Interest Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Expense [Abstract]
Interest expense before capitalization	$ 358	$ 398	$ 457
Capitalized interest	(23)	(26)	(19)
Interest Expense	335	372	438
Interest paid, net	$ 314	$ 351	$ 445