Leases - Components of Lease Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
ROU assets		$ 200
Lease liabilities	$ 156	$ 200
Finance lease cost
Amortization of ROU assets	4
Interest on lease liabilities	1
Operating lease cost	386
Short-term lease cost	9
Variable lease cost	128
Sublease income	(2)
Total lease cost	$ 526
Real estate
Lessee, Lease, Description [Line Items]
Average term (in years)	5 years
Equipment leases | Minimum
Lessee, Lease, Description [Line Items]
Average term (in years)	2 years
Equipment leases | Maximum
Lessee, Lease, Description [Line Items]
Average term (in years)	5 years
Property, plant and equipment
Lessee, Lease, Description [Line Items]
ROU assets	$ 149
Short-term debt
Lessee, Lease, Description [Line Items]
Lease liabilities	13
Long-term debt
Lessee, Lease, Description [Line Items]
Lease liabilities	$ 144