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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

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