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Segment Information - Major Customers (Details) - Electric operating revenues - customer | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
PNMR and PNM |  |  |
Concentration Risk [Line Items] |  |  |
Number of customers that make up more than 10% of total revenue | 0 |  |
PNMR and PNM | Maximum |  |  |
Concentration Risk [Line Items] |  |  |
Operating revenues from continuing operations | 10.00% |  |
Texas-New Mexico Power Company |  |  |
Concentration Risk [Line Items] |  |  |
Number of customers that make up more than 10% of total revenue | 3 |  |
Texas-New Mexico Power Company | REP A |  |  |
Concentration Risk [Line Items] |  |  |
Operating revenues from continuing operations | 16.00% | 16.00% | 16.00%
Texas-New Mexico Power Company | REP B |  |  |
Concentration Risk [Line Items] |  |  |
Operating revenues from continuing operations | 11.00% | 11.00% | 13.00%
Texas-New Mexico Power Company | REP C |  |  |
Concentration Risk [Line Items] |  |  |
Operating revenues from continuing operations | 10.00% | 11.00% | 11.00%

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