Schedule I - Condensed Financial Information of Parent Company - Statements of Earnings (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Operating Expenses									$ 1,138,518	$ 1,084,966	$ 1,314,732
Operating income	$ 22,936	$ 142,484	$ 85,105	$ 55,960	$ 63,584	$ 108,071	$ 64,822	$ 41,508	306,485	277,985	124,350
Other Income and Deductions:
Other income									19,515	17,796	26,833
Net other income and deductions									46,899	45,822	36,663
Interest Charges									127,625	128,633	114,860
Earnings before Income Taxes									225,759	195,174	46,153
Income Taxes									130,340	63,278	15,075
Net Earnings Attributable to Company	$ (54,282)	$ 73,739	$ 37,555	$ 22,862	$ 24,809	$ 54,418	$ 27,076	$ 10,546	79,874	116,849	15,640
PNM Resources
Condensed Financial Statements, Captions [Line Items]
Operating Revenues									0	0	0
Operating Expenses									2,902	2,871	1,221
Operating income									(2,902)	(2,871)	(1,221)
Other Income and Deductions:
Equity in earnings of subsidiaries									111,877	122,252	27,352
Other income									1,181	1,711	747
Net other income and deductions									113,058	123,963	28,099
Interest Charges									12,490	8,102	8,275
Earnings before Income Taxes									97,666	112,990	18,603
Income Taxes									17,792	(3,859)	2,963
Net Earnings Attributable to Company									$ 79,874	$ 116,849	$ 15,640