Revenue from Contracts with Customers Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Total revenues	$ 373,466	$ 356,009	$ 1,513,898	$ 1,422,143	$ 1,477,837
Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			627,100	684,400	654,500
Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			610,200	656,300	606,700
Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			47,600	47,400	41,300
Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			34,200	34,400	32,900
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			1,319,100	1,422,500	1,335,400
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			43,900	(130,600)	18,300
Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			97,100	78,400	77,800
Montana | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			509,000	544,400	509,700
Montana | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			468,900	505,100	447,900
South Dakota | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			96,900	104,500	109,000
South Dakota | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			129,900	129,100	136,700
Nebraska | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			21,200	35,500	35,800
Nebraska | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			11,400	22,100	22,100
Regulatory Amortization [Member]
Disaggregation of Revenue [Line Items]
Regulatory amortization			53,800	51,800	46,300
Total Revenue [Member]
Disaggregation of Revenue [Line Items]
Total revenues			1,513,900	1,422,100	1,477,800
Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			468,800	476,200	427,200
Electric | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			520,800	534,600	476,800
Electric | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			46,600	46,000	39,800
Electric | Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			32,800	32,700	31,000
Electric | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			1,069,000	1,089,500	974,800
Electric | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			24,900	(105,600)	46,400
Electric | Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			97,100	78,400	77,800
Electric | Montana | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			398,800	408,300	357,400
Electric | Montana | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			409,000	431,400	368,600
Electric | South Dakota | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			70,000	67,900	69,800
Electric | South Dakota | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			111,800	103,200	108,200
Electric | Nebraska | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			0	0	0
Electric | Nebraska | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			0	0	0
Electric | Regulatory Amortization [Member]
Disaggregation of Revenue [Line Items]
Regulatory amortization			9,700	6,500	7,600
Electric | Total Revenue [Member]
Disaggregation of Revenue [Line Items]
Total revenues			1,200,700	1,068,800	1,106,600
Natural gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			158,300	208,200	227,300
Natural gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			89,400	121,700	129,900
Natural gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			1,000	1,400	1,500
Natural gas | Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			1,400	1,700	1,900
Natural gas | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			250,100	333,000	360,600
Natural gas | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			19,000	(25,000)	(28,100)
Natural gas | Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			0	0	0
Natural gas | Montana | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			110,200	136,100	152,300
Natural gas | Montana | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			59,900	73,700	79,300
Natural gas | South Dakota | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			26,900	36,600	39,200
Natural gas | South Dakota | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			18,100	25,900	28,500
Natural gas | Nebraska | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			21,200	35,500	35,800
Natural gas | Nebraska | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			11,400	22,100	22,100
Natural gas | Regulatory Amortization [Member]
Disaggregation of Revenue [Line Items]
Regulatory amortization			44,100	45,300	38,700
Natural gas | Total Revenue [Member]
Disaggregation of Revenue [Line Items]
Total revenues			$ 313,200	$ 353,300	$ 371,200