Regulatory Assets and Liabilities Narrative (Details)	12 Months Ended
	Dec. 31, 2018
Electric Supply Costs | South Dakota
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	7.20%
Natural Gas Supply Costs | Montana
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	7.00%
Natural Gas Supply Costs | South Dakota
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	7.80%
Natural Gas Supply Costs | Nebraska
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	8.50%