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Fair Value Measurements - Hierarchy of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Assets:    
Total assets $ 99,165 $ 156,215
Total liabilities 2,576  
Money market funds [Member]    
Assets:    
Total assets 54 742
Commodity derivative instruments [Member]    
Assets:    
Total assets 99,111 155,473
Total liabilities 2,576  
Level 1 [Member]    
Assets:    
Total assets 54 742
Total liabilities 0  
Level 1 [Member] | Money market funds [Member]    
Assets:    
Total assets 54 742
Level 1 [Member] | Commodity derivative instruments [Member]    
Assets:    
Total assets 0 0
Total liabilities 0  
Level 2 [Member]    
Assets:    
Total assets 99,111 155,473
Total liabilities 2,576  
Level 2 [Member] | Money market funds [Member]    
Assets:    
Total assets 0 0
Level 2 [Member] | Commodity derivative instruments [Member]    
Assets:    
Total assets 99,111 155,473
Total liabilities 2,576  
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