Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Depreciation	$ 649	$ 574	$ 515
Net change in property and equipment included in accounts payable (2017 and 2015 - not material)		$ 35