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Fair Value of Derivative and Other Financial Instruments - Items Recorded and Presented by Level of Hierarchy (Details) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Transfers between levels |  |  | $ 0 | $ 0
Long-term debt | $ 2,642,154,000 |  | 2,642,154,000 | 2,554,836,000
Westmoreland Loan |  |  |  | 66,588,000
Other investments | 348,000 |  | 348,000 | 503,000
Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 | 0
Westmoreland Loan |  |  |  | 0
Other investments | 348,000 |  | 348,000 | 503,000
Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 2,642,154,000 |  | 2,642,154,000 | 2,554,836,000
Westmoreland Loan |  |  |  | 0
Other investments | 0 |  | 0 | 0
Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 | 0
Westmoreland Loan |  |  |  | 66,588,000
Other investments | 0 |  | 0 | 0
Carrying Amount |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 2,614,511,000 |  | 2,614,511,000 | 2,437,645,000
Westmoreland Loan |  |  |  | 56,640,000
Other investments | 348,000 |  | 348,000 | 503,000
PNM |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 1,665,064,000 |  | 1,665,064,000 | 1,727,135,000
Other investments | 142,000 |  | 142,000 | 283,000
PNM | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 | 0
Other investments | 142,000 |  | 142,000 | 283,000
PNM | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 1,665,064,000 |  | 1,665,064,000 | 1,727,135,000
Other investments | 0 |  | 0 | 0
PNM | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 | 0
Other investments | 0 |  | 0 | 0
PNM | Carrying Amount |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 1,656,102,000 |  | 1,656,102,000 | 1,657,910,000
Other investments | 142,000 |  | 142,000 | 283,000
TNMP |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 580,017,000 |  | 580,017,000 | 527,563,000
Other investments | 206,000 |  | 206,000 | 220,000
TNMP | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 | 0
Other investments | 206,000 |  | 206,000 | 220,000
TNMP | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 580,017,000 |  | 580,017,000 | 527,563,000
Other investments | 0 |  | 0 | 0
TNMP | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 | 0
Other investments | 0 |  | 0 | 0
TNMP | Carrying Amount |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 560,293,000 |  | 560,293,000 | 480,620,000
Other investments | 206,000 |  | 206,000 | 220,000
Recurring | PNM |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 331,746,000 |  | 331,746,000 | 323,524,000
Recurring | PNM | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 135,566,000 |  | 135,566,000 | 204,101,000
Recurring | PNM | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 193,126,000 |  | 193,126,000 | 119,423,000
Recurring | PNM | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 3,054,000 |  | 3,054,000 | 0
Investments, unrealized gain | 2,018,000 | $ 18,028,000 |  |
Recurring | PNM | Commodity derivatives |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity derivative assets | 3,824,000 |  | 3,824,000 | 4,644,000
Commodity derivative liabilities | (3,833,000) |  | (3,833,000) | (4,738,000)
Net | (9,000) |  | (9,000) | (94,000)
Recurring | PNM | Commodity derivatives | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity derivative assets | 0 |  | 0 | 0
Commodity derivative liabilities | 0 |  | 0 | 0
Net | 0 |  | 0 | 0
Recurring | PNM | Commodity derivatives | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity derivative assets | 3,824,000 |  | 3,824,000 | 4,644,000
Commodity derivative liabilities | (3,833,000) |  | (3,833,000) | (4,738,000)
Net | (9,000) |  | (9,000) | (94,000)
Recurring | PNM | Commodity derivatives | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity derivative assets | 0 |  | 0 | 0
Commodity derivative liabilities | 0 |  | 0 | 0
Net | 0 |  | 0 | 0
Recurring | PNM | Cash and cash equivalents |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Cash and cash equivalents | 3,527,000 |  | 3,527,000 | 52,636,000
Recurring | PNM | Cash and cash equivalents | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Cash and cash equivalents | 3,527,000 |  | 3,527,000 | 52,636,000
Recurring | PNM | Cash and cash equivalents | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Cash and cash equivalents | 0 |  | 0 | 0
Recurring | PNM | Cash and cash equivalents | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Cash and cash equivalents | 0 |  | 0 | 0
Recurring | PNM | Corporate stocks, common |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 40,017,000 |  | 40,017,000 |
Recurring | PNM | Corporate stocks, common | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 40,017,000 |  | 40,017,000 |
Recurring | PNM | Corporate stocks, common | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 0 |  | 0 |
