Accrued Expenses and Allowance for Doubtful Accounts - Summary of Other Accrued Expenses (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Professional liability risks	$ 429	$ 391
Interest	406	409
Taxes other than income	299	283
Other	849	952
Other accrued expenses	$ 1,983	$ 2,035