Commitment and contingencies variable interest entities (Details 4) (USD $)	Mar. 31, 2015	Dec. 01, 2014	Dec. 31, 2014	Mar. 31, 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
Project cost low range estimate	425,000,000
Project cost high range estimate	435,000,000
Term loan for project debt financing	75,000,000
Line of credit facility, maximum borrowing capacity		50,000,000
Dakota Prairie Refining, LLC [Member] | Cash and cash equivalents [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	10,784,000		21,376,000	22,996,000
Dakota Prairie Refining, LLC [Member] | Accounts receivable [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	2,335,000		2,759,000	0
Dakota Prairie Refining, LLC [Member] | Inventories [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	7,902,000		5,311,000	0
Dakota Prairie Refining, LLC [Member] | Other current assets [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	2,926,000		4,019,000	1,135,000
Dakota Prairie Refining, LLC [Member] | Total current assets [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	23,947,000		33,465,000	24,131,000
Dakota Prairie Refining, LLC [Member] | Net property, plant and equipment [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	425,944,000		398,984,000	207,260,000
Dakota Prairie Refining, LLC [Member] | Other [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	4,562,000		3,400,000	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	4,562,000		3,400,000	0
Dakota Prairie Refining, LLC [Member] | Total assets [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, assets	454,453,000		435,849,000	231,391,000
Dakota Prairie Refining, LLC [Member] | Short-term borrowings [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	16,100,000		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	3,000,000		3,000,000	3,000,000
Dakota Prairie Refining, LLC [Member] | Accounts payable [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	23,654,000		55,089,000	16,103,000
Dakota Prairie Refining, LLC [Member] | Taxes payable [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	569,000		648,000	113,000
Dakota Prairie Refining, LLC [Member] | Accrued compensation [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	683,000		727,000	164,000
Dakota Prairie Refining, LLC [Member] | Other accrued liabilities [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	1,016,000		899,000	580,000
Dakota Prairie Refining, LLC [Member] | Total current liabilities [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	45,022,000		60,363,000	19,960,000
Dakota Prairie Refining, LLC [Member] | Long-term debt [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	69,000,000		69,000,000	72,000,000
Dakota Prairie Refining, LLC [Member] | Total liabilities [Member]
Variable Interest Entities [Line Items]
Limited liability entity, consolidated, carrying amount, liabilities	114,022,000		129,363,000	91,960,000
Fuel contract [Member]
Variable Interest Entities [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$ 20,800,000
WBI Holdings [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Percent of credit agreement guaranteed		50.00%
Noncontrolling interest [Member] | Dakota Prairie Refining, LLC [Member]
Variable Interest Entities [Line Items]
Percent of credit agreement guaranteed		50.00%