Leases - Components of operating lease costs (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lease Cost [Abstract]
Operating lease cost	$ 1,413	$ 1,459
Short-term lease cost	145	178
Variable lease cost	228	276
Sublease income	(10)	(24)
Total lease cost	$ 1,776	$ 1,889