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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 15,276	$ 13,311	$ 43,688	$ 37,240
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	$ 2,645	$ 2,603	$ 7,816	$ 7,618
Revenues from third party payers, Ratio	17.30%	19.60%	17.90%	20.50%
Managed Medicare [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 2,124	$ 1,760	$ 6,281	$ 5,074
Revenues from third party payers, Ratio	13.90%	13.20%	14.40%	13.60%
Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 692	$ 445	$ 1,722	$ 1,423
Revenues from third party payers, Ratio	4.50%	3.30%	3.90%	3.80%
Managed Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 813	$ 707	$ 2,369	$ 1,904
Revenues from third party payers, Ratio	5.30%	5.30%	5.40%	5.10%
Managed Care and Insurers [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 7,998	$ 6,752	$ 22,300	$ 19,028
Revenues from third party payers, Ratio	52.40%	50.70%	51.00%	51.00%
International (managed care and insurers) [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 324	$ 307	$ 995	$ 838
Revenues from third party payers, Ratio	2.10%	2.30%	2.30%	2.30%
Other [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 680	$ 737	$ 2,205	$ 1,355
Other, Ratio	4.50%	5.60%	5.10%	3.70%