Valuation of debt and equity investments and certain liabilities - Investments at fair value (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 1,306		$ 1,627
Short-term Investments	1,880		2,202
Long-term Investments	219		216
Proceeds from sales, redemptions and maturities of short-term available-for-sale investments	2,491	3,564
Aggregate maturities of investments in debt securities classified as available for sale [Abstract]
One year or less	2,759
One to two years	200
Measured at fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	1,079		1,410
Short-term Investments	1,880		2,202
Long-term Investments	180		179
Measured at fair value | Available-for-sale securities | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	364		500
Short-term Investments	0		0
Long-term Investments	0		0
Measured at fair value | Available-for-sale securities | Corporate obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	90		123
Short-term Investments	345		217
Long-term Investments	0		0
Measured at fair value | Available-for-sale securities | U.S. Government agency and Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	625		787
Short-term Investments	1,535		1,985
Long-term Investments	0		0
Measured at fair value | Trading securities | Mutual funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	0		0
Short-term Investments	0		0
Long-term Investments	180		179
Other measurement basis | Equity-method investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	0		0
Short-term Investments	0		0
Long-term Investments	27		24
Other measurement basis | Cost-method investments
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 | Cash on hand
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	227		217
Short-term Investments	0		0
Long-term Investments	$ 0		$ 0