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Defined Benefit Plan Assets Measured at Fair Value Using Unobservable Inputs (Level Three) (Detail) - Level 3 - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets at beginning of year $ 287 $ 208
Actual return on plan assets, Sold during the year 7 4
Actual return on plan assets, Held at year end 16 3
Purchases, sales, issuances and settlements (net) 73 72
Fair value of plan assets at end of year $ 383 $ 287