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Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified			9 Months Ended				9 Months Ended
	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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Future obligations under supply agreements

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The amount committed to invest, expend and/or incur expenses in exchange for the transfer of certain assets.

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- Definition

The maximum duration period applicable to the existing commitments.

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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.

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-Publisher SEC

-Name Industry Guide

-Number 2

-Paragraph 8

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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.

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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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