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Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified |  |  | 9 Months Ended |  | 9 Months Ended
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Sep. 30, 2013 | Sep. 26, 2013 | Sep. 30, 2013 Long Term Commitments [Member] | Sep. 30, 2013 Long Term Commitments [Member] Williston Basin project [Member] | Sep. 30, 2013 Long Term Commitments [Member] Williston Basin project [Member] Crude oil [Member] MMBbls | Sep. 30, 2013 Long Term Commitments [Member] Williston Basin project [Member] Natural Gas [Member] MMcf | Sep. 30, 2013 Drilling contracts [Member]
Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Total rental commitments under leases | $ 11.5 |  |  |  |  |  |
Payments upon early contract termination |  |  |  |  |  |  | 24.9
Minimum quantity of delivery |  |  |  |  | 14.3 | 12,800.0 |
Maximum commitment term |  |  |  | 6 years |  |  |
Future obligations under supply agreements |  |  | 55.5 |  |  |  |
Investment commitment | $ 7.2 | $ 8.2 |  |  |  |  |

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- Definition Total amount of product committed to be supplied under existing contracts or agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Industry Guide -Number 2 -Paragraph 8 -Subparagraph A + Details
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- Definition Amount of required minimum rental payments for leases having an initial or remaining non-cancelable letter-terms in excess of one year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717 + Details
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