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, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	$ 208	$ 211
Corporate obligations	508	513
U.S. Government agency and Treasury securities	2,988	3,019
Mutual funds	159	159
Total assets	3,863	3,902
Deferred compensation	177	174
Total liabilities	177	174
Level 1 [Member]
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	208	211
Corporate obligations	0	0
U.S. Government agency and Treasury securities	1,352	1,145
Mutual funds	159	159
Total assets	1,719	1,515
Deferred compensation	177	174
Total liabilities	177	174
Level 2 [Member]
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	0	0
Corporate obligations	508	513
U.S. Government agency and Treasury securities	1,636	1,874
Mutual funds	0	0
Total assets	2,144	2,387
Deferred compensation	0	0
Total liabilities	$ 0	$ 0