Leases - Components of Lease Cost and Rent Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Leases [Abstract]
Operating lease cost	$ 51	$ 48	$ 101	$ 95
Short-term rent expense	27	27	54	59
Variable lease cost	6	7	11	10
Sublease income	(2)	(1)	(3)	(2)
Total operating lease cost	82	81	163	162
Finance lease cost:
Amortization of right-of-use assets	3	3	6	6
Interest on finance lease liabilities	2	2	4	4
Total finance lease cost	$ 5	$ 5	$ 10	$ 10