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Significant Accounting Policies - Additional Information (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Schedule of Premium Revenue by Health Plan Type [Line Items]    
Amounts due government agencies $ 1,844 $ 1,202
Premium deficiency reserve 100 30
Retained earnings 657 810
Adjustment | Accounting Standards Update 2014-09    
Schedule of Premium Revenue by Health Plan Type [Line Items]    
Retained earnings 50  
Medicaid Expansion    
Schedule of Premium Revenue by Health Plan Type [Line Items]    
Amounts due government agencies 114 244
Medical Premium Liability Due to Agency    
Schedule of Premium Revenue by Health Plan Type [Line Items]    
Medical premiums liability based on medical cost thresholds $ 136 $ 272