Leases - Information Related to Operating Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentNet	us-gaap:PropertyPlantAndEquipmentNet
Right-of-use assets	$ 8,006	$ 8,190
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent	us-gaap:AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent
Lease liabilities	$ 8,508	$ 8,505
Weighted average remaining lease term (in years)	8 years 8 months 12 days	8 years 9 months 18 days
Weighted average discount rate	3.48%	3.68%
Supplemental cash flow information
Cash paid for amounts included in the measurement of lease liabilities - operating cash flows	$ 1,626	$ 1,572
Supplemental non-cash information
Right-of-use assets obtained in exchange for operating lease obligations	1,350	1,413
Rental expense
Gross rental expense	2,094	2,057
Sublease rental income	(166)	(184)
Net rental expense	$ 1,928	$ 1,873