Leases - Operating Lease Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Operating Lease Expense
Long-term fixed lease	$ 65	$ 70	$ 128	$ 142
Long-term variable lease	8	9	16	20
Short-term lease	110	119	210	280
Total operating lease expense	$ 183	$ 198	$ 354	$ 442