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Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Commitments And Contingencies [Line Items]
Rental expense	$ 1.6	$ 1.5	$ 0.8
Long-term Purchase Commitment, Amount	5.0
Cost Sharing Commitments, Amount	12.7
Investment Commitment	7.0
Long Term Commitments [Member]
Commitments And Contingencies [Line Items]
Future obligations under supply agreements	49.3
Long Term Commitments [Member] | Williston Basin Project [Member] | Crude oil collar [Member]
Commitments And Contingencies [Line Items]
Minimum quantity of delivery	11,600,000
Long Term Commitments [Member] | Williston Basin Project [Member] | Natural Gas [Member]
Commitments And Contingencies [Line Items]
Minimum quantity of delivery	11,800
Drilling contracts [Member]
Commitments And Contingencies [Line Items]
Payments upon early contract termination	$ 26.2

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- Definition

Cost Sharing Commitments, Amount

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- Definition

Future obligations under supply agreements

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- Definition

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

+ References

No definition available.

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- Definition

The minimum amount the entity agreed to spend under the long-term purchase commitment.

+ 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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- 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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