Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
Segment Information (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Segment Reporting Information [Line Items]
Operating revenues	$ 321,090	$ 335,068	$ 1,066,134	$ 1,052,554
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	88,943	108,920	322,013	338,994
Utility margin	232,147	226,148	744,121	713,560
Operating and maintenance	53,240	54,654	163,941	160,785
Administrative and general	29,355	28,146	94,058	87,010
Property and other taxes	41,763	46,466	131,043	140,209
Depreciation and depletion	52,159	48,588	157,787	145,705
Operating income (loss)	55,630	48,294	197,292	179,851
Interest expense, net	(28,725)	(25,332)	(85,144)	(73,081)
Other income (expense), net	4,127	4,157	12,926	11,791
Income tax (expense) benefit	1,697	(249)	14,085	2,297
Net Income	29,335	27,368	110,989	116,264
Total assets	7,429,499	7,114,213	7,429,499	7,114,213	$ 7,317,783
Capital expenditures	157,163	151,901	420,525	386,339
Operating Segments \| Electricity, US Regulated
Segment Reporting Information [Line Items]
Operating revenues	280,030	292,270	804,604	807,415
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	77,995	95,553	198,492	230,872
Utility margin	202,035	196,717	606,112	576,543
Operating and maintenance	39,990	40,914	123,771	121,237
Administrative and general	20,682	20,739	67,285	63,591
Property and other taxes	33,740	36,353	103,013	109,204
Depreciation and depletion	43,230	40,647	130,447	121,256
Operating income (loss)	64,393	58,064	181,596	161,255
Interest expense, net	(21,300)	(18,225)	(61,584)	(56,031)
Other income (expense), net	3,380	2,944	9,700	7,245
Income tax (expense) benefit	3,223	1,006	13,366	2,790
Net Income	43,250	41,777	116,346	109,679
Total assets	5,963,950	5,741,879	5,963,950	5,741,879
Capital expenditures	110,804	122,522	326,313	312,804
Operating Segments \| Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Operating revenues	41,060	42,798	261,530	245,139
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	10,948	13,367	123,521	108,122
Utility margin	30,112	29,431	138,009	137,017
Operating and maintenance	13,250	13,740	40,170	39,548
Administrative and general	8,249	7,934	26,336	23,757
Property and other taxes	9,574	10,110	29,576	30,998
Depreciation and depletion	8,929	7,941	27,340	24,449
Operating income (loss)	(9,890)	(10,294)	14,587	18,265
Interest expense, net	(4,426)	(3,238)	(12,167)	(9,951)
Other income (expense), net	1,328	1,727	3,887	4,669
Income tax (expense) benefit	41	(1,119)	180	1,263
Net Income	(13,029)	(10,686)	6,127	11,720
Total assets	1,454,445	1,365,896	1,454,445	1,365,896
Capital expenditures	46,359	29,379	94,212	73,535
Operating Segments \| Product and Service, Other
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	0	0	0	0
Utility margin	0	0	0	0
Operating and maintenance	0	0	0	0
Administrative and general	424	(527)	437	(338)
Property and other taxes	(1,551)	3	(1,546)	7
Depreciation and depletion	0	0	0	0
Operating income (loss)	1,127	524	1,109	331
Interest expense, net	(2,999)	(3,869)	(11,393)	(7,099)
Other income (expense), net	(581)	(514)	(661)	(123)
Income tax (expense) benefit	1,567	(136)	539	(1,756)
Net Income	(886)	(3,723)	(11,484)	(5,135)
Total assets	11,104	6,438	11,104	6,438
Capital expenditures	0	0	0	0
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	0	0	0	0
Utility margin	0	0	0	0
Operating and maintenance	0	0	0	0
Administrative and general	0	0	0	0
Property and other taxes	0	0	0	0
Depreciation and depletion	0	0	0	0
Operating income (loss)	0	0	0	0
Interest expense, net	0	0	0	0
Other income (expense), net	0	0	0	0
Income tax (expense) benefit	0	0	0	0
Net Income	0	0	0	0
Total assets	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0

NORTHWESTERN CORP 10-Q