Basis of Presentation - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) $ / shares $ / MMBTU shares	Jun. 30, 2017 USD ($)	Mar. 31, 2018 USD ($)	Feb. 15, 2018 USD ($)
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 340		$ 340
Amortization expense	$ 9		$ 14
Share purchase plan, authorized amount						$ 750
Shares repurchased and retired during period (in shares) | shares	1.8		4.0
Repurchase price (in usd per share) | $ / shares	$ 35.15		$ 32.41
Revenues	$ 1,230	$ 1,059	$ 2,516	$ 2,095
Expenses	887	$ 3,359	1,465	$ 4,360
Stranded tax assets	7		$ 7
Minimum
Finite-Lived Intangible Assets [Line Items]
Intangible asset, useful life			7 years
Maximum
Finite-Lived Intangible Assets [Line Items]
Intangible asset, useful life			13 years
ASC 606
Finite-Lived Intangible Assets [Line Items]
Revenues	2
Expenses	7
West Africa
Finite-Lived Intangible Assets [Line Items]
Contractual sales price (in usd per MMBtu) | $ / MMBTU			0.25
Retained Earnings | ASC 606
Finite-Lived Intangible Assets [Line Items]
Cumulative effect of adoption	$ 2		$ 2		$ 7