Divestitures - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds	$ 7	$ 371
Net losses	$ 5	$ 24
Certain Onshore Domestic Assets Alaska [Member] | Exploration and Production [Member] | Asset Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds				$ 383
Net losses			$ 154	$ 30