Leases - Components of Lease Cost and Rent Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Leases [Abstract]
Operating lease cost	$ 50	$ 49	$ 152	$ 143
Short-term rent expense	27	28	80	87
Variable lease cost	9	3	20	13
Sublease income	(1)	(1)	(4)	(3)
Total operating lease cost	85	79	248	240
Finance lease cost:
Amortization of right-of-use assets	2	3	8	9
Interest on finance lease liabilities	2	2	6	6
Total finance lease cost	$ 4	$ 5	$ 14	$ 15