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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