Accounting Policies - Schedule of Revenues from Third Party Payers, Uninsured and Other Payers (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 12,274	$ 11,451	$ 11,529	$ 11,423	$ 11,562	$ 10,696	$ 10,733	$ 10,623	$ 46,677	$ 43,614	$ 41,490
Revenues ratio from third party payers									100.00%	100.00%	100.00%
Medicare [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 9,831	$ 9,285	$ 8,719
Revenues from third party payers, Ratio									21.10%	21.30%	21.00%
Managed Medicare [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 5,497	$ 4,680	$ 4,278
Revenues from third party payers, Ratio									11.80%	10.70%	10.30%
Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 1,358	$ 1,316	$ 1,278
Revenues from third party payers, Ratio									2.90%	3.00%	3.10%
Managed Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 2,403	$ 2,165	$ 2,317
Revenues from third party payers, Ratio									5.10%	5.00%	5.60%
Managed Care and Other Insurers [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 24,467	$ 23,342	$ 22,287
Revenues from third party payers, Ratio									52.40%	53.50%	53.70%
International (Managed Care and Other Insurers) [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 1,156	$ 1,097	$ 1,195
Revenues from third party payers, Ratio									2.50%	2.50%	2.90%
Product and Service, Other [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 1,965	$ 1,729	$ 1,416
Other, Ratio									4.20%	4.00%	3.40%