Debt (Details 1) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Item]
Total debt	$ 11,480us-gaap_LongTermDebt	$ 9,577us-gaap_LongTermDebt
Current portion of debt	(462)	(705)
Total long-term debt	11,018	8,872
Notes And Debentures [Member]
Debt Instrument [Line Item]
Total debt	9,266	8,068
Stated percentage rate range, minimum	2.30%
Stated percentage rate range, maximum	7.90%
Debt due through	Jan. 01, 2054
Capitalized Leases [Member]
Debt Instrument [Line Item]
Total debt	1,520	1,702
Stated percentage rate range, minimum	3.10%
Stated percentage rate range, maximum	8.40%
Debt due through	Dec. 10, 2028
Equipment Obligations [Member]
Debt Instrument [Line Item]
Total debt	597	110
Stated percentage rate range, minimum	3.20%
Stated percentage rate range, maximum	6.70%
Debt due through	Jan. 02, 2031
Receivables Securitization Facility [Member]
Debt Instrument [Line Item]
Total debt	400	0
Floating Rate Term Loan [Member]
Debt Instrument [Line Item]
Total debt	200	200
Debt due through	Oct. 31, 2016
Mortgage Bonds [Member]
Debt Instrument [Line Item]
Total debt	57	57
Stated percentage rate range, minimum	4.80%
Stated percentage rate range, maximum	4.80%
Debt due through	Jan. 01, 2030
Medium-Term Notes [Member]
Debt Instrument [Line Item]
Total debt	23	32
Stated percentage rate range, minimum	9.30%
Stated percentage rate range, maximum	10.00%
Debt due through	Apr. 15, 2020
Tax-Exempt Financings [Member]
Debt Instrument [Line Item]
Total debt	8	12
Debt due through	Dec. 01, 2015
Unamortized Discount [Member]
Debt Instrument [Line Item]
Total debt	$ (591)	$ (604)

Maximum contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Minimum contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Amount of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

+ References

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Amount of long-term debt and capital leases due within one year or the normal operating cycle, if longer.

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