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Fair Value Measurements - Hierarchy of Fair Value, Assets and Liabilities Measured on Recurring Basis (Detail) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified | Jun. 30, 2013 | Dec. 31, 2012
--- | --- | ---
Assets: |  |
Total assets | $ 54,349 | $ 90,384
Liabilities: |  |
Total liabilities | 291 | 1,428
Money market funds [Member] |  |
Assets: |  |
Total assets | 36,442 | 66,387
Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets | 17,907 | 23,997
Liabilities: |  |
Total liabilities | 291 | 1,428
Level 1 [Member] |  |
Assets: |  |
Total assets | 36,442 | 66,387
Liabilities: |  |
Total liabilities |  |
Level 1 [Member] | Money market funds [Member] |  |
Assets: |  |
Total assets | 36,442 | 66,387
Level 1 [Member] | Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets |  |
Liabilities: |  |
Total liabilities |  |
Level 2 [Member] |  |
Assets: |  |
Total assets | 17,907 | 23,997
Liabilities: |  |
Total liabilities | 291 | 1,428
Level 2 [Member] | Money market funds [Member] |  |
Assets: |  |
Total assets |  |
Level 2 [Member] | Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets | 17,907 | 23,997
Liabilities: |  |
Total liabilities | 291 | 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 Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + 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=25499696&loc=d3e19207-110258 + Details
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- Definition Fair value of financial and nonfinancial obligations. + 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=25499696&loc=d3e19207-110258 + Details
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