Debt, Interest Incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest and Debt Expense, Net of Capitalized Interest
Interest and debt expense	$ 539	$ 511	$ 504
Less capitalized interest	71	65	71
Interest and debt expense, net of capitalized interest	$ 468	$ 446	$ 433