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Pension and Other Postretirement Benefits - Other Postretirement Benefits (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
401(k) plan |  |  |
Defined Contribution Plan [Abstract] |  |  |
401(k) plan | $ 16,452 | $ 17,762 | $ 16,725
401(k) plan | Public Service Company of New Mexico |  |  |
Defined Contribution Plan [Abstract] |  |  |
401(k) plan | 12,120 | 13,397 | 12,679
401(k) plan | Texas-New Mexico Power Company |  |  |
Defined Contribution Plan [Abstract] |  |  |
401(k) plan | 4,332 | 4,365 | 4,046
Non-qualified plan |  |  |
Defined Contribution Plan [Abstract] |  |  |
Non-qualified plan | 3,702 | 2,017 | 1,436
Non-qualified plan | Public Service Company of New Mexico |  |  |
Defined Contribution Plan [Abstract] |  |  |
Non-qualified plan | 2,834 | 1,535 | 1,090
Non-qualified plan | Texas-New Mexico Power Company |  |  |
Defined Contribution Plan [Abstract] |  |  |
Non-qualified plan | $ 868 | $ 482 | $ 346

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