Summary of the Business and Significant Accounting Policies - Summarized Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Results of Operations
Operating revenues	$ 344,144	$ 433,586	$ 330,228	$ 349,645	$ 343,756	$ 422,666	$ 352,313	$ 317,878	$ 1,457,603	$ 1,436,613	$ 1,445,003
Net Earnings Attributable to PNMR	31,805	$ 102,771	$ (75,914)	$ 18,700	(55,077)	$ 87,521	$ 38,208	$ 14,990
Financial Position
Net property, plant, and equipment	161,106				194,427				161,106	194,427
Total assets	7,298,774				6,865,551				7,298,774	6,865,551	6,646,103
Total PNMR common stockholders’ equity	1,678,698				1,688,382				1,678,698	1,688,382
NMRD
Results of Operations
Operating revenues									3,662	3,147	100
Operating expenses									2,971	2,136	100
Net Earnings Attributable to PNMR									691	1,011	$ 100
Financial Position
Current assets	7,187				2,581				7,187	2,581
Net property, plant, and equipment	132,772				50,784				132,772	50,784
Total assets	139,959				53,365				139,959	53,365
Current liabilities	9,640				237				9,640	237
Total PNMR common stockholders’ equity	$ 130,319				$ 53,128				$ 130,319	$ 53,128