Recurring | PNM | Corporate stocks, common | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 0 |  | 0 |
Recurring | PNM | Corporate stocks, preferred |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 7,239,000 |  | 7,239,000 |
Recurring | PNM | Corporate stocks, preferred | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 1,587,000 |  | 1,587,000 |
Recurring | PNM | Corporate stocks, preferred | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 5,652,000 |  | 5,652,000 |
Recurring | PNM | Corporate stocks, preferred | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 0 |  | 0 |
Recurring | PNM | Mutual funds and other |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 75,035,000 |  | 75,035,000 |
Recurring | PNM | Mutual funds and other | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 75,035,000 |  | 75,035,000 |
Recurring | PNM | Mutual funds and other | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 0 |  | 0 |
Recurring | PNM | Mutual funds and other | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities | 0 |  | 0 |
Recurring | PNM | Domestic value |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 40,032,000
Recurring | PNM | Domestic value | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 40,032,000
Recurring | PNM | Domestic value | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 0
Recurring | PNM | Domestic value | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 0
Investments, unrealized gain |  | 4,011,000 |  |
Recurring | PNM | Domestic growth |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 35,456,000
Recurring | PNM | Domestic growth | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 35,456,000
Recurring | PNM | Domestic growth | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 0
Recurring | PNM | Domestic growth | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 0
Investments, unrealized gain |  | 3,995,000 |  |
Recurring | PNM | International and other |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 45,867,000
Recurring | PNM | International and other | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 42,332,000
Recurring | PNM | International and other | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 3,535,000
Recurring | PNM | International and other | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities |  |  |  | 0
Investments, unrealized gain |  | 6,810,000 |  |
Recurring | PNM | U.S. Government |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 25,689,000 |  | 25,689,000 | 34,317,000
Recurring | PNM | U.S. Government | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 15,400,000 |  | 15,400,000 | 33,645,000
Recurring | PNM | U.S. Government | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 10,289,000 |  | 10,289,000 | 672,000
Recurring | PNM | U.S. Government | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 0 |  | 0 | 0
Investments, unrealized gain | 23,000 | 273,000 |  |
Recurring | PNM | International Government |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 8,460,000 |  | 8,460,000 |
Recurring | PNM | International Government | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 0 |  | 0 |
Recurring | PNM | International Government | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 8,460,000 |  | 8,460,000 |
Recurring | PNM | International Government | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 0 |  | 0 |
Investments, unrealized gain | 90,000 |  |  |
Recurring | PNM | Municipals |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 51,280,000 |  | 51,280,000 | 48,076,000
Recurring | PNM | Municipals | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 0 |  | 0 | 0
Recurring | PNM | Municipals | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 51,280,000 |  | 51,280,000 | 48,076,000
Recurring | PNM | Municipals | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 0 |  | 0 | 0
Investments, unrealized gain | 78,000 | 1,225,000 |  |
Recurring | PNM | Corporate and other |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 120,499,000 |  | 120,499,000 | 67,140,000
Recurring | PNM | Corporate and other | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 0 |  | 0 | 0
Recurring | PNM | Corporate and other | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 117,445,000 |  | 117,445,000 | 67,140,000
Recurring | PNM | Corporate and other | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Available-for-sale debt securities | 3,054,000 |  | $ 3,054,000 | $ 0
Investments, unrealized gain | $ 1,827,000 | $ 1,714,000 |  |

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- Definition Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313 + Details
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