Variable Interest Entities (Details)		3 Months Ended		9 Months Ended
	Feb. 01, 2016 USD ($)	Sep. 30, 2018 USD ($) MW	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) MW	Sep. 30, 2017 USD ($)	May 22, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 31, 2016 USD ($)
Results of Operations
Earnings attributable to non-controlling interest		$ 3,920,000	$ 4,456,000	$ 11,706,000	$ 11,452,000
Financial Position
Current assets		363,925,000		363,925,000			$ 294,420,000
Total assets		6,848,995,000	6,697,254,000	6,848,995,000	6,697,254,000		6,646,103,000
Current liabilities		980,475,000		980,475,000			835,644,000
Owners’ equity – non-controlling interest		65,224,000		65,224,000			66,195,000
PNM
Results of Operations
Earnings attributable to non-controlling interest		3,920,000	4,456,000	11,706,000	11,452,000
Financial Position
Current assets		311,338,000		311,338,000			243,631,000
Total assets		5,042,761,000		5,042,761,000			4,921,563,000
Current liabilities		353,096,000		353,096,000			218,496,000
Owners’ equity – non-controlling interest		65,224,000		65,224,000			66,195,000
PNM | Valencia
Variable Interest Entity [Line Items]
Payment for fixed costs		4,900,000	4,900,000	14,700,000	14,700,000
Payment for variable costs		500,000	900,000	$ 1,400,000	1,200,000
Long-term contract option to purchase, ownership percentage (up to)				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		5,368,000	5,859,000	$ 16,047,000	15,880,000
Operating expenses		(1,448,000)	(1,403,000)	(4,341,000)	(4,428,000)
Earnings attributable to non-controlling interest		3,920,000	$ 4,456,000	11,706,000	$ 11,452,000
Financial Position
Current assets		3,449,000		3,449,000			2,688,000
Net property, plant, and equipment		62,698,000		62,698,000			64,109,000
Total assets		66,147,000		66,147,000			66,797,000
Current liabilities		923,000		923,000			602,000
Owners’ equity – non-controlling interest		$ 65,224,000		$ 65,224,000			$ 66,195,000
PNM | Valencia | Purchased through May 2028
Variable Interest Entity [Line Items]
Number of megawatts purchased (in megawatts) | MW		158		158
NM Capital | San Juan Generating Station | Coal supply
Variable Interest Entity [Line Items]
Loan agreement among several entities	$ 125,000,000.0
Cash used to support bank letter or credit arrangement		$ 30,300,000		$ 30,300,000				$ 30,300,000
NM Capital | San Juan Coal Company, Westmoreland | Coal supply
Variable Interest Entity [Line Items]
Repayment of line of credit						$ 50,100,000