Accounting Policies - Summary of Oil and Gas Revenue (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Provisional oil sales contracts	$ 31,683	$ (54,267)	$ (34,003)	$ (232,057)
Oil and gas revenue	357,036	242,833	1,049,759	585,220
Oil and gas revenue
Disaggregation of Revenue [Line Items]
Provisional oil sales contracts	4,668	3,075	1,881	3,584
Oil and gas revenue	357,036	242,833	1,049,759	585,220
Equatorial Guinea
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	54,894	0	207,173	0
Ghana
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	173,613	215,581	502,413	557,459
Gulf of Mexico
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 123,861	$ 24,177	$ 338,292	$ 24,177

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

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