Employee Benefit Obligations - Total (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Employee Benefit Obligations
Underfunded defined benefit pension liabilities	$ 885	$ 945
Less current portion	(25)	(27)
Long-term defined benefit pension liabilities	860	918
Long-term retiree medical liabilities	191	196
Deferred compensation plans	293	275
Other	77	74
Total non-current employee benefit obligations	$ 1,421	$ 1,463