Percentage of ownership interests (Details)	Dec. 31, 2012
Big Stone Station:
Jointly owned utility plant interests [Line Items]
Company's percentage ownership interests in the assets, liabilities and expenses of the joint ventures or affiliates.	22.70%
Coyote Station:
Jointly owned utility plant interests [Line Items]
Company's percentage ownership interests in the assets, liabilities and expenses of the joint ventures or affiliates.	25.00%
Wygen III:
Jointly owned utility plant interests [Line Items]
Company's percentage ownership interests in the assets, liabilities and expenses of the joint ventures or affiliates.	25.00%