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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 | Mar. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Assets: |  |
Total assets | $ 47,353 | $ 90,384
Liabilities: |  |
Total liabilities | 4,660 | 1,428
Money market funds [Member] |  |
Assets: |  |
Total assets | 36,422 | 66,387
Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets | 10,931 | 23,997
Liabilities: |  |
Total liabilities | 4,660 | 1,428
Level 1 [Member] |  |
Assets: |  |
Total assets | 36,422 | 66,387
Liabilities: |  |
Total liabilities |  |
Level 1 [Member] | Money market funds [Member] |  |
Assets: |  |
Total assets | 36,422 | 66,387
Level 1 [Member] | Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets |  |
Liabilities: |  |
Total liabilities |  |
Level 2 [Member] |  |
Assets: |  |
Total assets | 10,931 | 23,997
Liabilities: |  |
Total liabilities | 4,660 | 1,428
Level 2 [Member] | Money market funds [Member] |  |
Assets: |  |
Total assets |  |
Level 2 [Member] | Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets | 10,931 | 23,997
Liabilities: |  |
Total liabilities | 4,660 | 1,428
Level 3 [Member] |  |
Assets: |  |
Total assets |  |
Liabilities: |  |
Total liabilities |  |
Level 3 [Member] | Money market funds [Member] |  |
Assets: |  |
Total assets |  |
Level 3 [Member] | Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets |  |
Liabilities: |  |
Total liabilities |  |

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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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- Details
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