Income Taxes - Unrecognized Tax Benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of unrecognized tax benefits
Balance at January 1,	$ 540us-gaap_UnrecognizedTaxBenefits	$ 478us-gaap_UnrecognizedTaxBenefits	$ 416us-gaap_UnrecognizedTaxBenefits
Additions based on tax positions related to the current year	18	16	86
Reductions based on tax positions related to the current year	(10)	(5)	(31)
Additions for tax positions of prior years	5	66	32
Reductions for tax positions of prior years	(337)	(15)	(15)
Reductions related to settlements with taxing authorities	(202)	0	(10)
Balance at December 31,	$ 14us-gaap_UnrecognizedTaxBenefits	$ 540us-gaap_UnrecognizedTaxBenefits	$ 478us-gaap_UnrecognizedTaxBenefits

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Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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Amount of decrease in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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