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Fair Value of Derivative and Other Financial Instruments - Items Recorded and Presented by Level of Hierarchy (Details) - USD ($) $ in Thousands | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Jun. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | $ 2,661,283 |  | $ 2,554,836 |
Westmoreland Loan | 57,486 |  | 66,588 |
Other investments | 375 |  | 503 |
Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 |
Westmoreland Loan | 0 |  | 0 |
Other investments | 375 |  | 503 |
Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 2,661,283 |  | 2,554,836 |
Westmoreland Loan | 0 |  | 0 |
Other investments | 0 |  | 0 |
Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 |
Westmoreland Loan | 57,486 |  | 66,588 |
Other investments | 0 |  | 0 |
Carrying Amount |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 2,581,549 |  | 2,437,645 |
Westmoreland Loan | 50,991 |  | 56,640 |
Other investments | 375 |  | 503 |
PNM |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 6,021 | $ 8,645 |  |
Long-term debt | 1,703,098 |  | 1,727,135 |
Other investments | 155 |  | 283 |
PNM | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 |
Other investments | 155 |  | 283 |
PNM | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 1,703,098 |  | 1,727,135 |
Other investments | 0 |  | 0 |
PNM | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 |
Other investments | 0 |  | 0 |
PNM | Carrying Amount |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 1,658,431 |  | 1,657,910 |
Other investments | 155 |  | 283 |
TNMP |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 513,690 |  | 527,563 |
Other investments | 220 |  | 220 |
TNMP | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 |
Other investments | 220 |  | 220 |
TNMP | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 513,690 |  | 527,563 |
Other investments | 0 |  | 0 |
TNMP | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 0 |  | 0 |
Other investments | 0 |  | 0 |
TNMP | Carrying Amount |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt | 480,716 |  | 480,620 |
Other investments | 220 |  | 220 |
Fair Value, Measurements, Recurring | PNM |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 324,003 | 323,524 |  |
Fair Value, Measurements, Recurring | PNM | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 157,665 | 204,101 |  |
Fair Value, Measurements, Recurring | PNM | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 163,987 | 119,423 |  |
Fair Value, Measurements, Recurring | PNM | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 2,351 | 0 |  |
Investments, unrealized gain | 1,115 | 18,028 |  |
Fair Value, Measurements, Recurring | PNM | Commodity derivatives |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity derivative assets | 4,364 |  | 4,644 |
Commodity derivative liabilities | (4,605) |  | (4,738) |
Net | (241) |  | (94) |
Fair Value, Measurements, Recurring | PNM | Commodity derivatives | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity derivative assets | 0 |  | 0 |
Commodity derivative liabilities | 0 |  | 0 |
Net | 0 |  | 0 |
Fair Value, Measurements, Recurring | PNM | Commodity derivatives | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity derivative assets | 4,364 |  | 4,644 |
Commodity derivative liabilities | (4,605) |  | (4,738) |
Net | (241) |  | (94) |
Fair Value, Measurements, Recurring | PNM | Commodity derivatives | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity derivative assets |  |  | 0 | $ 0
Commodity derivative liabilities |  |  | 0 | 0
Net |  |  | $ 0 | $ 0
Fair Value, Measurements, Recurring | PNM | Cash and cash equivalents |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 22,810 | 52,636 |  |
Fair Value, Measurements, Recurring | PNM | Cash and cash equivalents | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 22,810 | 52,636 |  |
Fair Value, Measurements, Recurring | PNM | Cash and cash equivalents | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 | 0 |  |
Fair Value, Measurements, Recurring | PNM | Cash and cash equivalents | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 | 0 |  |
Fair Value, Measurements, Recurring | PNM | Corporate stocks, common |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 35,185 |  |  |
Fair Value, Measurements, Recurring | PNM | Corporate stocks, common | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 35,185 |  |  |
Fair Value, Measurements, Recurring | PNM | Corporate stocks, common | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 |  |  |
Fair Value, Measurements, Recurring | PNM | Corporate stocks, common | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 |  |  |
Fair Value, Measurements, Recurring | PNM | Corporate stocks, preferred |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 6,714 |  |  |
Fair Value, Measurements, Recurring | PNM | Corporate stocks, preferred | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 865 |  |  |
Fair Value, Measurements, Recurring | PNM | Corporate stocks, preferred | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 5,849 |  |  |
