Equity Method Investments - Summary of Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Costs and expenses:
Equity in earnings - KTIPI	$ 24,841	$ (4,804)	$ 59,637	$ (11,230)
Kosmos-Trident International Petroleum Inc.
Schedule of Equity Method Investments [Line Items]
Ownership percentage	50.00%		50.00%
Basis presented	100.00%		100.00%
Assets
Total current assets	$ 158,140		$ 158,140		$ 179,070
Property and equipment, net	291,960		291,960		345,611
Other assets	487		487		567
Total assets	450,587		450,587		525,248
Liabilities and shareholders' equity
Total current liabilities	196,338		196,338		106,769
Total long-term liabilities	541,881		541,881		565,591
Shareholders' equity:
Total shareholders' equity	(287,632)		(287,632)		(147,112)
Total liabilities and shareholders' equity	450,587		450,587		$ 525,248
Revenues and other income:
Oil and gas revenue	215,408		600,158
Other income	(72)		44
Total revenues and other income	215,336		600,202
Costs and expenses:
Oil and gas production	40,334		115,366
Depletion and depreciation	33,044		108,996
Other expenses, net	(58)		(211)
Total costs and expenses	73,320		224,151
Income before income taxes	142,016		376,051
Income tax expense	50,796		134,047
Net income	91,220		242,004
Kosmos' share of net income	45,610		121,002
Basis difference amortization	20,769		61,365
Equity in earnings - KTIPI	$ 24,841		$ 59,637