Property and Equipment - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation of property and equipment and amortization of computer software	$ 546	$ 488	$ 466
Capital leases, net of accumulated depreciation	296	244
Property and equipment acquired through capital leases	111	145	$ 71
Capitalized internally-developed software	2,605	2,664
Software Development
Property, Plant and Equipment [Line Items]
Capitalized internally-developed software	$ 263	$ 206