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Pension and Other Postretirement Benefit Plans - Narrative (Details) - Other Postretirement Benefits - USD ($)
$ in Millions
3 Months Ended 87 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Jun. 30, 2011
Jun. 30, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Reduction in postretirement benefit obligation     $ 138  
Net periodic cost $ 3 $ (9)   $ 89