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Medical Claims and Benefits Payable - Medical Claims and Future Benefits (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Insurance [Abstract]        
Fee-for-service claims incurred but not paid (IBNP) $ 1,717 $ 1,352 $ 1,191  
Pharmacy payable 112 112 88  
Capitation payable 67 37 140  
Other 296 428 266  
Medical claims and benefits payable $ 2,192 $ 1,929 $ 1,685 $ 1,201