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Income Taxes - Reconciliation of Unrecognized Tax Benefits (Expense) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
PNMR      
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Beginning balance $ 6,752 $ 6,455 $ 15,031
Additions based on tax positions 262 242 1,214
Additions (reductions) for tax positions of prior years     (9,790)
Additions (reductions) for tax positions of prior years 2,415 55  
Settlement payments 0 0 0
Ending balance 9,429 6,752 6,455
Unrecognized tax benefits that would impact effective tax rate 8,900    
Public Service Company of New Mexico      
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Beginning balance 3,949 3,652 12,228
Additions based on tax positions 262 242 1,214
Additions (reductions) for tax positions of prior years     (9,790)
Additions (reductions) for tax positions of prior years 2,352 55  
Settlement payments 0 0 0
Ending balance 6,563 3,949 3,652
Unrecognized tax benefits that would impact effective tax rate 6,100    
Texas-New Mexico Power Company      
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Beginning balance 0 0 0
Additions based on tax positions 0 0 0
Additions (reductions) for tax positions of prior years     0
Additions (reductions) for tax positions of prior years 63 0  
Settlement payments 0 0 0
Ending balance 63 $ 0 $ 0
Unrecognized tax benefits that would impact effective tax rate $ 100