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Significant Accounting Policies - Additional Information (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Amounts due government agencies $ 1,030 $ 1,030 $ 1,542  
Risk adjustment receivable (payable) 390 $ 390 912  
Additional medical care costs incurred 5      
Maturity period   10 years    
Average maturity period   3 years    
Adjustments to provisional valuation of deferred tax assets and liabilities 4      
Restricted cash and cash equivalents 94 $ 94   $ 91
Medicaid Expansion        
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Amounts due government agencies 144 144 96  
Medical Premium Liability Due to Agency        
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Medical premiums liability based on medical cost thresholds $ 198 198 $ 135  
CMS Subsidies        
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Operating expense   $ 81