Supplementary Information for Certain Balance Sheet Accounts - Other Non-current Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Incentives from lessors		$ 120
Deferred compensation plan liability	$ 137	125
Contingent and deferred consideration on acquisitions	24	22
Liabilities for uncertain tax positions	49	46
Finance leases	24	26
Other non-current liabilities	65	90
Total other non-current liabilities	$ 299	$ 429