Fair Value, Measurements, Recurring | PNM | Corporate stocks, preferred | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 |  |  |
Fair Value, Measurements, Recurring | PNM | Mutual funds and other |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 74,976 |  |  |
Fair Value, Measurements, Recurring | PNM | Mutual funds and other | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 74,976 |  |  |
Fair Value, Measurements, Recurring | PNM | Mutual funds and other | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 |  |  |
Fair Value, Measurements, Recurring | PNM | Mutual funds and other | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 |  |  |
Fair Value, Measurements, Recurring | PNM | U.S. Government |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 23,829 | 34,317 |  |
Fair Value, Measurements, Recurring | PNM | U.S. Government | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 23,829 | 33,645 |  |
Fair Value, Measurements, Recurring | PNM | U.S. Government | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 | 672 |  |
Fair Value, Measurements, Recurring | PNM | U.S. Government | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 | 0 |  |
Investments, unrealized gain | 271 | 273 |  |
Fair Value, Measurements, Recurring | PNM | International Government | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 9,894 |  |  |
Fair Value, Measurements, Recurring | PNM | International Government | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments, unrealized gain | 61 |  |  |
Fair Value, Measurements, Recurring | PNM | Municipals |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 35,782 | 48,076 |  |
Fair Value, Measurements, Recurring | PNM | Municipals | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 | 0 |  |
Fair Value, Measurements, Recurring | PNM | Municipals | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 35,782 | 48,076 |  |
Fair Value, Measurements, Recurring | PNM | Municipals | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 | 0 |  |
Investments, unrealized gain | 103 | 1,225 |  |
Fair Value, Measurements, Recurring | PNM | Domestic value |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 40,032 |  |
Fair Value, Measurements, Recurring | PNM | Domestic value | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 40,032 |  |
Fair Value, Measurements, Recurring | PNM | Domestic value | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 0 |  |
Fair Value, Measurements, Recurring | PNM | Domestic value | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 0 |  |
Investments, unrealized gain |  | 4,011 |  |
Fair Value, Measurements, Recurring | PNM | Domestic growth |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 35,456 |  |
Fair Value, Measurements, Recurring | PNM | Domestic growth | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 35,456 |  |
Fair Value, Measurements, Recurring | PNM | Domestic growth | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 0 |  |
Fair Value, Measurements, Recurring | PNM | Domestic growth | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 0 |  |
Investments, unrealized gain |  | 3,995 |  |
Fair Value, Measurements, Recurring | PNM | International and other |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 45,867 |  |
Fair Value, Measurements, Recurring | PNM | International and other | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 42,332 |  |
Fair Value, Measurements, Recurring | PNM | International and other | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 3,535 |  |
Fair Value, Measurements, Recurring | PNM | International and other | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments |  | 0 |  |
Investments, unrealized gain |  | 6,810 |  |
Fair Value, Measurements, Recurring | PNM | Corporate and other |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 114,813 | 67,140 |  |
Fair Value, Measurements, Recurring | PNM | Corporate and other | Level 1 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 0 | 0 |  |
Fair Value, Measurements, Recurring | PNM | Corporate and other | Level 2 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 112,462 | 67,140 |  |
Fair Value, Measurements, Recurring | PNM | Corporate and other | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Investments | 2,351 | 0 |  |
Investments, unrealized gain | $ 680 | $ 1,714 |  |

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- Definition Amount of unrealized gain before deducting unrealized loss on investments in debt securities classified as available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other 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 assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313 + Details
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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 Reference 3: http://www.xbrl.org/2003/role/presentationRef -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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 + Details
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