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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Power Purchase Agreements [Line Items] |  |  |
Long-term Contract for Purchase of Electric Power, Number of Wind PPAs | 2 |  |
Interest Expense | $ 43.6 | $ 39.2 | $ 33.8
Minnkota Power Sales Agreement [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration | 2025 |  |
Long-term Contract for Purchase of Electric Power, Anticipated Date of New Transmission Line | 2013 |  |
Oliver Wind I PPA [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW) | 50 |  |
Long-term Contract for Purchase of Electric Power, Start Date of Contract | 2006 |  |
Oliver Wind II PPA [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW) | 48 |  |
Long-term Contract for Purchase of Electric Power, Start Date of Contract | 2007 |  |
Oliver Wind I and II PPA [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Long-term Contract for Purchase of Electric Power, Term of Contract (Years) | 25 |  |
fixed capacity charge | 0 |  |
Manitoba Hydro PPA (expires April 2015) [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW) | 50 |  |
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration | April 2015 |  |
Manitoba Hydro PPA (expires April 2022) [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Long-term Contract for Purchase of Electric Power, Start Date of Contract | May 2011 |  |
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration | April 2022 |  |
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MWh) | 1,000,000 |  |
Long-term Contract for Purchase of Electric Power, Date of MPUC Approval | March 31, 2011 |  |
Manitoba Hydro PPA (expires 2035) [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Long Term Contract For Purchase Of Electric Power Date Agreement Was Executed | May 19, 2011 |  |
Long-term Contract for Purchase of Electric Power, Term of Contract (Years) | 15 |  |
Long-term Contract for Purchase of Electric Power, Start Date of Contract | 2020 |  |
Long-term Contract for Purchase of Electric Power, Generating Unit Capacity (MW) | 250 |  |
Long-term Contract for Purchase of Electric Power, Date of MPUC Approval | January 26, 2012 |  |
Square Butte PPA [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Cost of Purchased Power | 61.2 | 55.2 | 53.9
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration | 2026 |  |
Long-term Contract for Purchase of Electric Power, Generating Unit Capacity (MW) | 455 |  |
Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding | 451.4 |  |
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased | 50.00% |  |
Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required | 44 |  |
Interest Expense | $ 11.1 | $ 10.2 | $ 11.0
DC Transmission Line from Square Butte [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Transmission Line Purchase Date | December 2009 |  |

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- Definition Anticipated date that a new transmission line will be built as part of 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 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 SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section D Reference 2: 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 + Details
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