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Medical Claims Liability - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Insurance [Abstract]    
Amounts recorded as an adjustment to premium revenues related to minimum HBR and return of premium programs $ 8 $ (13)
Short-duration insurance contracts, Incurred but not reported and expected development on reported claims $ 5,569