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Medical Claims and Benefits Payable - Components of Change in Medical Claims and Benefits Payable (Details) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Insurance Claims    
Medical claims and benefits payable, beginning balance $ 2,192 $ 1,929
Components of medical care costs related to:    
Current period 7,794 8,633
Prior periods (222) (31)
Total medical care costs 7,572 8,602
Change in non-risk provider payables 56 (114)
Payments for medical care costs related to:    
Current period 6,248 6,883
Prior periods 1,652 1,457
Total paid 7,900 8,340
Medical claims and benefits payable, ending balance $ 1,920 $ 2,077