Segment Information - Summarized Financial Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment	1
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	$ 333,622	$ 349,645
Other operating expenses	118,201	125,940
Depreciation and amortization	68,973	65,356
Operating income	47,738	36,723
Interest income	3,423	3,588
Other income (deductions)	(34,006)	14,208
Interest charges	(30,434)	(31,634)
Earnings (Loss) before Income Taxes	(13,279)	22,885
Income taxes (benefit)	(1,880)	1,223
Net Earnings (Loss)	(11,399)	21,662
Valencia non-controlling interest	(3,729)	(2,830)
Subsidiary preferred stock dividends	(132)	(132)
Net Earnings (Loss) Available for PNM Common Stock	(15,260)	18,700
Total Assets	7,375,119	7,250,758	$ 7,298,774
Goodwill	278,297	278,297	$ 278,297
PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	98,555	106,462
Depreciation and amortization	41,449	39,224
Operating income	33,605	24,293
Interest income	3,496	3,656
Other income (deductions)	(34,027)	14,358
Interest charges	(17,629)	(18,360)
Earnings (Loss) before Income Taxes	(14,555)	23,947
Income taxes (benefit)	(2,359)	1,973
Net Earnings (Loss)	(12,196)	21,974
Valencia non-controlling interest	(3,729)	(2,830)
Subsidiary preferred stock dividends	(132)	(132)
Net Earnings (Loss) Available for PNM Common Stock	(16,057)	19,012
Total Assets	5,260,528	5,202,039
Goodwill	51,632	51,632
TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	25,122	25,241
Depreciation and amortization	21,836	20,214
Operating income	14,345	12,585
Interest income	0	0
Other income (deductions)	561	585
Interest charges	(7,172)	(8,800)
Earnings (Loss) before Income Taxes	7,734	4,370
Income taxes (benefit)	642	272
Net Earnings (Loss)	7,092	4,098
Valencia non-controlling interest	0	0
Subsidiary preferred stock dividends	0	0
Net Earnings (Loss) Available for PNM Common Stock	7,092	4,098
Total Assets	1,913,270	1,871,780
Goodwill	226,665	226,665
Corporate and Other
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	(5,476)	(5,763)
Depreciation and amortization	5,688	5,918
Operating income	(212)	(155)
Interest income	(73)	(68)
Other income (deductions)	(540)	(735)
Interest charges	(5,633)	(4,474)
Earnings (Loss) before Income Taxes	(6,458)	(5,432)
Income taxes (benefit)	(163)	(1,022)
Net Earnings (Loss)	(6,295)	(4,410)
Valencia non-controlling interest	0	0
Subsidiary preferred stock dividends	0	0
Net Earnings (Loss) Available for PNM Common Stock	(6,295)	(4,410)
Total Assets	201,321	176,939
Goodwill	0	0
Cost of energy
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	333,622	349,645
Cost of energy	98,710	121,626
Utility margin	234,912	228,019
Cost of energy | PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	248,133	269,318
Cost of energy	74,524	99,339
Utility margin	173,609	169,979
Cost of energy | TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	85,489	80,327
Cost of energy	24,186	22,287
Utility margin	61,303	58,040
Cost of energy | Corporate and Other
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	0	0
Cost of energy	0	0
Utility margin	$ 0	$ 0