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Summary of the Business and Significant Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2018
Accounting Policies [Abstract]  
Schedule of Average Rates Used Allocated Between Depreciation Expense and Construction Expense Projects Based on Use of Equipment
Average straight-line rates used were as follows:
 
Year ended December 31
 
2018
 
2017
 
2016
PNM
 
 
 
 
 
Electric plant
2.40
%
 
2.52
%
 
2.33
%
Common, intangible, and general plant
8.18
%
 
8.36
%
 
5.40
%
TNMP
3.49
%
 
3.57
%
 
3.66
%
Schedule of Inventory
Inventories consisted of the following at December 31:
 
PNMR
 
PNM
 
TNMP
 
2018
 
2017
 
2018
 
2017
 
2018
 
2017
 
(In thousands)
Coal
$
22,777

 
$
16,714

 
$
22,777

 
$
16,714

 
$
—

 
$
—

Materials and supplies
49,057

 
49,788

 
44,320

 
44,145

 
4,737

 
5,643

 
$
71,834

 
$
66,502

 
$
67,097

 
$
60,859

 
$
4,737

 
$
5,643