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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
Sep. 30, 2015
Dec. 31, 2014
Assets:    
Total assets $ 136,188 $ 316,249
Total liabilities 90  
Money market funds [Member]    
Assets:    
Total assets 742 742
Commodity derivative instruments [Member]    
Assets:    
Total assets 135,446 315,507
Total liabilities 90  
Level 1 [Member]    
Assets:    
Total assets 742 742
Total liabilities 0  
Level 1 [Member] | Money market funds [Member]    
Assets:    
Total assets 742 742
Level 1 [Member] | Commodity derivative instruments [Member]    
Assets:    
Total assets 0 0
Total liabilities 0  
Level 2 [Member]    
Assets:    
Total assets 135,446 315,507
Total liabilities 90  
Level 2 [Member] | Money market funds [Member]    
Assets:    
Total assets 0 0
Level 2 [Member] | Commodity derivative instruments [Member]    
Assets:    
Total assets 135,446 315,507
Total liabilities 90  
Level 3 [Member]    
Assets:    
Total assets 0 0
Total liabilities 0  
Level 3 [Member] | Money market funds [Member]    
Assets:    
Total assets 0 0
Level 3 [Member] | Commodity derivative instruments [Member]    
Assets:    
Total assets 0 $ 0
Total liabilities $ 0