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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, 2018
Mar. 31, 2017
Revenues From Third Party Payers [Line Items]    
Other $ 291 $ 304
Revenues $ 11,423 $ 10,623
Other, Ratio 2.40% 3.00%
Revenues ratio from third party payers 100.00% 100.00%
Medicare [Member]    
Revenues From Third Party Payers [Line Items]    
Revenues from third party payers $ 2,524 $ 2,361
Revenues from third party payers, Ratio 22.10% 22.20%
Managed Medicare [Member]    
Revenues From Third Party Payers [Line Items]    
Revenues from third party payers $ 1,399 $ 1,183
Revenues from third party payers, Ratio 12.30% 11.10%
Medicaid [Member]    
Revenues From Third Party Payers [Line Items]    
Revenues from third party payers $ 281 $ 294
Revenues from third party payers, Ratio 2.50% 2.80%
Managed Medicaid [Member]    
Revenues From Third Party Payers [Line Items]    
Revenues from third party payers $ 561 $ 589
Revenues from third party payers, Ratio 4.90% 5.50%
Managed Care and Insurers [Member]    
Revenues From Third Party Payers [Line Items]    
Revenues from third party payers $ 6,062 $ 5,623
Revenues from third party payers, Ratio 53.10% 52.90%
International (Managed Care and Other Insurers) [Member]    
Revenues From Third Party Payers [Line Items]    
Revenues from third party payers $ 305 $ 269
Revenues from third party payers, Ratio 2.70% 2.50%