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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