Revenue from Contracts with Customers (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 341,502,000	$ 367,312,000
Residential
Disaggregation of Revenue [Line Items]
Revenues	169,600,000	171,700,000
Residential | Montana
Disaggregation of Revenue [Line Items]
Revenues	128,100,000	134,600,000
Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenues	30,100,000	28,100,000
Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenues	11,400,000	9,000,000
Commercial
Disaggregation of Revenue [Line Items]
Revenues	142,200,000	144,700,000
Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenues	104,200,000	110,000,000
Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenues	31,900,000	29,800,000
Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenues	6,100,000	4,900,000
Industrial
Disaggregation of Revenue [Line Items]
Revenues	11,300,000	11,400,000
Other
Disaggregation of Revenue [Line Items]
Revenues	5,500,000	5,700,000
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenues	328,600,000	333,500,000
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenues	28,100,000	48,500,000
Revenue from Contract with Customer
Disaggregation of Revenue [Line Items]
Revenues	356,700,000	382,000,000
Regulatory amortization
Disaggregation of Revenue [Line Items]
Revenues	(15,200,000)	(14,700,000)
Total Revenues
Disaggregation of Revenue [Line Items]
Revenues	341,500,000	367,300,000
Electric Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenues	238,342,000	266,239,000
Electric Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenues	105,900,000	108,100,000
Electric Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenues	87,200,000	90,800,000
Electric Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenues	18,700,000	17,300,000
Electric Domestic Regulated [Member] | Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenues	0	0
Electric Domestic Regulated [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenues	107,600,000	110,500,000
Electric Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenues	83,600,000	88,100,000
Electric Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenues	24,000,000	22,400,000
Electric Domestic Regulated [Member] | Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenues	0	0
Electric Domestic Regulated [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenues	10,800,000	10,900,000
Electric Domestic Regulated [Member] | Other
Disaggregation of Revenue [Line Items]
Revenues	5,000,000	5,200,000
Electric Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenues	229,300,000	234,700,000
Electric Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenues	17,800,000	37,400,000
Electric Domestic Regulated [Member] | Revenue from Contract with Customer
Disaggregation of Revenue [Line Items]
Revenues	247,100,000	272,100,000
Electric Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Revenues	(8,800,000)	(5,900,000)
Electric Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Revenues	238,300,000	266,200,000
Gas Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenues	103,160,000	101,073,000
Gas Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenues	63,700,000	63,600,000
Gas Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenues	40,900,000	43,800,000
Gas Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenues	11,400,000	10,800,000
Gas Domestic Regulated [Member] | Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenues	11,400,000	9,000,000
Gas Domestic Regulated [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenues	34,600,000	34,200,000
Gas Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenues	20,600,000	21,900,000
Gas Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenues	7,900,000	7,400,000
Gas Domestic Regulated [Member] | Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenues	6,100,000	4,900,000
Gas Domestic Regulated [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenues	500,000	500,000
Gas Domestic Regulated [Member] | Other
Disaggregation of Revenue [Line Items]
Revenues	500,000	500,000
Gas Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenues	99,300,000	98,800,000
Gas Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenues	10,300,000	11,100,000
Gas Domestic Regulated [Member] | Revenue from Contract with Customer
Disaggregation of Revenue [Line Items]
Revenues	109,600,000	109,900,000
Gas Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Revenues	(6,400,000)	(8,800,000)
Gas Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Revenues	$ 103,200,000	$ 101,100,000