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, 2014	Mar. 31, 2013	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 1,565		$ 1,627
Short-term Investments	2,467		2,202
Long-term Investments	212		216
Proceeds from sales, redemptions and maturities of short-term available-for-sale investments	785	615
Aggregate maturities of investments in debt securities classified as available for sale [Abstract]
One year or less	3,530
One to two years	260
Measured at fair value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	1,323		1,410
Short-term Investments	2,467		2,202
Long-term Investments	176		179
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	542		500
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	176		123
Short-term Investments	312		217
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	605		787
Short-term Investments	2,155		1,985
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	176		179
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	24		24
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	12		13
Other measurement basis [Member] | Cash on hand [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	242		217
Short-term Investments	0		0
Long-term Investments	$ 0		$ 0