Revenue from Contracts with Customers Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									$ 1,231,100	$ 1,187,200
Total revenues	$ 328,135	$ 274,836	$ 270,719	$ 384,220	$ 308,816	$ 279,874	$ 261,817	$ 341,502	1,257,910	1,192,009	$ 1,305,652
Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									526,700	503,000
Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									530,700	505,200
Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									44,600	43,800
Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									31,600	30,600
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									1,133,600	1,082,600
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									97,500	104,600
Montana | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									418,200	390,000
Montana | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									403,800	381,300
South Dakota | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									88,300	89,600
South Dakota | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									116,400	112,000
Nebraska | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									20,200	23,400
Nebraska | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									10,500	11,900
Regulatory Amortization [Member]
Disaggregation of Revenue [Line Items]
Regulatory amortization									26,800	4,800
Total Revenue [Member]
Disaggregation of Revenue [Line Items]
Total revenues									1,257,900	1,192,000
Electric
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									952,400	912,700
Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									371,300	351,500
Electric | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									445,200	423,600
Electric | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									43,600	42,600
Electric | Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									30,600	29,600
Electric | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									890,700	847,300
Electric | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									61,700	65,400
Electric | Montana | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									308,800	287,300
Electric | Montana | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									348,100	329,600
Electric | South Dakota | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									62,500	64,200
Electric | South Dakota | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									97,100	94,000
Electric | Nebraska | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									0	0
Electric | Nebraska | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									0	0
Electric | Regulatory Amortization [Member]
Disaggregation of Revenue [Line Items]
Regulatory amortization									28,800	8,400
Electric | Total Revenue [Member]
Disaggregation of Revenue [Line Items]
Total revenues									981,200	921,100
Natural gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									278,700	274,500
Natural gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									155,400	151,500
Natural gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									85,500	81,600
Natural gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									1,000	1,200
Natural gas | Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									1,000	1,000
Natural gas | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									242,900	235,300
Natural gas | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									35,800	39,200
Natural gas | Montana | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									109,400	102,700
Natural gas | Montana | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									55,700	51,700
Natural gas | South Dakota | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									25,800	25,400
Natural gas | South Dakota | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									19,300	18,000
Natural gas | Nebraska | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									20,200	23,400
Natural gas | Nebraska | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer									10,500	11,900
Natural gas | Regulatory Amortization [Member]
Disaggregation of Revenue [Line Items]
Regulatory amortization									(2,000)	(3,600)
Natural gas | Total Revenue [Member]
Disaggregation of Revenue [Line Items]
Total revenues									$ 276,700	$ 270,900