Debt Debt Outstanding (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Total Debt	[1]	$ 21,737	$ 19,204
Total Debt, Weighted Average Interest Rate		4.90%
Current portion of debt	[1]	$ (389)	$ (366)
Short-term Debt, Weighted Average Interest Rate	[1]	6.40%	5.20%
Total long-term debt	[1]	$ 21,348	$ 18,838
Notes and Debentures [Member]
Debt Instrument [Line Items]
Total Debt	[1]	$ 20,082	$ 17,838
Total Debt, Weighted Average Interest Rate	[1]	4.90%	5.00%
Equipment Obligations [Member]
Debt Instrument [Line Items]
Total Debt	[1]	$ 564	$ 96
Total Debt, Weighted Average Interest Rate	[1]	3.70%	5.50%
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Total Debt	[1]	$ 660	$ 745
Total Debt, Weighted Average Interest Rate	[1]	6.10%	6.10%
Mortgage Bonds [Member]
Debt Instrument [Line Items]
Total Debt	[1]	$ 81	$ 81
Total Debt, Weighted Average Interest Rate	[1]	4.50%	4.50%
Financing Obligation [Member]
Debt Instrument [Line Items]
Total Debt	[1]	$ 237	$ 251
Total Debt, Weighted Average Interest Rate	[1]	6.30%	6.30%
Unamortized Fair Value Adjustment [Member]
Debt Instrument [Line Items]
Total Debt	[1]	$ 113	$ 193

[1] Amounts represent debt outstanding and weighted average effective interest rates for 2015 and 2014, respectively. Maturities are as of December 31, 2015.