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Components of Accumulated Other Comprehensive Income, Net of Tax (Parenthetical) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Increase in prior service credits $ 0 $ 30 $ 0
Increase in actuarial losses $ 306 $ 560 $ 78