Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 265,800	$ 266,700	$ 611,900	$ 638,600
Total Revenues	269,360	270,719	604,615	654,939
Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	110,900	104,900	274,700	285,300
Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	88,100	81,700	215,000	221,400
Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	19,300	19,000	48,500	50,100
Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3,500	4,200	11,200	13,800
Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	113,200	119,800	256,200	267,200
Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	85,600	92,400	190,800	202,100
Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	26,100	25,300	59,900	57,700
Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,500	2,100	5,500	7,400
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,300	10,300	18,400	22,400
Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,500	8,000	15,100	13,600
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	242,900	243,000	564,400	588,500
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	22,900	23,700	47,500	50,100
Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	3,500	4,100	(7,300)	16,300
Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	269,300	270,800	604,600	654,900
Electric Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	218,500	213,900	467,500	468,800
Total Revenues	217,938	219,659	462,563	492,696
Electric Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	85,200	76,300	192,700	188,400
Electric Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	70,600	62,900	159,200	157,000
Electric Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	14,600	13,400	33,500	31,400
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	100,600	104,600	213,100	214,500
Electric Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	77,400	82,900	163,400	169,600
Electric Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	23,200	21,700	49,700	44,900
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	10,200	18,000	21,800
Electric Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,200	7,800	14,500	12,900
Electric Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	204,200	198,900	438,300	437,600
Electric Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	14,300	15,000	29,200	31,200
Electric Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	(600)	5,800	(5,000)	23,900
Electric Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	217,900	219,700	462,500	492,700
Gas Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	47,300	52,800	144,400	169,800
Total Revenues	51,422	51,060	142,052	162,243
Gas Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	25,700	28,600	82,000	96,900
Gas Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	17,500	18,800	55,800	64,400
Gas Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,700	5,600	15,000	18,700
Gas Domestic Regulated [Member] | Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3,500	4,200	11,200	13,800
Gas Domestic Regulated [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	12,600	15,200	43,100	52,700
Gas Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8,200	9,500	27,400	32,500
Gas Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,900	3,600	10,200	12,800
Gas Domestic Regulated [Member] | Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,500	2,100	5,500	7,400
Gas Domestic Regulated [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	100	100	400	600
Gas Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	300	200	600	700
Gas Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	38,700	44,100	126,100	150,900
Gas Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8,600	8,700	18,300	18,900
Gas Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	4,100	(1,700)	(2,300)	(7,600)
Gas Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	$ 51,400	$ 51,100	$ 142,100	$ 162,200