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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) $ in Millions | 6 Months Ended
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Jun. 30, 2016 USD ($) MW | Jun. 30, 2015 USD ($)
Square Butte [Member] | Square Butte PPA [Member] |  |
Power Purchase Agreements [Line Items] |  |
PPA Counterparty Debt Outstanding | $ | $ 361.9 |
Cost of Power Purchased | $ | 37.7 | $ 39.5
Pro Rata Share of Interest Expense | $ | $ 4.8 | $ 5.0
Square Butte [Member] | Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member] |  |
Power Purchase Agreements [Line Items] |  |
Generating Capacity (MW) | 455 |
Output Entitlement | 50.00% |
Minnkota Power [Member] | Square Butte Coal-fired Unit [Member] | Minnkota Power Sales Agreement [Member] |  |
Power Purchase Agreements [Line Items] |  |
Output Entitlement | 28.00% | 28.00%
Silver Bay Power [Member] | Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] | Minimum [Member] |  |
Power Purchase Agreements [Line Items] |  |
Output Being Sold (MW) | 50 |
Silver Bay Power [Member] | Silver Bay Power Sales Agreement through 2031 (Years 2020-2031) [Member] |  |
Power Purchase Agreements [Line Items] |  |
Output Being Sold (MW) | 90 |
Silver Bay Power [Member] | Silver Bay Power Self-Generation [Member] |  |
Power Purchase Agreements [Line Items] |  |
Generating Capacity (MW) | 90 |
Shell Energy [Member] | Shell Energy PPA [Member] |  |
Power Purchase Agreements [Line Items] |  |
Output Being Purchased (MW) | 50 |

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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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