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Medical Claims and Benefits Payable - Components of the Change (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]      
Medical claims and benefits payable, beginning balance $ 1,929 $ 1,685 $ 1,201
Components of medical care costs related to:      
Current period 17,037 14,966 11,935
Prior periods 36 (192) (141)
Total medical care costs 17,073 14,774 11,794
Change in non-risk provider payables (106) 58 48
Payments for medical care costs related to:      
Current period 15,130 13,304 10,448
Prior periods 1,574 1,284 910
Total paid 16,704 14,588 11,358
Medical claims and benefits payable, ending balance $ 2,192 $ 1,929 $ 1,685