Jointly Owned Facilities (Details)	Dec. 31, 2011
Big Stone Station [Member]
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 [Member]
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 [Member]
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%