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Medical Claims and Benefits Payable - Components of Change in Medical Claims and Benefits Payable (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Insurance Claims    
Medical claims and benefits payable, beginning balance $ 2,192 $ 1,929
Components of medical care costs related to:    
Current period 11,589 12,813
Prior periods (227) 9
Total medical care costs 11,362 12,822
Change in non-risk provider payables 60 172
Payments for medical care costs related to:    
Current period 9,866 10,944
Prior periods 1,706 1,501
Total paid 11,572 12,445
Medical claims and benefits payable, ending balance $ 2,042 $ 2,478