Pension plans - Schedule of information for plans with PBOs and ABOs in excess of plan assets (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
PBOs	$ 12,775	$ 12,625
Plan assets	9,018	9,627
ABOs	10,875	10,617
Plan assets	$ 7,820	$ 8,367