Jointly Owned Facilities Jointly Owned Facilities (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Big Stone Station [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Utility plant in service	$ 63,715		$ 60,404
Less accumulated depreciation	42,475		41,136
Utility plant in services net	21,240		19,268
Coyote Station [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Utility plant in service	131,719		131,395
Less accumulated depreciation	86,788		84,710
Utility plant in services net	44,931		46,685
Wygen III [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Utility plant in service	63,300	[1]	63,215	[1]
Less accumulated depreciation	2,106	[1]	838	[1]
Utility plant in services net	$ 61,194	[1]	$ 62,377	[1]

[1] Began commercial operation on April 1, 2010.