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Commitments And Contingencies (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Supply Commitment [Line Items]
Total rental commitments under leases	$ 14.7
Long Term Commitments [Member]
Supply Commitment [Line Items]
Future obligations under supply agreements	73.4
Williston Basin Project [Member] | Long Term Commitments [Member]
Supply Commitment [Line Items]
Minimum quantity of delivery	16.5
Williston Basin Project [Member] | Long Term Commitments [Member] | Millions Of Barrels [Member]
Supply Commitment [Line Items]
Minimum quantity of delivery	21,200,000.0
Drilling Contracts [Member]
Supply Commitment [Line Items]
Payments upon early contract termination	58.5
Fracturing Services [Member]
Supply Commitment [Line Items]
Payments upon early contract termination	$ 31.4

X

- Definition

Future obligations under supply agreements

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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