Basis of Presentation - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares $ / MMBTU shares	Feb. 15, 2018 USD ($)	Dec. 31, 2017 USD ($)
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 5
Stranded tax assets	$ 7
Share purchase plan, authorized amount		$ 750
Shares repurchased and retired during period (in shares) | shares	2.2
Repurchase price (in usd per share) | $ / shares	$ 30.21
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 | Retained Earnings
Finite-Lived Intangible Assets [Line Items]
Cumulative effect of adoption			$ 5
West Africa
Finite-Lived Intangible Assets [Line Items]
Contractual sales price (in usd per MMBtu) | $ / MMBTU	0.25