Segment Information - Summarized Financial Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			1
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	$ 422,666	$ 419,900	$ 1,092,857	$ 1,112,398
Utility margin	309,130	316,152	799,054	801,580
Other operating expenses	119,560	112,709	365,238	338,788
Depreciation and amortization	61,580	58,821	180,365	172,829
Operating income	127,990	144,622	253,451	289,963
Interest income	3,400	3,582	11,862	12,348
Other income (deductions)	3,574	4,972	3,214	14,984
Interest charges	(30,492)	(32,106)	(96,868)	(96,137)
Earnings before Income Taxes	104,472	121,070	171,659	221,158
Income taxes (benefit)	12,899	42,743	18,838	75,154
Net Earnings	91,573	78,327	152,821	146,004
Valencia non-controlling interest	(3,920)	(4,456)	(11,706)	(11,452)
Subsidiary preferred stock dividends	(132)	(132)	(396)	(396)
Net Earnings Available for PNM Common Stock	87,521	73,739	140,719	134,156
Total Assets	6,848,995	6,697,254	6,848,995	6,697,254	$ 6,646,103
Goodwill	278,297	278,297	278,297	278,297	$ 278,297
PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Utility margin	238,990	244,887	602,569	608,274
Other operating expenses	98,000	92,733	305,569	281,886
Depreciation and amortization	38,474	36,764	113,314	109,228
Operating income	102,516	115,390	183,686	217,160
Interest income	3,472	1,782	9,340	6,457
Other income (deductions)	2,515	4,204	588	13,510
Interest charges	(18,063)	(20,451)	(58,881)	(62,393)
Earnings before Income Taxes	90,440	100,925	134,733	174,734
Income taxes (benefit)	9,012	35,642	11,009	58,865
Net Earnings	81,428	65,283	123,724	115,869
Valencia non-controlling interest	(3,920)	(4,456)	(11,706)	(11,452)
Subsidiary preferred stock dividends	(132)	(132)	(396)	(396)
Net Earnings Available for PNM Common Stock	77,376	60,695	111,622	104,021
Total Assets	5,042,761	5,023,816	5,042,761	5,023,816
Goodwill	51,632	51,632	51,632	51,632
TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Utility margin	70,140	71,265	196,485	193,306
Other operating expenses	25,140	25,367	73,624	72,188
Depreciation and amortization	17,176	16,424	49,676	47,392
Operating income	27,824	29,474	73,185	73,726
Interest income	0	0	0	0
Other income (deductions)	1,151	1,228	3,067	2,392
Interest charges	(8,241)	(7,704)	(23,771)	(22,619)
Earnings before Income Taxes	20,734	22,998	52,481	53,499
Income taxes (benefit)	4,634	8,271	11,602	18,964
Net Earnings	16,100	14,727	40,879	34,535
Valencia non-controlling interest	0	0	0	0
Subsidiary preferred stock dividends	0	0	0	0
Net Earnings Available for PNM Common Stock	16,100	14,727	40,879	34,535
Total Assets	1,628,842	1,465,219	1,628,842	1,465,219
Goodwill	226,665	226,665	226,665	226,665
Corporate and Other
Segment Reporting Information, Profit (Loss) [Abstract]
Utility margin	0	0	0	0
Other operating expenses	(3,580)	(5,391)	(13,955)	(15,286)
Depreciation and amortization	5,930	5,633	17,375	16,209
Operating income	(2,350)	(242)	(3,420)	(923)
Interest income	(72)	1,800	2,522	5,891
Other income (deductions)	(92)	(460)	(441)	(918)
Interest charges	(4,188)	(3,951)	(14,216)	(11,125)
Earnings before Income Taxes	(6,702)	(2,853)	(15,555)	(7,075)
Income taxes (benefit)	(747)	(1,170)	(3,773)	(2,675)
Net Earnings	(5,955)	(1,683)	(11,782)	(4,400)
Valencia non-controlling interest	0	0	0	0
Subsidiary preferred stock dividends	0	0	0	0
Net Earnings Available for PNM Common Stock	(5,955)	(1,683)	(11,782)	(4,400)
Total Assets	177,392	208,219	177,392	208,219
Goodwill	0	0	0	0
Electricity
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	422,666	419,900	1,092,857	1,112,398
Cost of energy	113,536	103,748	293,803	310,818
Electricity | PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	331,374	327,254	832,116	854,909
Cost of energy	92,384	82,367	229,547	246,635
Electricity | TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	91,292	92,646	260,741	257,489
Cost of energy	21,152	21,381	64,256	64,183
Electricity | Corporate and Other
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	0	0	0	0
Cost of energy	$ 0	$ 0	$ 0	$ 0