Basis of Presentation and Significant Accounting Policies - Schedule of Revenues from Third Party Payers, Uninsured and Other Payers (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 11,451	$ 10,696	$ 34,403	$ 32,052
Revenues ratio from third party payers	100.00%	100.00%	100.00%	100.00%
Medicare [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 2,404	$ 2,298	$ 7,353	$ 6,931
Revenues from third party payers, Ratio	21.00%	21.50%	21.40%	21.60%
Managed Medicare [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 1,344	$ 1,122	$ 4,088	$ 3,463
Revenues from third party payers, Ratio	11.70%	10.50%	11.90%	10.80%
Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 338	$ 286	$ 976	$ 956
Revenues from third party payers, Ratio	3.00%	2.70%	2.80%	3.00%
Managed Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 622	$ 537	$ 1,769	$ 1,653
Revenues from third party payers, Ratio	5.40%	5.00%	5.10%	5.20%
Managed Care and Insurers [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 6,026	$ 5,605	$ 18,081	$ 16,957
Revenues from third party payers, Ratio	52.60%	52.40%	52.60%	52.90%
International (Managed Care and Other Insurers) [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 273	$ 276	$ 873	$ 814
Revenues from third party payers, Ratio	2.40%	2.60%	2.50%	2.50%
Product and Service, Other [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 444	$ 572	$ 1,263	$ 1,278
Other, Ratio	3.90%	5.30%	3.70%	4.00%
Health Care, Patient Service [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 11,451	$ 10,696	$ 34,403	$ 32,052