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Commitments And Contingencies (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Oil and Gas Delivery Commitments and Contracts [Line Items]
Total rental commitments under leases	$ 14.0
Long Term Commitments [Member]
Oil and Gas Delivery Commitments and Contracts [Line Items]
Future obligations under supply agreements	75.4
Williston Basin Project [Member] | Long Term Commitments [Member] | Crude Oil Collar [Member]
Oil and Gas Delivery Commitments and Contracts [Line Items]
Minimum quantity of delivery	24.7
Williston Basin Project [Member] | Long Term Commitments [Member] | Natural Gas [Member]
Oil and Gas Delivery Commitments and Contracts [Line Items]
Minimum quantity of delivery	15.7
Drilling Contracts [Member]
Oil and Gas Delivery Commitments and Contracts [Line Items]
Payments upon early contract termination	$ 50.4

X

- Definition

Future obligations under supply agreements

+ References

No definition available.

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X

- 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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X

- 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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X

- Definition

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 Statement of Financial Accounting Standard (FAS)

-Number 13

-Paragraph 16

-Subparagraph b(i)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 13

-Paragraph 122

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 3: 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



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717

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