Valuation of debt and equity investments and certain liabilities (Investments at fair value) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Proceeds from sales, redemptions and maturities of short-term available-for-sale investments	$ 615	$ 613
Cash and Cash Equivalents	1,393		1,416
Short-term Investments	2,469		2,549
Long-term Investments	204		215
Aggregate maturities of investments in money market funds and other debt securities classified as available for sale [Abstract]
One year or less	3,363
One to three years	341
Measured at fair value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	1,235		1,194
Short-term Investments	2,469		2,549
Long-term Investments	159		159
Measured at fair value [Member] \| Available-for-sale securities [Member] \| Money market funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	208		211
Short-term Investments	0		0
Long-term Investments	0		0
Measured at fair value [Member] \| Available-for-sale securities [Member] \| Corporate obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	200		188
Short-term Investments	308		325
Long-term Investments	0		0
Measured at fair value [Member] \| Available-for-sale securities [Member] \| U.S. Government agency and Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	827		795
Short-term Investments	2,161		2,224
Long-term Investments	0		0
Measured at fair value [Member] \| Trading securities [Member] \| Mutual funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	0		0
Short-term Investments	0		0
Long-term Investments	159		159
Other measurement basis [Member] \| Equity-method investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	0		0
Short-term Investments	0		0
Long-term Investments	23		34
Other measurement basis [Member] \| Cost-method investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	0		0
Short-term Investments	0		0
Long-term Investments	22		22
Other measurement basis [Member] \| Cash on hand [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	158		222
Short-term Investments	0		0
Long-term Investments	$ 0		$ 0

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

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The cash inflow from sales, maturities, prepayments, calls and collections of all investments, including securities and other assets, having ready marketability and intended by management to be liquidated, if necessary, within the current operating cycle. Includes cash flows from securities classified as trading securities that were acquired for reasons other than sale in the short-term.

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