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Segment Information - Major Customers (Details) - Customer Concentration Risk - Texas-New Mexico Power Company - Electric operating revenues | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
REP A |  |  |
Concentration Risk [Line Items] |  |  |
Operating revenues from continuing operations | 21.00% | 22.00% | 21.00%
REP B |  |  |
Concentration Risk [Line Items] |  |  |
Operating revenues from continuing operations | 18.00% | 17.00% | 15.00%
REP C |  |  |
Concentration Risk [Line Items] |  |  |
Operating revenues from continuing operations | 11.00% | 12.00% | 12.00%

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