Revenue from Contracts with Customers Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			$ 1,459,400	$ 1,326,800	$ 1,206,800
Total revenues	$ 425,283	$ 347,341	1,477,837	1,372,316	1,198,670
Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			654,500	573,600	529,300
Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			606,700	554,400	513,100
Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			41,300	39,000	37,700
Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			32,900	33,500	32,700
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			1,335,400	1,200,500	1,112,800
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			124,000	126,300	94,000
Montana | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			509,700	460,600	424,300
Montana | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			447,900	421,400	389,600
South Dakota | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			109,000	92,000	88,100
South Dakota | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			136,700	121,600	115,400
Nebraska | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			35,800	21,000	16,900
Nebraska | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			22,100	11,400	8,100
Regulatory Amortization [Member]
Disaggregation of Revenue [Line Items]
Regulatory amortization			18,400	45,500	(8,100)
Total Revenue [Member]
Disaggregation of Revenue [Line Items]
Total revenues			1,477,800	1,372,300	1,198,700
Electric
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			1,060,500	1,018,700	953,900
Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			427,200	400,000	387,400
Electric | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			476,800	459,200	439,400
Electric | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			39,800	37,900	36,800
Electric | Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			31,000	32,100	31,800
Electric | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			974,800	929,200	895,400
Electric | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			85,700	89,500	58,500
Electric | Montana | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			357,400	334,600	320,800
Electric | Montana | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			368,600	356,700	338,300
Electric | South Dakota | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			69,800	65,400	66,600
Electric | South Dakota | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			108,200	102,500	101,100
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			46,100	33,500	(13,100)
Electric | Total Revenue [Member]
Disaggregation of Revenue [Line Items]
Total revenues			1,106,600	1,052,200	940,800
Natural gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			398,900	308,100	252,900
Natural gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			227,300	173,600	141,900
Natural gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			129,900	95,200	73,700
Natural gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			1,500	1,100	900
Natural gas | Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			1,900	1,400	900
Natural gas | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			360,600	271,300	217,400
Natural gas | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			38,300	36,800	35,500
Natural gas | Montana | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			152,300	126,000	103,500
Natural gas | Montana | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			79,300	64,700	51,300
Natural gas | South Dakota | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			39,200	26,600	21,500
Natural gas | South Dakota | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			28,500	19,100	14,300
Natural gas | Nebraska | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			35,800	21,000	16,900
Natural gas | Nebraska | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer			22,100	11,400	8,100
Natural gas | Regulatory Amortization [Member]
Disaggregation of Revenue [Line Items]
Regulatory amortization			(27,700)	12,000	5,000
Natural gas | Total Revenue [Member]
Disaggregation of Revenue [Line Items]
Total revenues			$ 371,200	$ 320,100	$ 257,900