Debt and Credit Agreements - Schedule of Long-term Debt Instruments (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Subtotal Long-term Debt (including current portion)	$ 14,757	$ 11,809
Less Debt Due within One Year	(18)	(19)
Long-term Debt (excluding current portion)	$ 14,739	11,790
Notes
Debt Instrument [Line Items]
Average Interest Rates at December	4.50%
Subtotal Long-term Debt (including current portion)	$ 14,558	11,591
Equipment Obligations
Debt Instrument [Line Items]
Average Interest Rates at December	6.30%
Subtotal Long-term Debt (including current portion)	$ 195	213
Capital Leases
Debt Instrument [Line Items]
Average Interest Rates at December	16.20%
Subtotal Long-term Debt (including current portion)	$ 4	$ 5

Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

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