Jointly Owned Facilities Cost of utility plant in service (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Big Stone Station:
Public utility, property, plant and equipment [Line Items]
Utility plant in service	$ 63,146	$ 63,715
Less accumulated depreciation	40,859	42,475
Utility plant in services net	22,287	21,240
Coyote Station:
Public utility, property, plant and equipment [Line Items]
Utility plant in service	135,073	131,719
Less accumulated depreciation	87,524	86,788
Utility plant in services net	47,549	44,931
Wygen III:
Public utility, property, plant and equipment [Line Items]
Utility plant in service	63,462	63,300
Less accumulated depreciation	3,368	2,106
Utility plant in services net	$ 60,094	$ 61,194