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, 2013	Sep. 30, 2012	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 1,435		$ 1,416
Short-term Investments	2,158		2,549
Long-term Investments	210		215
Proceeds from sales, redemptions and maturities of short-term available-for-sale investments	3,564	1,639
Aggregate maturities of investments in debt securities classified as available for sale [Abstract]
One year or less	3,230
One to three years	265
Measured at fair value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	1,337		1,194
Short-term Investments	2,158		2,549
Long-term Investments	168		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	582		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	75		188
Short-term Investments	317		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	680		795
Short-term Investments	1,841		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	168		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	24		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	18		22
Other measurement basis [Member] | Cash on hand [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	98		222
Short-term Investments	0		0
Long-term Investments	$ 0		$ 0