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Medical Claims and Benefits Payable - Additional Information (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Other Liabilities Disclosure [Abstract]      
Non-risk provider payables $ 158   $ 122
Liability for Claims and Claims Adjustment Expense [Line Items]      
Expense (recovery) for prior period claims development $ 227 $ (9)  
Minimum      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Percentage of IBNP not expected to be paid 8.00%    
Maximum      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Percentage of IBNP not expected to be paid 10.00%