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Medical Claims Payable ((Favorable) Unfavorable Developments By Changes In Key Assumptions) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Medical Claims Payable [Abstract]      
Assumed trend factors $ (599) $ (325) $ (515)
Assumed completion factors (38) (175) (415)
Total $ (637) $ (500) $ (930)