Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 307,300	$ 310,300	$ 1,039,700	$ 978,200
Total Revenues	335,068	325,955	1,052,554	1,024,975
Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	120,600	118,900	457,300	418,100
Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	96,000	95,400	344,600	333,800
Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	21,400	21,000	86,700	69,700
Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3,200	2,500	26,000	14,600
Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	133,200	133,400	434,500	406,800
Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	99,700	101,300	312,300	309,500
Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	31,200	30,600	106,200	89,600
Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,300	1,500	16,000	7,700
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,800	9,200	29,300	28,800
Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	12,800	13,200	26,800	27,800
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	276,400	274,700	947,900	881,500
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,900	35,600	91,800	96,700
Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	27,800	15,700	12,900	46,800
Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	335,100	326,000	1,052,600	1,025,000
Electric Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	270,500	277,500	772,300	770,100
Electric Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	104,200	104,300	307,900	302,400
Electric Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	85,200	85,500	252,900	251,400
Electric Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	19,000	18,800	55,000	51,000
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	121,700	124,000	346,600	343,600
Electric Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	92,600	95,200	263,400	266,600
Electric Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	29,100	28,800	83,200	77,000
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	9,700	9,200	28,400	28,100
Electric Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	12,600	13,100	25,400	26,800
Electric Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	248,200	250,600	708,300	700,900
Electric Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	22,300	26,900	64,000	69,200
Electric Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	21,800	10,000	35,100	28,900
Electric Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	292,300	287,500	807,400	799,000
Gas Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	36,800	32,800	267,400	208,100
Gas Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	16,400	14,600	149,400	115,700
Gas Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	10,800	9,900	91,700	82,400
Gas Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,400	2,200	31,700	18,700
Gas Domestic Regulated [Member] | Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3,200	2,500	26,000	14,600
Gas Domestic Regulated [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	11,500	9,400	87,900	63,200
Gas Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7,100	6,100	48,900	42,900
Gas Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,100	1,800	23,000	12,600
Gas Domestic Regulated [Member] | Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,300	1,500	16,000	7,700
Gas Domestic Regulated [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	100	0	900	700
Gas Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	200	100	1,400	1,000
Gas Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	28,200	24,100	239,600	180,600
Gas Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8,600	8,700	27,800	27,500
Gas Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	6,000	5,700	(22,200)	17,900
Gas Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	$ 42,800	$ 38,500	$ 245,200	$ 226,000