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Benefit Plans - Components of Net Periodic Postretirement Health Care Costs (Details) - Postretirement Health Care Plan - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]        
Service cost   $ 6 $ 7 $ 12
Interest cost   14 15 20
Net loss from experience differences $ (10) 15 14 10
Prior service credit [1]   (39) (40) (20)
Net periodic postretirement health care costs   $ (4) $ (4) $ 22
Curtailment gain $ 9      
[1] In the fourth quarter of 2016, the prior service credit included a one-time $9 million curtailment gain related to a change in the eligibility requirement resulting in ongoing amortization of $10 million. We continued to amortize the prior service credit and recorded $39 million in 2018 and $40 million in 2017.