Leases - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
Operating lease assets	$ 2,245
Operating lease liabilities	2,244
Capital leases, net of accumulated depreciation		$ 296
Property and equipment acquired through capital leases		111	$ 145
Additions of property and equipment, build-to-suit leases	39
Operating leases that have not yet commenced, undiscounted lease payments	$ 176
Rent expense		$ 820	$ 716
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Operating lease assets				$ 2,100
Operating lease liabilities				$ 2,100
Minimum
Lessee, Lease, Description [Line Items]
Operating leases that have not yet commenced, term	4 years
Maximum
Lessee, Lease, Description [Line Items]
Operating leases that have not yet commenced, term	15 years