Employee Benefit Obligations - Total (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Employee Benefit Obligations
Underfunded defined benefit pension liabilities	$ 973	$ 954
Less current portion	(24)	(25)
Long-term defined benefit pension liabilities	949	929
Long-term retiree medical liabilities	163	157
Deferred compensation plans	329	291
Other	38	78
Total non-current employee benefit obligations	$ 1,479	$ 1,455