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Benefit Plans - Postretirement Benefit Plans (Details) - Postretirement Benefit Plans - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 1 $ 1 $ 4 $ 4
Interest cost 4 4 11 11
Net loss from experience differences 2 4 5 11
Prior service credit (10) (10) (29) (29)
Net periodic benefit costs $ (3) $ (1) $ (9) $ (3)