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Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Commitments And Contingencies [Line Items]
Total rental commitments under leases	$ 12.9
Long Term Commitments [Member]
Commitments And Contingencies [Line Items]
Future obligations under supply agreements	62.1
Long Term Commitments [Member] | Williston Basin project [Member] | Crude oil [Member]
Commitments And Contingencies [Line Items]
Minimum quantity of delivery	20.1
Long Term Commitments [Member] | Williston Basin project [Member] | Natural Gas [Member]
Commitments And Contingencies [Line Items]
Minimum quantity of delivery	14.2
Drilling contracts [Member]
Commitments And Contingencies [Line Items]
Payments upon early contract termination	$ 37.2

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- Definition

Future obligations under supply agreements

+ 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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- 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

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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

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