Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 279,000	$ 264,700	$ 890,800	$ 903,100
Total Revenues	280,610	274,836	885,225	929,775
Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	110,700	96,800	385,400	382,200
Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	88,400	77,300	303,400	298,700
Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	20,600	17,700	69,100	67,800
Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,700	1,800	12,900	15,700
Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	123,800	121,700	380,000	388,900
Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	94,700	93,200	285,500	295,300
Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	28,400	27,600	88,300	85,300
Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	700	900	6,200	8,300
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,300	10,700	27,700	33,100
Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	12,000	12,500	27,100	25,800
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	255,800	241,700	820,200	830,000
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	23,200	23,000	70,600	73,100
Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	1,700	10,100	(5,600)	26,600
Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	280,700	274,800	885,200	929,700
Electric Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	249,900	236,700	717,400	705,400
Total Revenues	244,155	241,237	706,718	733,933
Electric Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	97,400	84,400	290,100	272,800
Electric Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	78,500	68,400	237,700	225,400
Electric Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	18,900	16,000	52,400	47,400
Electric Domestic Regulated [Member] | Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Electric Domestic Regulated [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	116,500	114,000	329,600	328,500
Electric Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	89,100	87,700	252,500	257,300
Electric Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	27,400	26,300	77,100	71,200
Electric Domestic Regulated [Member] | Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Electric Domestic Regulated [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,200	10,600	27,200	32,400
Electric Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	11,900	12,400	26,400	25,200
Electric Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	235,000	221,400	673,300	658,900
Electric Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	14,900	15,300	44,100	46,500
Electric Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	(5,700)	4,500	(10,700)	28,500
Electric Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	244,200	241,200	706,700	733,900
Gas Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	29,100	28,000	173,400	197,700
Total Revenues	36,455	33,599	178,507	195,842
Gas Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	13,300	12,400	95,300	109,400
Gas Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,900	8,900	65,700	73,300
Gas Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,700	1,700	16,700	20,400
Gas Domestic Regulated [Member] | Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,700	1,800	12,900	15,700
Gas Domestic Regulated [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7,300	7,700	50,400	60,400
Gas Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,600	5,500	33,000	38,000
Gas Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,000	1,300	11,200	14,100
Gas Domestic Regulated [Member] | Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	700	900	6,200	8,300
Gas Domestic Regulated [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	100	100	500	700
Gas Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	100	100	700	600
Gas Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	20,800	20,300	146,900	171,100
Gas Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8,300	7,700	26,500	26,600
Gas Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	7,400	5,600	5,100	(1,900)
Gas Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	$ 36,500	$ 33,600	$ 178,500	$ 195,800