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CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME (Parenthetical) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]        
Changes in unamortized actuarial loss, tax expense (benefit) $ 8 $ 12 $ 128 $ 94
Changes in unamortized net prior service credit, tax expense (benefit) $ 0 $ 0 $ 0 $ 1