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Benefit Plans - Postretirement Benefit Plans (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]    
Gain related to a change in the eligibility requirement and a change in benefits to Medicare-eligible participants $ 8 $ 8
Postretirement Benefit Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 1 2
Interest cost 4 4
Net loss from experience differences 2 4
Prior service credit [1] (10) (10)
Net periodic benefit costs $ (3) $ 0
[1] $8 million for the three months ended March 31, 2019 and March 31, 2018 related to a change in the eligibility requirement and a change in benefits to Medicare-eligible participants.