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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) (USD $) | 6 Months Ended
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Jun. 30, 2013 | Jun. 30, 2012
Great Northern Transmission Line [Member] |  |
Power Purchase Agreements [Line Items] |  |
Transmission Line Capacity (kV) | 500 |
Square Butte PPA [Member] |  |
Power Purchase Agreements [Line Items] |  |
PPA Counterparty Debt Outstanding | $ 404,900,000 |
Annual Debt Service | 44,000,000 |
Power Purchased under Long-term Contracts | 33,000,000 | 33,000,000
Interest Expense | 5,300,000 | 5,600,000
Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member] |  |
Power Purchase Agreements [Line Items] |  |
Generating Capacity (MW) | 455 |
Output Entitlement | 50.00% |
Minnkota PPA [Member] |  |
Power Purchase Agreements [Line Items] |  |
Output Being Purchased (MW) | 50 |
Oliver Wind I PPA [Member] |  |
Power Purchase Agreements [Line Items] |  |
Output Being Purchased (MW) | 50 |
Term of Contract (Years) | 25 |
Fixed Capacity Carges | 0 |
Oliver Wind II PPA [Member] |  |
Power Purchase Agreements [Line Items] |  |
Output Being Purchased (MW) | 48 |
Term of Contract (Years) | 25 |
Fixed Capacity Carges | 0 |
Manitoba Hydro PPA (expires April 2015) [Member] |  |
Power Purchase Agreements [Line Items] |  |
Output Being Purchased (MW) | 50 |
Manitoba Hydro PPA (expires April 2022) [Member] |  |
Power Purchase Agreements [Line Items] |  |
Output Being Purchased (MWh) | 1,000,000 |
Manitoba Hydro PPA (expires 2035) [Member] |  |
Power Purchase Agreements [Line Items] |  |
Output Being Purchased (MW) | 250 |
Term of Contract (Years) | 15 |
Minimum [Member] | Great Northern Transmission Line [Member] |  |
Power Purchase Agreements [Line Items] |  |
Estimated Capital Expenditures | 200,000,000 |
Maximum [Member] | Great Northern Transmission Line [Member] |  |
Power Purchase Agreements [Line Items] |  |
Estimated Capital Expenditures | $ 400,000,000 |

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- Definition Share of plant output being purchased in megawatts under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. + References No definition available. + Details
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- Definition Share of plant output being purchased in megawatthours under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. + References No definition available. + Details
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- Definition Term in years of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. + References No definition available. + Details
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- Definition Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.D) -URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section D + Details
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- Definition Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service. If the utility is obligated to pay certain minimum amounts which cover debt service requirements whether or not the plant is operating, the amount can be disclosed here. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.D) -URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section D + Details
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