Segment Information - Reconciliation of Operating Revenues (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	$ 30,447	$ 32,071	$ 62,057	$ 64,199
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	28,595	29,763	58,041	59,630
Corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	1,967	2,412	4,239	4,747
Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating revenues	$ (115)	$ (104)	$ (223)	$ (178)