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Income Taxes - Federal Income Tax Reform (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($)
Income Taxes [Line Items] |
Net increase in regulatory liabilities | $ 548,952
Net decrease in deferred income tax liabilities (deferred income tax assets) | 491,491
Net deferred income tax expense | 57,461
Reclassification of stranded income taxes to retained earnings | (17,586)
Retained Earnings |
Income Taxes [Line Items] |
Reclassification of stranded income taxes to retained earnings | 17,586
PNM |
Income Taxes [Line Items] |
Net increase in regulatory liabilities | 402,501
Net decrease in deferred income tax liabilities (deferred income tax assets) | 372,895
Net deferred income tax expense | 29,606
Reclassification of stranded income taxes to retained earnings | (17,794)
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 | 146,451
Net decrease in deferred income tax liabilities (deferred income tax assets) | 138,586
Net deferred income tax expense | 7,865
Corporate and Other |
Income Taxes [Line Items] |
Net increase in regulatory liabilities | 0
Net decrease in deferred income tax liabilities (deferred income tax assets) | (19,990)
Net deferred income tax expense | $ 19,990

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