Suspended Well Costs (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 project	Dec. 31, 2012 project
Suspended Well Costs
Capitalized exploratory well costs subsequently expensed in the same period	$ 8,100,000
Reconciliation of capitalized exploratory well costs on completed wells
Balance at the beginning of the period	372,492,000
Additions to capitalized exploratory well costs pending the determination of proved reserves	937,000
Capitalized exploratory well costs charged to expense	(1,705,000)
Balance at the end of the period	371,724,000
Aging of capitalized exploratory well costs and number of projects for which exploratory well costs were capitalized for more than one year
Exploratory well costs capitalized for a period of one year or less	56,903,000	106,635,000
Exploratory well costs capitalized for a period one to two years	215,320,000	179,933,000
Exploratory well costs capitalized for a period three to four years	99,501,000	85,924,000
Ending balance	$ 371,724,000
Number of projects that have exploratory well costs that have been capitalized for a period greater than one year	7	7
Mahogany East Area | Minimum
Projects with exploratory well costs capitalized for more than one year
Period of negotiation and consultation of notice of dispute issued to Ministry of Energy by the entity	30 days
Teak-1 Discovery
Projects with exploratory well costs capitalized for more than one year
Number of appraisal wells which were drilled	2
Teak-1 Discovery | Maximum
Projects with exploratory well costs capitalized for more than one year
Submission of PoD to Ghana's Ministry of Energy, period	6 months
Teak-2 Discovery | Maximum
Projects with exploratory well costs capitalized for more than one year
Submission of PoD to Ghana's Ministry of Energy, period	6 months
Akasa Discovery | Maximum
Projects with exploratory well costs capitalized for more than one year
Submission of PoD to Ghana's Ministry of Energy, period	6 months