Valuation of debt and equity investments and certain liabilities (Fair value assets and liabilities measured on recurring basis) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	$ 98	$ 55
Corporate obligations	186	294
U.S. Government agency and Treasury securities	1,763	2,121
Auction-rate securities	40	134
Mutual funds	151	169
Total assets	2,238	2,773
Deferred compensation	166	191
Total liabilities	166	191
Level 1 [Member]
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	98	55
Corporate obligations	0	0
U.S. Government agency and Treasury securities	785	606
Auction-rate securities	0	0
Mutual funds	151	169
Total assets	1,034	830
Deferred compensation	166	191
Total liabilities	166	191
Level 2 [Member]
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	0	0
Corporate obligations	186	294
U.S. Government agency and Treasury securities	978	1,515
Auction-rate securities	0	0
Mutual funds	0	0
Total assets	1,164	1,809
Deferred compensation	0	0
Total liabilities	0	0
Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	0	0
Corporate obligations	0	0
U.S. Government agency and Treasury securities	0	0
Auction-rate securities	40	134
Mutual funds	0	0
Total assets	40	134
Deferred compensation	0	0
Total liabilities	$ 0	$ 0