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CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (Parenthetical) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Pension and retiree medical benefits:    
Amortization of losses included in net periodic benefit cost, tax $ 0 $ 1
Derivative instruments:    
Reclassification of losses to net income, tax $ 0 $ 1