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Commitment and contingencies variable interest entities (Details 4) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
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%