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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016
401(k) plan |  |  |
Defined Contribution Plan [Abstract] |  |  |
401(k) plan | $ 16,677 | $ 16,452 | $ 17,762
401(k) plan | Public Service Company of New Mexico |  |  |
Defined Contribution Plan [Abstract] |  |  |
401(k) plan | 12,052 | 12,120 | 13,397
401(k) plan | Texas-New Mexico Power Company |  |  |
Defined Contribution Plan [Abstract] |  |  |
401(k) plan | 4,625 | 4,332 | 4,365
Non-qualified plan |  |  |
Defined Contribution Plan [Abstract] |  |  |
Non-qualified plan | 865 | 3,702 | 2,017
Non-qualified plan | Public Service Company of New Mexico |  |  |
Defined Contribution Plan [Abstract] |  |  |
Non-qualified plan | 621 | 2,834 | 1,535
Non-qualified plan | Texas-New Mexico Power Company |  |  |
Defined Contribution Plan [Abstract] |  |  |
Non-qualified plan | $ 244 | $ 868 | $ 482

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