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, 2013	Dec. 31, 2012
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	$ 277	$ 211
Corporate obligations	344	513
U.S. Government agency and Treasury securities	2,468	3,019
Mutual funds	159	159
Total assets	3,248	3,902
Deferred compensation	178	174
Total liabilities	178	174
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	277	211
Corporate obligations	0	0
U.S. Government agency and Treasury securities	1,517	1,145
Mutual funds	159	159
Total assets	1,953	1,515
Deferred compensation	178	174
Total liabilities	178	174
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Financial Statement Captions [Line Items]
Money market funds	0	0
Corporate obligations	344	513
U.S. Government agency and Treasury securities	951	1,874
Mutual funds	0	0
Total assets	1,295	2,387
Deferred compensation	0	0
Total liabilities	$ 0	$ 0