Benefit Plans - Information for Plans With Projected and Accumulated Benefit Obligations in Excess of Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 2,825	$ 3,132
Accumulated benefit obligation	2,813	3,117
Fair value of plan assets	2,201	2,462
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 18	$ 21