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Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Commitments And Contingencies [Line Items] |  |  |
Rental expense | $ 1.5 | $ 0.8 | $ 0.6
Long Term Commitments [Member] |  |  |
Commitments And Contingencies [Line Items] |  |  |
Future obligations under supply agreements | 70.5 |  |
Long Term Commitments [Member] | Williston Basin Project [Member] | Crude oil collar [Member] |  |  |
Commitments And Contingencies [Line Items] |  |  |
Minimum quantity of delivery | 22.4 |  |
Long Term Commitments [Member] | Williston Basin Project [Member] | Natural Gas [Member] |  |  |
Commitments And Contingencies [Line Items] |  |  |
Minimum quantity of delivery | 14,800 |  |
Drilling contracts [Member] |  |  |
Commitments And Contingencies [Line Items] |  |  |
Payments upon early contract termination | $ 43.7 |  |

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- Details
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- Definition Future obligations under supply agreements + References No definition available. + Details
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- Definition The loss recognized on termination of a contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 912 -SubTopic 275 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6471736&loc=d3e54681-109401 + Details
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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 Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717 + Details
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