Benefit Plans - Information for Plans With Projected and Accumulated Benefit Obligations in Excess of Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 3,021	$ 2,825
Accumulated benefit obligation	3,014	2,813
Fair value of plan assets	2,456	2,201
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 16	$ 18