Pension and Other Postretirement Benefits - Schedule of Pension Plan Benefit Obligations (Detail) - Pension benefits - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 7,359	$ 7,026	$ 14,247
Accumulated benefit obligation	7,169	6,850
The aggregate projected benefit obligation and fair value of plan assets for pension plans with projected benefit obligations in excess of plan assets was as follows:
Projected benefit obligation	6,600	6,995
Fair value of plan assets	4,016	4,629
The aggregate accumulated benefit obligation and fair value of plan assets for pension plans with accumulated benefit obligations in excess of plan assets was as follows:
Accumulated benefit obligation	6,422	6,104
Fair value of plan assets	$ 3,998	$ 3,894