Variable Interest Entities (Details) $ in Thousands		3 Months Ended
	Jan. 31, 2016 USD ($)	Mar. 31, 2017 USD ($) MW	Mar. 31, 2016 USD ($)	May 02, 2017 USD ($)	May 01, 2017 USD ($)	Apr. 24, 2017 USD ($)	Dec. 31, 2016 USD ($)
Results of Operations
Earnings attributable to non-controlling interest		$ 3,452	$ 3,287
Financial Position
Current assets		326,769					$ 378,039
Total assets		6,477,679	6,273,812				6,471,080
Current liabilities		962,136					805,108
Owners’ equity – non-controlling interest		68,344					68,920
Public Service Company of New Mexico
Results of Operations
Earnings attributable to non-controlling interest		3,452	3,287
Financial Position
Current assets		281,309					314,245
Total assets		4,870,201					4,867,546
Current liabilities		451,156					457,361
Owners’ equity – non-controlling interest		68,344					68,920
Public Service Company of New Mexico | Valencia
Variable Interest Entity [Line Items]
Payment for fixed costs		4,900	4,800
Payment for variable costs		$ 100	200
Long-term contract option to purchase, ownership percentage		50.00%
Long-term contract option to purchase, purchase price - percentage of adjusted NBV		50.00%
Long-term contract option to purchase, purchase price - percentage of FMV		50.00%
Results of Operations
Operating revenues		$ 4,927	4,937
Operating expenses		(1,475)	(1,650)
Earnings attributable to non-controlling interest		3,452	$ 3,287
Financial Position
Current assets		2,841					2,551
Net property, plant, and equipment		66,237					66,947
Total assets		69,078					69,498
Current liabilities		734					578
Owners’ equity – non-controlling interest		$ 68,344					$ 68,920
Public Service Company of New Mexico | Valencia | Purchased through May 2028
Variable Interest Entity [Line Items]
Number of megawatts purchased (in megawatts) | MW		158
NM Capital | San Juan Generating Station | Coal supply
Variable Interest Entity [Line Items]
Loan agreement among several entities	$ 125,000
Cash used to support bank letter or credit arrangement	30,300	$ 30,300
NM Capital | San Juan Coal Company, Westmoreland | Coal supply
Variable Interest Entity [Line Items]
Loan receivable	$ 125,000	85,400
Interest receivable		$ 1,400
NM Capital | San Juan Coal Company, Westmoreland | Coal supply | Subsequent event
Variable Interest Entity [Line Items]
Scheduled principal payment proceeds				$ 9,600	$ 9,600
Scheduled interest payment proceeds				$ 2,200	$ 2,200
Unaffiliated third party | San Juan Coal Company, Westmoreland | Coal supply | Subsequent event
Variable Interest Entity [Line Items]
Amount held in a restricted bank account						$ 11,800