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Goodwill (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Apr. 01, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Apr. 01, 2016 | Apr. 01, 2015
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Schedule of Goodwill and Other Intangible Assets [Line Items] |  |  |  |  |  |
Goodwill | $ 278,297 |  | $ 278,297 | $ 278,297 |  |
Public Service Company of New Mexico |  |  |  |  |  |
Schedule of Goodwill and Other Intangible Assets [Line Items] |  |  |  |  |  |
Goodwill | 51,632 | $ 51,600 | 51,632 |  | $ 51,600 | $ 51,600
Percentage of fair value in excess of carrying amount |  | 19.00% |  |  | 25.00% |
Percentage increase in expected return on equity |  |  |  |  | 0.50% | 0.50%
Reduced percentage of fair value in excess of carrying value |  |  |  |  | 18.00% |
Texas-New Mexico Power Company |  |  |  |  |  |
Schedule of Goodwill and Other Intangible Assets [Line Items] |  |  |  |  |  |
Goodwill | $ 226,665 | $ 226,700 | $ 226,665 |  | $ 226,700 |
Percentage of fair value in excess of carrying amount |  | 32.00% |  |  | 32.00% |
Percentage increase in expected return on equity |  |  |  |  | 0.50% |
Reduced percentage of fair value in excess of carrying value |  |  |  |  | 21.00% |

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- Definition Goodwill, Reporting Unit, Sensitivity analysis, Reduced Percentage of Fair Value in Excess of Carrying Value + References No definition available. + Details
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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
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