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Fair Value Measurements - Hierarchy of Fair Value, Assets and Liabilities Measured on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | ---
Assets: |  |
Total assets | $ 90,384 | $ 254,781
Liabilities: |  |
Total liabilities | 1,428 | 9,412
Money market funds [Member] |  |
Assets: |  |
Total assets | 66,387 | 250,419
Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets | 23,997 | 4,362
Liabilities: |  |
Total liabilities | 1,428 | 9,412
Level 1 [Member] |  |
Assets: |  |
Total assets | 66,387 | 250,419
Level 1 [Member] | Money market funds [Member] |  |
Assets: |  |
Total assets | 66,387 | 250,419
Level 2 [Member] |  |
Assets: |  |
Total assets | 23,997 |
Liabilities: |  |
Total liabilities | 1,428 |
Level 2 [Member] | Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets | 23,997 |
Liabilities: |  |
Total liabilities | 1,428 |
Level 3 [Member] |  |
Assets: |  |
Total assets |  | 4,362
Liabilities: |  |
Total liabilities |  | 9,412
Level 3 [Member] | Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets |  | 4,362
Liabilities: |  |
Total liabilities |  | $ 9,412

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- Definition This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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- Definition This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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