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Commitment and contingencies variable interest entities (Details 4) - USD ($) $ in Thousands | Mar. 31, 2016 | Dec. 31, 2015 | Mar. 31, 2015
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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 |  |
Dakota Prairie Refining, LLC [Member] | Cash and cash equivalents [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, assets | 478 | $ 851 | $ 10,784
Dakota Prairie Refining, LLC [Member] | Accounts receivable [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, assets | 11,169 | 7,693 | 2,335
Dakota Prairie Refining, LLC [Member] | Inventories [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, assets | 17,056 | 13,176 | 7,902
Dakota Prairie Refining, LLC [Member] | Other current assets [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, assets | 6,124 | 6,215 | 2,926
Dakota Prairie Refining, LLC [Member] | Total current assets [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, assets | 34,827 | 27,935 | 23,947
Dakota Prairie Refining, LLC [Member] | Net property, plant and equipment [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, assets | 419,492 | 425,123 | 425,944
Dakota Prairie Refining, LLC [Member] | Other [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, assets | 8,941 | 9,626 | 4,562
Dakota Prairie Refining, LLC [Member] | Total deferred charges and other assets [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, assets | 8,941 | 9,626 | 4,562
Dakota Prairie Refining, LLC [Member] | Total assets [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, assets | 463,260 | 462,684 | 454,453
Dakota Prairie Refining, LLC [Member] | Short-term borrowings [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 63,200 | 45,500 | 16,100
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 | 6,375 | 5,250 | 3,000
Dakota Prairie Refining, LLC [Member] | Accounts payable [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 27,697 | 24,766 | 23,654
Dakota Prairie Refining, LLC [Member] | Taxes payable [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 1,001 | 1,391 | 569
Dakota Prairie Refining, LLC [Member] | Accrued compensation [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 717 | 938 | 683
Dakota Prairie Refining, LLC [Member] | Other accrued liabilities [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 7,155 | 4,953 | 1,016
Dakota Prairie Refining, LLC [Member] | Total current liabilities [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 106,145 | 82,798 | 45,022
Dakota Prairie Refining, LLC [Member] | Long-term debt [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 62,625 | 63,750 | 69,000
Dakota Prairie Refining, LLC [Member] | Total liabilities [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Limited liability entity, consolidated, carrying amount, liabilities | 168,770 | $ 146,548 | $ 114,022
Fuel contract [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Variable interest entity, reporting entity involvement, maximum loss exposure, amount | 43,100 |  |
WBI Holdings [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Cumulative capital contributions, net of distributions | 230,400 |  |
Line of credit facility, maximum borrowing capacity | 25,000 |  |
Notes outstanding | 1,700 |  |
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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