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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, 2017
Sep. 30, 2016
Insurance Claims    
Medical claims and benefits payable, beginning balance $ 1,929 $ 1,685
Components of medical care costs related to:    
Current period 12,813 11,120
Prior periods 9 (190)
Total medical care costs 12,822 10,930
Change in non-risk provider payables 172 70
Payments for medical care costs related to:    
Current period 10,944 9,536
Prior periods 1,501 1,278
Total paid 12,445 10,814
Medical claims and benefits payable, ending balance $ 2,478 $ 1,871
Benefit from prior period as a percentage of:    
Balance at beginning of period (0.50%) 11.30%
Premium revenue, trailing twelve months 0.00% 1.20%
Medical care costs, trailing twelve months (0.10%) 1.30%