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Medical Claims and Benefits Payable - Additional Information (Details)
member in Thousands, $ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
member
Dec. 31, 2015
USD ($)
member
Insurance Claims [Line Items]      
Medical claims and benefits payable | $ $ 122 $ 225 $ 167
Reinsurance recoverable for unpaid claims | $ 16 61 46
Prior period claims expense (benefit) recognized | $ $ 36 $ (192) $ (141)
Minimum      
Insurance Claims [Line Items]      
Estimated discount percentage of final payment on claims incurred but not paid 8.00%    
Maximum      
Insurance Claims [Line Items]      
Estimated discount percentage of final payment on claims incurred but not paid 10.00%    
Ohio | Medicaid Expansion      
Insurance Claims [Line Items]      
Number of members added, approximately (in member) | member     61
Ohio | MMP Program      
Insurance Claims [Line Items]      
Number of members added, approximately (in member) | member     17
California | Medicaid Expansion      
Insurance Claims [Line Items]      
Number of members added, approximately (in member) | member   55 100