Leases (Components of Lease Cost and Rent Expense) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Leases [Abstract]
Operating lease cost	$ 51	$ 48
Short-term rent expense	26	30
Variable lease cost	5	3
Sublease Income	(1)	(1)
Total operating lease cost	81	80
Finance lease cost:
Amortization of right-of-use assets	3	3
Interest on finance lease liabilities	2	2
Total finance lease cost	$ 5	$ 5