Accrued Expenses and Allowance for Doubtful Accounts - Summary of Allowance of Doubtful Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Payables and Accruals [Abstract]
Balance at Beginning of Year	$ 4,988	$ 5,326	$ 5,011
Provision for Doubtful Accounts	4,039	3,257	3,913
Accounts Written off, Net of Recoveries	(3,539)	(3,595)	(3,598)
Balance at End of Year	$ 5,488	$ 4,988	$ 5,326