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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues From Third Party Payers [Line Items]
Other	$ 11,529	$ 10,733	$ 22,952	$ 21,356
Other, Ratio	4.60%	3.70%	3.50%	3.30%
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	$ 2,425	$ 2,272	$ 4,949	$ 4,633
Revenues from third party payers, Ratio	21.00%	21.20%	21.60%	21.70%
Managed Medicare [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 1,345	$ 1,158	$ 2,744	$ 2,341
Revenues from third party payers, Ratio	11.70%	10.80%	12.00%	11.00%
Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 357	$ 376	$ 638	$ 670
Revenues from third party payers, Ratio	3.10%	3.50%	2.80%	3.10%
Managed Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 586	$ 527	$ 1,147	$ 1,116
Revenues from third party payers, Ratio	5.10%	4.90%	5.00%	5.20%
Managed Care and Insurers [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 5,993	$ 5,729	$ 12,055	$ 11,352
Revenues from third party payers, Ratio	51.90%	53.40%	52.50%	53.20%
International (Managed Care and Other Insurers) [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 295	$ 269	$ 600	$ 538
Revenues from third party payers, Ratio	2.60%	2.50%	2.60%	2.50%
Product and Service, Other [Member]
Revenues From Third Party Payers [Line Items]
Other	$ 528	$ 402	$ 819	$ 706
Health Care, Patient Service [Member]
Revenues From Third Party Payers [Line Items]
Other	$ 11,529	$ 10,733	$ 22,952	$ 21,356