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Benefit Plans - Postretirement Health Care Costs (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
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 2 2
Interest cost 4 4
Net loss from experience differences 4 3
Prior service credit (10) (10)
Net periodic benefit costs $ 0 $ (1)