Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Disaggregation of Revenue [Line Items]
Total Revenues	$ 342,713	$ 319,929	$ 809,343	$ 795,271
Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	126,100	130,000	343,600	339,400
Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	99,800	104,900	266,200	270,900
Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	21,800	21,300	59,700	54,200
Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,500	3,800	17,700	14,300
Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	138,400	142,400	310,100	312,300
Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	104,300	110,400	228,100	237,000
Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	31,700	30,000	72,200	67,100
Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,400	2,000	9,800	8,200
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	10,000	11,500	20,600	23,600
Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,700	8,900	14,800	14,200
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	284,200	292,800	689,100	689,500
Regulatory amortization
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	15,500	(7,200)	33,800	36,100
Transmission customers
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	28,100	22,400	54,700	44,800
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Regulatory amortization	14,900	11,900	31,700	24,900
Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	342,700	319,900	809,300	795,300
Electric Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	98,000	101,400	235,300	238,100
Electric Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	81,800	86,000	196,800	203,400
Electric Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	16,200	15,400	38,500	34,700
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	126,000	248,000	255,300
Electric Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	93,900	99,700	190,900	201,200
Electric Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	27,800	26,300	57,100	54,100
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	9,900	11,300	20,000	23,000
Electric Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,400	8,600	14,000	13,300
Electric Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	239,000	247,300	517,300	529,700
Electric Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	10,300	(10,900)	38,000	25,500
Electric Domestic Regulated [Member] | Transmission customers
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	28,100	22,400	54,700	44,800
Electric Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Regulatory amortization	2,100	1,300	5,000	3,300
Electric Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	279,500	260,100	615,000	603,300
Gas Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	28,100	28,600	108,300	101,300
Gas Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	18,000	18,900	69,400	67,500
Gas Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,600	5,900	21,200	19,500
Gas Domestic Regulated [Member] | Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,500	3,800	17,700	14,300
Gas Domestic Regulated [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	16,700	16,400	62,100	57,000
Gas Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	10,400	10,700	37,200	35,800
Gas Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3,900	3,700	15,100	13,000
Gas Domestic Regulated [Member] | Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,400	2,000	9,800	8,200
Gas Domestic Regulated [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	100	200	600	600
Gas Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	300	300	800	900
Gas Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	45,200	45,500	171,800	159,800
Gas Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,200	3,700	(4,200)	10,600
Gas Domestic Regulated [Member] | Transmission customers
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Gas Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Regulatory amortization	12,800	10,600	26,700	21,600
Gas Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	$ 63,200	$ 59,800	$ 194,300	$ 192,000