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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues From Third Party Payers [Line Items]
Other									$ 3,492	$ 2,786	$ 3,688
Revenues before provision for doubtful accounts									47,653	44,747	43,591
Provision for doubtful accounts									(4,039)	(3,257)	(3,913)
Revenues	$ 11,562	$ 10,696	$ 10,733	$ 10,623	$ 10,641	$ 10,270	$ 10,319	$ 10,260	$ 43,614	$ 41,490	$ 39,678
Other, Ratio									8.00%	6.70%	9.30%
Revenues before provision for doubtful accounts, Ratio									109.20%	107.80%	109.90%
Provision for doubtful accounts, Ratio									(9.20%)	(7.80%)	(9.90%)
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									$ 9,483	$ 8,895	$ 8,654
Revenues from third party payers, Ratio									21.70%	21.40%	21.80%
Managed Medicare [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 4,788	$ 4,355	$ 4,133
Revenues from third party payers, Ratio									11.00%	10.50%	10.40%
Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 1,631	$ 1,597	$ 1,705
Revenues from third party payers, Ratio									3.70%	3.80%	4.30%
Managed Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 2,349	$ 2,478	$ 2,234
Revenues from third party payers, Ratio									5.40%	6.00%	5.60%
Managed Care and Other Insurers [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 24,813	$ 23,441	$ 21,882
Revenues from third party payers, Ratio									56.90%	56.50%	55.20%
International (Managed Care and Other Insurers) [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers									$ 1,097	$ 1,195	$ 1,295
Revenues from third party payers, Ratio									2.50%	2.90%	3.30%