Basis of Presentation and Significant Accounting Policies - Schedule of Estimated Cost of Uncompensated Care (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accounting Policies [Abstract]
Patient care costs (salaries and benefits, supplies, other operating expenses and depreciation and amortization)	$ 9,946	$ 9,472	$ 29,684	$ 27,798
Cost-to-charges ratio (patient care costs as percentage of gross patient charges)	12.70%	13.40%	12.60%	13.10%
Total uncompensated care	$ 6,786	$ 6,089	$ 19,524	$ 17,137
Multiply by the cost-to-charges ratio	12.70%	13.40%	12.60%	13.10%
Estimated cost of total uncompensated care	$ 862	$ 816	$ 2,460	$ 2,245