595,000 |
Unrealized Gains | 3,995,000 | 3,032,000 |
PNMR and PNM | International and other |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 45,867,000 | 27,481,000 |
Unrealized Gains | 6,810,000 | 2,029,000 |
PNMR and PNM | U.S. Government |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 34,317,000 | 40,962,000 |
Unrealized Gains | 273,000 | 115,000 |
PNMR and PNM | Municipals |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 48,076,000 | 43,789,000 |
Unrealized Gains | 1,225,000 | 585,000 |
PNMR and PNM | Corporate and other |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 67,140,000 | 54,671,000 |
Unrealized Gains | 1,714,000 | 553,000 |
PNMR and PNM | Measured on a recurring basis |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 323,524,000 | 272,977,000 |
PNMR and PNM | Measured on a recurring basis | Nuclear Decommissioning Trust |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 293,700,000 | 253,900,000 |
PNMR and PNM | Measured on a recurring basis | Mine Reclamation Trust |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 29,800,000 | 19,100,000 |
PNMR and PNM | Measured on a recurring basis | Cash and cash equivalents |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 52,636,000 | 23,683,000 |
PNMR and PNM | Measured on a recurring basis | Domestic value |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 40,032,000 | 34,796,000 |
PNMR and PNM | Measured on a recurring basis | Domestic growth |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 35,456,000 | 47,595,000 |
PNMR and PNM | Measured on a recurring basis | International and other |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 45,867,000 | 27,481,000 |
PNMR and PNM | Measured on a recurring basis | U.S. Government |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 34,317,000 | 40,962,000 |
PNMR and PNM | Measured on a recurring basis | Municipals |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | 48,076,000 | 43,789,000 |
PNMR and PNM | Measured on a recurring basis | Corporate and other |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Fair Value | $ 67,140,000 | $ 54,671,000 |

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