Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 346,000	$ 371,900
Total Revenues	335,255	384,220
Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	163,800	180,400
Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	126,900	139,700
Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	29,200	31,100
Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7,700	9,600
Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	143,000	147,400
Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	105,200	109,700
Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	33,800	32,400
Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,000	5,300
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,100	12,100
Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,600	5,600
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	321,500	345,500
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	24,500	26,400
Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	(10,800)	12,300
Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	335,200	384,200
Electric Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	249,000	254,900
Total Revenues	244,625	273,037
Electric Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	107,500	112,100
Electric Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	88,600	94,100
Electric Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	18,900	18,000
Electric Domestic Regulated [Member] | Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Electric Domestic Regulated [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	112,500	109,900
Electric Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	86,000	86,700
Electric Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	26,500	23,200
Electric Domestic Regulated [Member] | Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Electric Domestic Regulated [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8,800	11,600
Electric Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,300	5,100
Electric Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	234,100	238,700
Electric Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	14,900	16,200
Electric Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	(4,400)	18,100
Electric Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	244,600	273,000
Gas Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	97,000	117,000
Total Revenues	90,630	111,183
Gas Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	56,300	68,300
Gas Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	38,300	45,600
Gas Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	10,300	13,100
Gas Domestic Regulated [Member] | Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7,700	9,600
Gas Domestic Regulated [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,500	37,500
Gas Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	19,200	23,000
Gas Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7,300	9,200
Gas Domestic Regulated [Member] | Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,000	5,300
Gas Domestic Regulated [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	300	500
Gas Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	300	500
Gas Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	87,400	106,800
Gas Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,600	10,200
Gas Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	(6,400)	(5,800)
Gas Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	$ 90,600	$ 111,200