Commitments and Contingencies (Details) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Nov. 30, 2018 GBP (£)
Guarantor Obligations [Line Items]
Surety Bond, Notional Amount | £				£ 92
Surety Bond Exposure, Liability	$ 6	$ 6
Contingent Royalty Liability	93
Contingent Capital And Expense Receivable, Noncurrent	20
Commitments to acquire property, plant and equipment	41		$ 57
Override Royalty, Amount Earned Directed To Corresponding Asset Retirement Obligations	46
Override Royalty - GOM Future Abandonment	$ 70