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Commitment and contingencies variable interest entities (Details 4) - USD ($) $ in Thousands | Sep. 30, 2015 | Dec. 31, 2014 | Dec. 01, 2014 | Sep. 30, 2014
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Dakota Prairie Refining, LLC [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Percent of ownership | 50.00% |  |  |
Total project costs | $ 300,000 |  |  |
Portion of capital commitment | 150,000 |  |  |
Partner portion of capital commitment | 75,000 |  |  |
Excess capital commitments | 300,000 |  |  |
Term loan for project debt financing | 75,000 |  |  |
Line of credit facility, maximum borrowing capacity | 75,000 |  | $ 50,000 |
Dakota Prairie Refining, LLC [Member] | Cash and cash equivalents [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, assets | 625 | $ 21,376 |  | $ 16,723
Dakota Prairie Refining, LLC [Member] | Accounts receivable [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, assets | 14,648 | 2,759 |  | 150
Dakota Prairie Refining, LLC [Member] | Inventories [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, assets | 12,354 | 5,311 |  | 0
Dakota Prairie Refining, LLC [Member] | Other current assets [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, assets | 7,125 | 4,019 |  | 4,187
Dakota Prairie Refining, LLC [Member] | Total current assets [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, assets | 34,752 | 33,465 |  | 21,060
Dakota Prairie Refining, LLC [Member] | Net property, plant and equipment [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, assets | 428,383 | 398,984 |  | 314,551
Dakota Prairie Refining, LLC [Member] | Other [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, assets | 5,052 | 3,400 |  | 0
Dakota Prairie Refining, LLC [Member] | Total deferred charges and other assets [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, assets | 5,052 | 3,400 |  | 0
Dakota Prairie Refining, LLC [Member] | Total assets [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, assets | 468,187 | 435,849 |  | 335,611
Dakota Prairie Refining, LLC [Member] | Short-term borrowings [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 29,500 | 0 |  | 0
Dakota Prairie Refining, LLC [Member] | Long-term debt due within one year [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 4,125 | 3,000 |  | 3,000
Dakota Prairie Refining, LLC [Member] | Accounts payable [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 21,686 | 55,089 |  | 36,541
Dakota Prairie Refining, LLC [Member] | Taxes payable [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 1,630 | 648 |  | 323
Dakota Prairie Refining, LLC [Member] | Accrued compensation [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 1,059 | 727 |  | 617
Dakota Prairie Refining, LLC [Member] | Other accrued liabilities [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 1,217 | 899 |  | 633
Dakota Prairie Refining, LLC [Member] | Total current liabilities [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 59,217 | 60,363 |  | 41,114
Dakota Prairie Refining, LLC [Member] | Long-term debt [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 64,875 | 69,000 |  | 69,000
Dakota Prairie Refining, LLC [Member] | Total liabilities [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 124,092 | $ 129,363 |  | $ 110,114
Fuel contract [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Variable interest entity, reporting entity involvement, maximum loss exposure, amount | 33,900 |  |  |
WBI Holdings [Member] | Dakota Prairie Refining, LLC [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Cumulative capital contributions, net of distributions | 230,400 |  |  |
Noncontrolling interest [Member] | Dakota Prairie Refining, LLC [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Cumulative capital contributions, net of distributions | $ 163,600 |  |  |
Percent of credit agreement guaranteed | 50.00% |  |  |
Centennial Energy Holdings, Inc [Member] | Dakota Prairie Refining, LLC [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Percent of credit agreement guaranteed | 50.00% |  |  |

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