Regulatory Assets and Liabilities Narrative (Details)	12 Months Ended
	Dec. 31, 2012
State & local taxes & fees [Member]
Regulatory Assets And Liabilities [Line Items]
Percentage of estimated increase in local taxes and fees authorized for recovery by MPSC	60.00%
Electric Supply Costs [Member] | South Dakota
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	10.60%
Natural Gas Supply Costs [Member] | South Dakota
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	7.79%
Natural Gas Supply Costs [Member] | Nebraska
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	8.49%
Supply costs [Member] | Montana
Regulatory Assets And Liabilities [Line Items]
Percentage of interest earned on electric and natural gas supply costs	7.92%
Regulatory Assets [Member]
Regulatory Assets And Liabilities [Line Items]
Required percentage of disclosure coverage of regulatory assets and liabilites	98.00%
Regulatory Liabilities [Member]
Regulatory Assets And Liabilities [Line Items]
Required percentage of disclosure coverage of regulatory assets and liabilites	92.00%