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	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	$ 69	$ 55
Corporate obligations	320	294
U.S. Government agency and Treasury securities	1,951	2,121
Auction-rate securities	118	134
Mutual funds	159	169
Total assets	2,617	2,773
Deferred compensation	173	191
Total liabilities	173	191
Level 1 [Member]
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	69	55
Corporate obligations	0	0
U.S. Government agency and Treasury securities	730	606
Auction-rate securities	0	0
Mutual funds	159	169
Total assets	958	830
Deferred compensation	173	191
Total liabilities	173	191
Level 2 [Member]
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	0	0
Corporate obligations	320	294
U.S. Government agency and Treasury securities	1,221	1,515
Auction-rate securities	0	0
Mutual funds	0	0
Total assets	1,541	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	118	134
Mutual funds	0	0
Total assets	118	134
Deferred compensation	0	0
Total liabilities	$ 0	$ 0