Quarterly report pursuant to Section 13 or 15(d)

Impairment of Proved Properties (Detail)

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Impairment of Proved Properties (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Fair value of fields $ 399,485       $ 399,485   $ 338,641
Impairment of proved properties 17,988       29,208    
Impairment Of Oil And Gas Properties 18,000 $ 500   $ 0 29,200 $ 0  
North Tchibala [Member]              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Fair value of fields 200       200    
Impairment Of Oil And Gas Properties 13,500            
North Tchibala [Member] | Fair Value, Inputs, Level 3 [Member]              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Fair value of fields $ 200       $ 200    
Southeast Etame And North Tchibala field              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Impairment Of Oil And Gas Properties   $ 5,800 $ 5,400