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Income Taxes - Federal Income Tax Reform (Details) - USD ($) $ in Thousands |  | 12 Months Ended
--- | --- | ---
May 23, 2017 | Dec. 31, 2018 | Dec. 31, 2017
Income Taxes [Line Items] |  |  |
Net increase in regulatory liabilities |  | $ 7,175 | $ 548,952
Net decrease in deferred income tax liabilities (deferred income tax assets) |  | 1,910 | 491,491
Net increase in affiliate receivables (affiliate payables) |  | 0 |
Net deferred income tax expense |  | 5,265 | 57,461
Reclassification of stranded income taxes to retained earnings |  |  | 0
Retained Earnings |  |  |
Income Taxes [Line Items] |  |  |
Reclassification of stranded income taxes to retained earnings |  | 17,600 | 17,586
PNM |  |  |
Income Taxes [Line Items] |  |  |
Net increase in regulatory liabilities |  | 11,244 | 402,501
Net decrease in deferred income tax liabilities (deferred income tax assets) |  | (2,175) | 372,895
Net increase in affiliate receivables (affiliate payables) |  | 12,300 |
Net deferred income tax expense |  | 1,119 | 29,606
Reclassification of stranded income taxes to retained earnings |  |  | 0
PNM | Retained Earnings |  |  |
Income Taxes [Line Items] |  |  |
Reclassification of stranded income taxes to retained earnings |  |  | 17,794
TNMP |  |  |
Income Taxes [Line Items] |  |  |
Net increase in regulatory liabilities |  | (4,069) | 146,451
Net decrease in deferred income tax liabilities (deferred income tax assets) |  | (9,784) | 138,586
Net increase in affiliate receivables (affiliate payables) |  | 4,042 |
Net deferred income tax expense |  | 1,673 | 7,865
Corporate and Other |  |  |
Income Taxes [Line Items] |  |  |
Net increase in regulatory liabilities |  | 0 | 0
Net decrease in deferred income tax liabilities (deferred income tax assets) |  | 13,869 | (19,990)
Net increase in affiliate receivables (affiliate payables) |  | (16,342) |
Net deferred income tax expense |  | $ 2,473 | $ 19,990
NMPRC |  |  |
Income Taxes [Line Items] |  |  |
Proposed term for providing benefits to customers related to reduction in state corporate tax | 21 years |  |
NMPRC | PNM |  |  |
Income Taxes [Line Items] |  |  |
Proposed term for providing benefits to customers related to reduction in state corporate tax | 3 years |  |

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