Leases - Lease Expense (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Leases [Abstract]
Operating and short-term lease cost	$ 166	$ 335
Finance lease cost:
Amortization of right-of-use assets	11	23
Interest on lease liability	3	7
Total finance lease cost	14	30
Total lease cost	$ 180	$ 365