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
	Mar. 31, 2021	Mar. 31, 2020
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 13,977	$ 12,861
Revenues ratio from third party payers	100.00%	100.00%
Medicare [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 2,559	$ 2,743
Revenues from third party payers, Ratio	18.30%	21.30%
Managed Medicare [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 2,053	$ 1,826
Revenues from third party payers, Ratio	14.70%	14.20%
Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 527	$ 414
Revenues from third party payers, Ratio	3.80%	3.20%
Managed Medicaid [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 725	$ 666
Revenues from third party payers, Ratio	5.20%	5.20%
Managed Care and Insurers [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 6,885	$ 6,645
Revenues from third party payers, Ratio	49.10%	51.60%
International (managed care and insurers) [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 333	$ 292
Revenues from third party payers, Ratio	2.40%	2.30%
Other [Member]
Revenues From Third Party Payers [Line Items]
Revenues from third party payers	$ 895	$ 275
Other, Ratio	6.50%	2.20%