Regulatory Assets and Liabilities Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ (28,842)	$ 39,957	$ (7,034)
Electric Supply Costs | South Dakota
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	6.80%
Natural Gas Supply Costs | Montana
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	6.70%
Natural Gas Supply Costs | South Dakota
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	6.90%
Natural Gas Supply Costs | Nebraska
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	8.50%
Regulatory Assets [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$ 804,265	773,571
Regulatory Assets [Member] | Flow-through income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	596,265	553,452
Regulatory Assets [Member] | Wildfire mitigation
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	17,368	1,623
Regulatory Liabilities [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory Liabilities	693,203	718,555
Regulatory Liabilities [Member] | State & local taxes & fees
Regulatory Assets And Liabilities [Line Items]
Regulatory Liabilities	$ 251	$ 30,576