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Variable interest entities (Details 6) - USD ($)	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Dec. 01, 2014
Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Percent of ownership	50.00%
Total project costs	$ 300,000,000
Portion of capital commitment	150,000,000
Partner portion of capital commitment	75,000,000
Excess capital commitments	300,000,000
Term loan for project debt financing	75,000,000
Facility limit, maximum borrowing capacity		$ 75,000,000		$ 50,000,000.0
Fuel contract [Member]
Variable Interest Entities [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	40,100,000
Centennial Energy Holdings, Inc [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Percent of credit agreement guaranteed		50.00%
WBI Holdings [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Cumulative capital contributions, net of distributions	230,400,000
Noncontrolling interest | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Cumulative capital contributions, net of distributions	163,600,000
Percent of credit agreement guaranteed		50.00%
Cash and cash equivalents [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets	851,000		$ 21,376,000
Accounts receivable [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets	7,693,000		2,759,000
Inventories [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets	13,176,000		5,311,000
Other current assets [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets	6,215,000		4,019,000
Total current assets [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets	27,935,000		33,465,000
Net property, plant and equipment [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets	425,123,000		398,984,000
Other [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets	9,626,000		3,400,000
Total deferred charges and other assets [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets	9,626,000		3,400,000
Total assets [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, assets	462,684,000		435,849,000
Short-term borrowings [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities	45,500,000		0
Long-term debt due within one year [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities	5,250,000		3,000,000
Accounts payable [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities	24,766,000		55,089,000
Taxes payable [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities	1,391,000		648,000
Accrued compensation [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities	938,000		727,000
Other accrued liabilities [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities	4,953,000		899,000
Total current liabilities [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities	82,798,000		60,363,000
Long-term debt [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities	63,750,000		69,000,000
Total liabilities [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities	$ 146,548,000		$ 129,363,000

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