Debt - Addtional Disclosures (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Maturities Of Debt Excluding Capital Leases [Abstract]
Maturities of Debt in 2015	$ 25
Maturities of Debt in 2016	600
Maturities of Debt in 2017	250
Maturities of Debt in 2018	131
Maturities of Debt in 2019	251
Maturities of Debt in years after 2019	3,731
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Maturities of debt under capital lease arrangements in 2015	22
Maturities of debt under capital lease arrangements in 2016	22
Maturities of debt under capital lease arrangements in 2017	11
Maturities of debt under capital lease arrangements in 2018	4
Maturities of debt under capital lease arrangements in 2019	0
Maturities of debt under capital lease arrangements in years after2019	0
Interest Expense [Abstract]
Interest expense on long-term and short-term debt	$ 265us-gaap_InterestExpenseDebt	$ 270us-gaap_InterestExpenseDebt	$ 268us-gaap_InterestExpenseDebt

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Amount of minimum lease payments maturing in the next fiscal year following the latest fiscal year for capital leases.

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

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 30

-Section 50

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Amount of minimum lease payments maturing in the fifth fiscal year following the latest fiscal year for capital leases.

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Amount of minimum lease payments maturing in the forth fiscal year following the latest fiscal year for capital leases.

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Amount of minimum lease payments maturing in the third fiscal year following the latest fiscal year for capital leases.

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Amount of minimum lease payments maturing in the second fiscal year following the latest fiscal year for capital leases.

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Amount of minimum lease payments maturing after the fifth fiscal year following the latest fiscal year for capital leases.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

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Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year.

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-Publisher FASB

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Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year.

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Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year.

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Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year.

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Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year.

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Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year.

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