Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total Revenues	$ 384,220	$ 341,502
Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	180,400	169,600
Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	139,700	128,100
Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	31,100	30,100
Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,600	11,400
Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	147,400	142,200
Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	109,700	104,200
Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	32,400	31,900
Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,300	6,100
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	12,100	11,300
Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,600	5,500
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	345,500	328,600
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	26,400	28,100
Revenue from Contract with Customer
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	371,900	356,700
Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	12,300	(15,200)
Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	384,200	341,500
Electric Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Total Revenues	273,037	238,342
Electric Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	112,100	105,900
Electric Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	94,100	87,200
Electric Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	18,000	18,700
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	109,900	107,600
Electric Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	86,700	83,600
Electric Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	23,200	24,000
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	11,600	10,800
Electric Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,100	5,000
Electric Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	238,700	229,300
Electric Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	16,200	17,800
Electric Domestic Regulated [Member] | Revenue from Contract with Customer
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	254,900	247,100
Electric Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	18,100	(8,800)
Electric Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	273,000	238,300
Gas Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Total Revenues	111,183	103,160
Gas Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	68,300	63,700
Gas Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	45,600	40,900
Gas Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	13,100	11,400
Gas Domestic Regulated [Member] | Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,600	11,400
Gas Domestic Regulated [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	37,500	34,600
Gas Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	23,000	20,600
Gas Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,200	7,900
Gas Domestic Regulated [Member] | Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,300	6,100
Gas Domestic Regulated [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	500	500
Gas Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	500	500
Gas Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	106,800	99,300
Gas Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	10,200	10,300
Gas Domestic Regulated [Member] | Revenue from Contract with Customer
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	117,000	109,600
Gas Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	(5,800)	(6,400)
Gas Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	$ 111,200	$ 103,200