Segment Information - Summarized Financial Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment	1
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	$ 349,645	$ 317,878
Utility margin	228,019	225,322
Other operating expenses	125,940	120,468
Depreciation and amortization	65,356	58,722
Operating income	36,723	46,132
Interest income	3,619	4,124
Other income (deductions)	14,177	2,381
Interest charges	(31,634)	(33,055)
Earnings before Income Taxes	22,885	19,582
Income taxes (benefit)	1,223	783
Net Earnings	21,662	18,799
Valencia non-controlling interest	(2,830)	(3,677)
Subsidiary preferred stock dividends	(132)	(132)
Net Earnings Available for PNM Common Stock	18,700	14,990
Total Assets	7,250,758	6,676,393	$ 6,865,551
Goodwill	278,297	278,297	$ 278,297
PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Utility margin	169,979	165,430
Other operating expenses	106,462	100,511
Depreciation and amortization	39,224	36,627
Operating income	24,293	28,292
Interest income	3,656	2,487
Other income (deductions)	14,358	1,217
Interest charges	(18,360)	(20,830)
Earnings before Income Taxes	23,947	11,166
Income taxes (benefit)	1,973	(348)
Net Earnings	21,974	11,514
Valencia non-controlling interest	(2,830)	(3,677)
Subsidiary preferred stock dividends	(132)	(132)
Net Earnings Available for PNM Common Stock	19,012	7,705
Total Assets	5,202,039	4,925,741
Goodwill	51,632	51,632
TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Utility margin	58,040	59,892
Other operating expenses	25,241	24,973
Depreciation and amortization	20,214	16,387
Operating income	12,585	18,532
Interest income	31	0
Other income (deductions)	554	1,085
Interest charges	(8,800)	(7,729)
Earnings before Income Taxes	4,370	11,888
Income taxes (benefit)	272	2,475
Net Earnings	4,098	9,413
Valencia non-controlling interest	0	0
Subsidiary preferred stock dividends	0	0
Net Earnings Available for PNM Common Stock	4,098	9,413
Total Assets	1,871,780	1,526,434
Goodwill	226,665	226,665
Corporate and Other
Segment Reporting Information, Profit (Loss) [Abstract]
Utility margin	0	0
Other operating expenses	(5,763)	(5,016)
Depreciation and amortization	5,918	5,708
Operating income	(155)	(692)
Interest income	(68)	1,637
Other income (deductions)	(735)	79
Interest charges	(4,474)	(4,496)
Earnings before Income Taxes	(5,432)	(3,472)
Income taxes (benefit)	(1,022)	(1,344)
Net Earnings	(4,410)	(2,128)
Valencia non-controlling interest	0	0
Subsidiary preferred stock dividends	0	0
Net Earnings Available for PNM Common Stock	(4,410)	(2,128)
Total Assets	176,939	224,218
Goodwill	0	0
Electricity
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	349,645	317,878
Cost of energy	121,626	92,556
Electricity | PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	269,318	236,232
Cost of energy	99,339	70,802
Electricity | TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	80,327	81,646
Cost of energy	22,287	21,754
Electricity | Corporate and Other
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	0	0
Cost of energy	$ 0	$ 0