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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, 2013
Dec. 31, 2012
Assets:    
Total assets $ 4,339 $ 90,384
Liabilities:    
Total liabilities 8,327 1,428
Money market funds [Member]
    
Assets:    
Total assets 742 66,387
Commodity derivative instruments [Member]
    
Assets:    
Total assets 3,597 23,997
Liabilities:    
Total liabilities 8,327 1,428
Level 1 [Member] | Money market funds [Member]
    
Assets:    
Total assets 742 66,387
Level 2 [Member]
    
Assets:    
Total assets   23,997
Liabilities:    
Total liabilities 8,327 1,428
Level 2 [Member] | Commodity derivative instruments [Member]
    
Assets:    
Total assets 3,597 23,997
Liabilities:    
Total liabilities $ 8,327 $ 1,428