Leases (Supplemental Balance Sheet Information Related to Leases) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Operating Leases
Operating lease right-of-use assets	$ 24,546	$ 24,039
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccountsPayableAndAccruedLiabilitiesCurrent	us-gaap:AccountsPayableAndAccruedLiabilitiesCurrent
Accounts payable and accrued liabilities	$ 3,483	$ 3,451
Operating lease obligation	22,056	21,804
Total operating lease obligation	25,539	25,255
Finance Leases
Property, plant and equipment, at cost	3,485	3,534
Accumulated depreciation and amortization	(1,363)	(1,296)
Property, plant and equipment, net	$ 2,122	$ 2,238
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization	us-gaap:PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization
Current portion of long-term debt	$ 174	$ 162
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:DebtCurrent	us-gaap:DebtCurrent
Long-term debt	$ 1,732	$ 1,872
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtNoncurrent	us-gaap:LongTermDebtNoncurrent
Total finance lease obligation	$ 1,906	$ 2,034
Weighted-Average Remaining Lease Term (years), Operating leases	8 years 6 months		8 years 8 months 12 days
Weighted-Average Remaining Lease Term (years), Finance leases	10 years 1 month 6 days		10 years 4 months 24 days
Weighted-Average Discount Rate, Operating leases	4.10%		4.30%
Weighted-Average Discount Rate, Finance leases	8.10%		8.40%