Leases - Components of Lease Cost and Rent Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Leases [Abstract]
Operating lease cost	$ 48	$ 51
Short-term rent expense	25	26
Variable lease cost	6	5
Sublease income	(1)	(1)
Total operating lease cost	78	81
Finance lease cost:
Amortization of right-of-use assets	3	3
Interest on finance lease liabilities	1	2
Total finance lease cost	$ 4	$ 5