Employee Benefit Obligations - Total (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Employee Benefit Obligations
Underfunded defined benefit pension liabilities	$ 809	$ 955
Less current portion	(23)	(24)
Long-term defined benefit pension liabilities	786	931
Long-term retiree medical liabilities	152	156
Deferred compensation plans	399	439
Other	58	87
Total non-current employee benefit obligations	$ 1,395	$ 1,613