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Affordable Care Act (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Affordable Care Act [Abstract]    
Risk adjustment and reinsurance, pre-tax benefit, period increase (decrease) $ 79 $ 48
Risk adjustment (928) (677)
Reinsurance 1 15
Risk corridor 4 6
Minimum MLR (265) (22)
Cost sharing reductions $ (50) $ (96)