Property and Equipment - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 8,831	$ 6,362	$ 4,949
Amortization of property and equipment acquired under capital leases	5,400	3,800	$ 2,700
Gross assets under capital leases	26,400	17,000
Accumulated depreciation associated with capital leases	13,400	8,500
Gross assets under finance leases	5,400	2,900
Accumulated depreciation associated with finance leases	$ 635	$ 361