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Income Taxes - Federal Income Tax Reform (Details)
$ in Thousands
 12 Months Ended
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