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Goodwill (Details) - USD ($) |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Apr. 01, 2017 | Dec. 31, 2017 | Sep. 30, 2018 | Apr. 01, 2018 | Sep. 30, 2017
Goodwill [Line Items] |  |  |  |  |
Goodwill |  | $ 278,297,000 | $ 278,297,000 |  | $ 278,297,000
Impairments of goodwill | $ 0 | 0 |  |  |
PNM |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill |  | 51,632,000 | 51,632,000 | $ 51,600,000 |
Goodwill fair value exceeded by its carrying value |  |  |  | 19.00% |
TNMP |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill |  | $ 226,665,000 | $ 226,665,000 | $ 226,700,000 |
Goodwill fair value exceeded by its carrying value |  |  |  | 32.00% |

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