Leases - Supplemental Cash Flow Information Related to Leases (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash paid for amounts included in the measurement of lease obligations:
Operating Leases, Operating cash flows	$ 218	$ 419
Finance lease cost: Interest on lease obligations	20	21
Finance Leases, Financing cash flows	17	55
Noncash transactions:
Operating Leases, Leased assets recognized for new lease obligations incurred	51	14
Finance Leases, Leased assets recognized for new lease obligations incurred	0	0
Operating Leases, Changes in leased assets and lease obligations due to lease modifications	123	14
Finance Leases, Changes in leased assets and lease obligations due to lease modifications	$ 0	$ 0