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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, 2017
Jun. 30, 2016
Insurance Claims    
Medical claims and benefits payable, beginning balance $ 1,929 $ 1,685
Components of medical care costs related to:    
Current period 8,633 7,371
Prior periods (31) (189)
Total medical care costs 8,602 7,182
Change in non-risk provider payables (114) 24
Payments for medical care costs related to:    
Current period 6,883 5,885
Prior periods 1,457 1,240
Total paid 8,340 7,125
Medical claims and benefits payable, ending balance $ 2,077 $ 1,766
Benefit from prior period as a percentage of:    
Balance at beginning of period 1.60% 11.30%
Premium revenue, trailing twelve months 0.20% 1.30%
Medical care costs, trailing twelve months 0.20% 1.40%