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Medical Claims and Benefits Payable - Components of Change in Medical Claims and Benefits Payable (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Insurance Claims [Roll Forward]    
Medical claims and benefits payable, beginning balance $ 1,929 $ 1,685
Components of medical care costs related to:    
Current period 4,253 3,755
Prior periods (142) (167)
Total medical care costs 4,111 3,588
Change in non-risk provider payables (96) 24
Payments for medical care costs related to:    
Current period 2,683 2,241
Prior periods 1,335 1,116
Total paid 4,018 3,357
Medical claims and benefits payable, ending balance $ 1,926 $ 1,940
Benefit from prior period as a percentage of:    
Balance at beginning of period 7.40% 10.00%
Premium revenue, trailing twelve months 0.80% 1.20%
Medical care costs, trailing twelve months 0.90% 1.30%