Leases - Components of Lease Cost and Rent Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Leases [Abstract]
Operating lease cost	$ 50	$ 51	$ 99	$ 101
Short-term rent expense	23	27	47	54
Variable lease cost	6	6	12	11
Sublease income	(1)	(2)	(2)	(3)
Total operating lease cost	78	82	156	163
Finance lease cost:
Amortization of right-of-use assets	1	3	4	6
Interest on finance lease liabilities	2	2	3	4
Total finance lease cost	$ 3	$ 5	$ 7	$ 10