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Short-term borrowings (Details 2) - Revolving credit facility [Member] - Short-term debt [Member] - Dakota Prairie Refining, LLC [Member] - USD ($) $ in Millions | Dec. 31, 2015 | Sep. 30, 2015 | Dec. 01, 2014
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Short-term debt [Line Items] |  |  |
Facility limit, maximum borrowing capacity | $ 75.0 | $ 75.0 | $ 50.0
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00% |  |
Ratio of total funded debt to total capitalization as specified in debt covenants | 65.00% |  |

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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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