Pension Plans, Other Postretirement Benefits, and Defined-Contribution Plans - Accumulated Benefit Obligations in Excess of Plan Assets Table (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 2,079	$ 2,175
Accumulated benefit obligation	1,749	1,866
Fair value of plan assets	$ 1,227	$ 1,171