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Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Commitments And Contingencies [Line Items]
Total rental commitments under leases	$ 12.2
Long Term Commitments [Member]
Commitments And Contingencies [Line Items]
Future obligations under supply agreements	61.1
Long Term Commitments [Member] | Williston Basin project [Member]
Commitments And Contingencies [Line Items]
Maximum commitment term	6 years
Long Term Commitments [Member] | Williston Basin project [Member] | Crude oil [Member]
Commitments And Contingencies [Line Items]
Minimum quantity of delivery	18.2
Long Term Commitments [Member] | Williston Basin project [Member] | Natural Gas [Member]
Commitments And Contingencies [Line Items]
Minimum quantity of delivery	13,600
Drilling contracts [Member]
Commitments And Contingencies [Line Items]
Payments upon early contract termination	$ 30.2

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Future obligations under supply agreements

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The maximum duration period applicable to the existing commitments.

+ References

No definition available.

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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

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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

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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



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