Employee Benefit Obligations - Total (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Employee Benefit Obligations
Underfunded defined benefit pension liabilities	$ 945	$ 945
Less current portion	(26)	(27)
Long-term defined benefit pension liabilities	919	918
Long-term retiree medical liabilities	195	196
Deferred compensation plans	264	275
Other	74	74
Total non-current employee benefit obligations	$ 1,452	$ 1,463