Debt and Credit Agreements - Schedule of Long-term Debt Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Finance Leases Average Interest Rate	14.50%
Subtotal Long-term Debt (including current portion)	$ 16,705	$ 16,238
Finance Leases	3	4
Less Debt Due within One Year	(401)	(245)	[1]
Long-term Debt (excluding current portion)	$ 16,304	15,993	[1]
Notes
Debt Instrument [Line Items]
Debt Average Interest Rates	4.00%
Subtotal Long-term Debt (including current portion)	$ 16,542	16,056
Equipment Obligations
Debt Instrument [Line Items]
Debt Average Interest Rates	6.30%
Subtotal Long-term Debt (including current portion)	$ 160	$ 178

[1] Certain prior year data has been reclassified to conform to the current presentation.