Debt Debt Outstanding (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total Debt	[1]	$ 23,226	$ 22,499
Total Debt, Weighted Average Effective Interest Rate		4.70%
Current portion of long-term debt		$ (830)	$ (740)	[1]
Current portion of long-term debt, Weighted Average Effective Interest Rate	[1]	4.80%	5.70%
Total long-term debt		$ 22,396	$ 21,759	[1]
Notes and Debentures, due 2018 to 2097 [Member]
Debt Instrument [Line Items]
Total Debt	[1]		21,245
Total Debt, Weighted Average Effective Interest Rate	[1]	4.70%
Equipment Obligations, due 2018 to 2028 [Member]
Debt Instrument [Line Items]
Total Debt	[1]	$ 470	$ 493
Total Debt, Weighted Average Effective Interest Rate	[1]		3.60%
Capital Lease Obligations, due 2018 to 2029 [Member]
Debt Instrument [Line Items]
Total Debt	[1]	$ 419	$ 466
Total Debt, Weighted Average Effective Interest Rate	[1]	6.10%
Mortgage Bonds, due 2020 to 2047 [Member]
Debt Instrument [Line Items]
Total Debt	[1]	$ 81	$ 81
Total Debt, Weighted Average Effective Interest Rate	[1]		4.50%
Financing Obligations, due 2018 to 2029 [Member]
Debt Instrument [Line Items]
Total Debt	[1]	$ 195	$ 211
Total Debt, Weighted Average Effective Interest Rate	[1]	6.10%	6.30%
Unamortized Fair Value Adjustment Under Acquisition Method Accounting [Member]
Debt Instrument [Line Items]
Total Debt	[1]	$ (33)	$ 3

[1] Amounts represent debt outstanding and weighted average effective interest rates for 2018 and 2017, respectively. Maturities are as of December 31, 2018.