Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Unrecognized tax benefits that would impact effective tax rate	$ 3,500	$ 2,600	$ 2,100
Amount of potential increase or decrease in gross balance of unrecognized tax benefits	1,300
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Balance at January 1,	3,450	3,497	4,911
Increases based on tax positions related to the current period	1,355	262	408
Increases based on tax positions related to prior periods	626	583	1,028
Decreases based on tax positions related to prior periods	(350)	(785)	(2,646)
Decreases related to cash settlements with taxing authorities	(334)	(56)	(204)
Decreases related to a lapse of applicable statute of limitations	0	(51)	0
Balance at December 31,	4,747	3,450	3,497
Income tax expense, penalties and interest expense
Penalties and interest expense/(benefit)	102	86	$ (156)
Penalties and interest accrued	$ 639	$ 687

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The amount of estimated penalties and interest accrued as of the balance sheet date arising from income tax examinations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)
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The sum of the amounts of estimated penalties and interest recognized in the period arising from income tax examinations.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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The amount of the unrecognized tax benefit of a position taken for which it is reasonably possible that the total amount thereof will significantly increase or decrease within twelve months of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount of unrecognized tax benefits.

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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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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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