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Jointly Owned Facilities Jointly Owned Facilities (Details 2) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2011 | Dec. 31, 2010
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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.

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