Accounting Policies - Schedule of Revenues from Third Party Payers, Uninsured and Other Payers (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 51,533	$ 51,336	$ 46,677
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	$ 10,420	$ 10,798	$ 9,831
Revenues from third party payers, Ratio	20.20%	21.00%	21.10%
Managed Medicare [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 6,997	$ 6,452	$ 5,497
Revenues from third party payers, Ratio	13.60%	12.60%	11.80%
Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 1,965	$ 1,572	$ 1,358
Revenues from third party payers, Ratio	3.80%	3.10%	2.90%
Managed Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 2,621	$ 2,450	$ 2,403
Revenues from third party payers, Ratio	5.10%	4.80%	5.10%
Managed Care and Other Insurers [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 26,535	$ 26,544	$ 24,467
Revenues from third party payers, Ratio	51.50%	51.60%	52.40%
International (Managed Care and Other Insurers) [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 1,120	$ 1,162	$ 1,156
Revenues from third party payers, Ratio	2.20%	2.30%	2.50%
Product and Service, Other [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 1,875	$ 2,358	$ 1,965
Other, Ratio	3.60%	4.60%	4.20%