Summary of the Business and Significant Accounting Policies - Inventories/Depreciation and Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Public Utilities, Inventory [Line Items]
Inventory	$ 66,502	$ 73,027
Coal
Public Utilities, Inventory [Line Items]
Inventory	16,714	19,940
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventory	49,788	53,087
Public Service Company of New Mexico
Public Utilities, Inventory [Line Items]
Inventory	60,859	64,401
Public Service Company of New Mexico | Coal
Public Utilities, Inventory [Line Items]
Inventory	16,714	19,940
Public Service Company of New Mexico | Materials and supplies
Public Utilities, Inventory [Line Items]
Inventory	$ 44,145	$ 44,461
Public Service Company of New Mexico | Electric plant
Public Utilities, Inventory [Line Items]
Depreciation average rates used	2.52%	2.33%	2.27%
Public Service Company of New Mexico | Common, intangible, and general plant
Public Utilities, Inventory [Line Items]
Depreciation average rates used	8.36%	5.40%	4.66%
Texas-New Mexico Power Company
Public Utilities, Inventory [Line Items]
Depreciation average rates used	3.57%	3.66%	3.65%
Inventory	$ 5,643	$ 8,626
Texas-New Mexico Power Company | Coal
Public Utilities, Inventory [Line Items]
Inventory	0	0
Texas-New Mexico Power Company | Materials and supplies
Public Utilities, Inventory [Line Items]
Inventory	$ 5,643	$ 8,626