Acquisitions and Divestitures (Details) a in Millions, MMBbls in Millions		1 Months Ended										2 Months Ended	3 Months Ended	11 Months Ended	12 Months Ended
	Feb. 01, 2016	Jan. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) item MMBbls	Oct. 31, 2016 km²	Sep. 30, 2016 USD ($)	May 31, 2016	Apr. 30, 2016 km²	Nov. 30, 2015	Oct. 31, 2015	Mar. 31, 2015 USD ($) item	Aug. 31, 2014 USD ($)	Oct. 31, 2016 km²	Mar. 31, 2014 USD ($) agreement item	Nov. 30, 2016	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Oct. 31, 2016 a
Acquisitions and Divestitures
Proceeds on sale of assets															$ 210,000	$ 28,692,000	$ 58,315,000
Gain on sale of assets																$ 24,651,000	$ 23,769,000
Farm-in agreement | Camarao, Ameijoa, Mexilhao and Ostra blocks
Acquisitions and Divestitures
Participating interests (as a percent)										31.00%
Farm-in agreement | Block 11
Acquisitions and Divestitures
Participation interest acquired (as a percent)									85.00%
Farm-in agreement | Block 6
Acquisitions and Divestitures
Participation interest acquired (as a percent)								45.00%
Petroleum agreement | Block C6 Related To Mauritania
Acquisitions and Divestitures
3 D seismic requirements (in square kilometers) | km²				2,000
Initial exploration period				4 years
Area of petroleum exploration				4,300								4,300						1.1
Sales and purchase agreement | Kosmos BP Senegal Limited | Cayar Offshore Profond And Saint Louis Offshore Profond Blocks
Acquisitions and Divestitures
Participating interests (as a percent)			65.00%
Timis Corporation Limited | Farm-in agreement | Cayar Offshore Profond And Saint Louis Offshore Profond Blocks
Acquisitions and Divestitures
Participation interest acquired (as a percent)											60.00%
Participating interests (as a percent)														60.00%
Maximum cost per contingent exploration well			$ 120,000,000								$ 120,000,000
Participating interest for carrying the full cost of third contingent exploration or appraisal well (as a percent)			65.00%												65.00%
Chevron | Farm-out agreements | Block C8 Block C12 And Block C13
Acquisitions and Divestitures
Participating interest reassigned (as a percent)	30.00%
Proceeds on sale of assets										$ 28,700,000
Gain on sale of assets										$ 24,700,000
Number of exploration wells for which a third party will pay a disproportionate amount | item										1
Chevron | Farm-out agreements | Block 42
Acquisitions and Divestitures
Participating interests (as a percent)							33.33%
Hess Corporation | Farm-out agreements | Block 42
Acquisitions and Divestitures
3 D seismic requirements (in square kilometers) | km²							6,500
ANP STP | Farm-in agreement | Block 5
Acquisitions and Divestitures
Carried interest held by third party as a percent															15.00%
ANP STP | Farm-in agreement | Block 12
Acquisitions and Divestitures
Carried interest held by third party as a percent															12.50%
ANP STP | Farm-in agreement | Block 11
Acquisitions and Divestitures
Carried interest held by third party as a percent									15.00%
Staatsolie | Farm-out agreements | Block 42
Acquisitions and Divestitures
Maximum percentage interest available upon approval (as a percent)							10.00%
Parent company | Essaouira Offshore Block
Acquisitions and Divestitures
Participating interests (as a percent)												75.00%
Parent company | Farm-in agreement | Block 5
Acquisitions and Divestitures
Participating interests (as a percent)															45.00%
Parent company | Farm-in agreement | Block 12
Acquisitions and Divestitures
Participating interests (as a percent)															45.00%
Parent company | Farm-out agreements | Block 42
Acquisitions and Divestitures
Participating interests (as a percent)							33.33%
Parent company | Petroleum agreement | Block C6 Related To Mauritania
Acquisitions and Divestitures
Participating interests (as a percent)				28.00%
Parent company | Petroleum agreement | Boujdour Maritime block
Acquisitions and Divestitures
Participating interests (as a percent)						55.00%
BP | Essaouira Offshore Block
Acquisitions and Divestitures
Participating interest reassigned (as a percent)					45.00%
Amount due in lieu of drilling exploration well					$ 30,000,000
Amount received in lieu of drilling exploration well		$ 30,000,000
BP | Farm-out agreements | Block C6 Block C8 Block C12 and Block C13 Mauritania
Acquisitions and Divestitures
Number of blocks covered by farm-out agreements | item			4
Participation interest acquired (as a percent)			62.00%
BP | Farm-out agreements | Essaouira Offshore, Foum Assaka Offshore and Tarhazoute Offshore blocks
Acquisitions and Divestitures
Number of farm-out agreements | agreement													3
Number of blocks covered by farm-out agreements | item													3
Proceeds on sale of assets													$ 56,900,000
Gain on sale of assets													23,800,000
BP | Farm-out agreements | Mauritania And Senegal Offshore Block
Acquisitions and Divestitures
Amount of potential and variable consideration per barrel			$ 2
BP | Farm-out agreements | Mauritania And Senegal Offshore Block | Maximum
Acquisitions and Divestitures
Number of barrels | MMBbls			1,000
BP | Sales and purchase agreement | Kosmos BP Senegal Limited
Acquisitions and Divestitures
Participation interest acquired (as a percent)			49.99%
BP | Sales and purchase agreement and farm-out agreements | Mauritania And Senegal Offshore Block
Acquisitions and Divestitures
Upfront amount to be received in cash			$ 162,000,000
Spending by third party for exploration and appraisal costs			221,000,000
Spending by third party for Kosmos' development costs			$ 533,000,000
Cairn | Farm-out agreements | Cap Boujdour Offshore block
Acquisitions and Divestitures
Reimbursement of shared costs previously incurred													$ 1,500,000
Cairn | Petroleum agreement | Boujdour Maritime block
Acquisitions and Divestitures
Participating interests (as a percent)						20.00%
ONHYM | Petroleum agreement | Boujdour Maritime block
Acquisitions and Divestitures
Carried interest held by third party as a percent						25.00%
SMHPM | Petroleum agreement | Block C6 Related To Mauritania
Acquisitions and Divestitures
Carried interest held by third party as a percent				10.00%
GALP | Farm-out agreements | Block 5, Block 11, and Block 12
Acquisitions and Divestitures
Non operated interest			20.00%