Segment Information - Reconciliation of Operating Revenues (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	$ 31,543	$ 32,894	$ 93,600	$ 97,093
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	29,485	30,591	87,526	90,221
Corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	2,203	2,400	6,442	7,147
Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	$ (145)	$ (97)	$ (368)	$ (275)