Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance			$ 1,761,448
Balance, as adjusted					$ 1,761,448
Amounts reclassified from AOCI (pre-tax)			2,297	$ (5,765)
Income tax impact of amounts reclassified			(584)	2,237
Other OCI changes (pre-tax)			5,303	22,024
Income tax impact of other OCI changes			(1,348)	(8,546)
Total Other Comprehensive Income	$ 2,277	$ 3,094	5,668	9,950
Ending balance	1,836,522		1,836,522
Fair Value Adjustment for Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance			1,153	(23)
Cumulative effect adjustment (Note 7)					0
Balance, as adjusted					1,153
Amounts reclassified from AOCI (pre-tax)			102	484
Income tax impact of amounts reclassified			(27)	(187)
Other OCI changes (pre-tax)			2,431	(278)
Income tax impact of other OCI changes			(618)	108
Total Other Comprehensive Income			1,888	127
Ending balance	3,041	104	3,041	104
AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance			(95,940)	(92,451)
Cumulative effect adjustment (Note 7)					(11,208)
Balance, as adjusted					(107,148)
Total Other Comprehensive Income			5,668
Ending balance	(101,480)	(82,501)	(101,480)	(82,501)
PNM
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance			1,488,369
Balance, as adjusted					1,488,369
Amounts reclassified from AOCI (pre-tax)			2,195	(6,249)
Income tax impact of amounts reclassified			(557)	2,424
Other OCI changes (pre-tax)			2,872	22,302
Income tax impact of other OCI changes			(730)	(8,654)
Total Other Comprehensive Income	2,188	2,989	3,780	9,823
Ending balance	1,602,800		1,602,800
PNM | Unrealized Gains on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance			13,169	4,320
Cumulative effect adjustment (Note 7)					(11,208)
Balance, as adjusted					1,961
Amounts reclassified from AOCI (pre-tax)			(3,483)	(11,088)
Income tax impact of amounts reclassified			885	4,302
Other OCI changes (pre-tax)			2,872	22,302
Income tax impact of other OCI changes			(730)	(8,654)
Total Other Comprehensive Income			(456)	6,862
Ending balance	1,505	11,182	1,505	11,182
PNM | Pension Liability Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance			(110,262)	(96,748)
Cumulative effect adjustment (Note 7)					0
Balance, as adjusted					(110,262)
Amounts reclassified from AOCI (pre-tax)			5,678	4,839
Income tax impact of amounts reclassified			(1,442)	(1,878)
Total Other Comprehensive Income			4,236	2,961
Ending balance	(106,026)	(93,787)	(106,026)	(93,787)
PNM | AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance			(97,093)	(92,428)
Cumulative effect adjustment (Note 7)					(11,208)
Balance, as adjusted					$ (108,301)
Total Other Comprehensive Income			3,780
Ending balance	$ (104,521)	$ (82,605)	$ (104,521)	$ (82,605)