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