Available-for-sale investments (Fair Values by Classification) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Fair values of available-for-sale investments by classification in the Consolidated Balance Sheets
Cash and cash equivalents	$ 2,864	$ 3,731	$ 3,687	$ 3,805
Marketable securities	24,254	23,295
Other assets — noncurrent	1,779us-gaap_OtherAssetsNoncurrent	1,592us-gaap_OtherAssetsNoncurrent
Total available-for-sale investments	26,674us-gaap_AvailableForSaleSecurities	26,732us-gaap_AvailableForSaleSecurities
Available-for-sale investments [Member]
Fair values of available-for-sale investments by classification in the Consolidated Balance Sheets
Cash and cash equivalents	2,262	3,293
Other assets — noncurrent	$ 158	$ 144

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Aggregate carrying amount, as of the balance sheet date, of noncurrent assets not separately disclosed in the balance sheet. Noncurrent assets are expected to be realized or consumed after one year (or the normal operating cycle, if longer).

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