Variable Interest Entities (Details)			12 Months Ended
	Feb. 01, 2016 USD ($)	Jan. 31, 2016 USD ($)	Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Results of Operations
Earnings attributable to non-controlling interest			$ 14,241,000	$ 15,112,000	$ 15,017,000
Financial Position
Current assets			294,010,000	302,524,000
Total assets			7,298,774,000	6,865,551,000	6,646,103,000
Current liabilities			967,481,000	512,453,000
Owners’ equity – non-controlling interest			63,052,000	64,212,000
Public Service Company of New Mexico
Results of Operations
Earnings attributable to non-controlling interest			14,241,000	15,112,000	15,017,000
Financial Position
Current assets			259,195,000	270,319,000
Total assets			5,242,991,000	5,035,883,000
Current liabilities			593,236,000	223,023,000
Owners’ equity – non-controlling interest			$ 63,052,000	64,212,000
Public Service Company of New Mexico | Valencia
Variable Interest Entity [Line Items]
Number of megawatts purchased (in megawatts) | MW			158
Payment for fixed charges			$ 19,900,000	19,600,000	19,600,000
Payment for variable charges			$ 1,200,000	1,400,000	1,300,000
Purchase price, percentage of the book value reduced by related indebtedness			50.00%
Purchase price, percentage of fair market value			50.00%
Results of Operations
Operating revenues			$ 21,073,000	21,025,000	20,887,000
Operating expenses			(6,832,000)	(5,913,000)	(5,870,000)
Earnings attributable to non-controlling interest			14,241,000	15,112,000	$ 15,017,000
Financial Position
Current assets			5,094,000	2,684,000
Net property, plant and equipment			58,581,000	62,066,000
Total assets			63,675,000	64,750,000
Current liabilities			623,000	538,000
Owners’ equity – non-controlling interest			$ 63,052,000	$ 64,212,000
Public Service Company of New Mexico | Valencia | Maximum
Variable Interest Entity [Line Items]
Option to purchase a percentage of the plant or VIE (up to )			50.00%
NM Capital | Coal Supply | San Juan Generating Station
Financial Position
Loan agreement	$ 125,000,000.0	$ 125,000,000.0
Issuance in letters of credit		$ 30,300,000	$ 30,300,000