Fair value measurements (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Fair value measurements using quoted prices in active markets for identical financial instruments (Level 1)
Financial assets
Money market funds	$ 2,214	$ 2,216
U.S. treasuries with original maturities of three months or less		332
U.S. treasuries and government sponsored agency debt securities	596	672
ARS held through Morgan Stanley Smith Barney LLC	0	0
Foreign exchange contract derivatives, assets		0
Total financial assets at fair value	2,810	3,220
Fair value measurements using significant other observable inputs (Level 2)
Financial assets
Money market funds	0	0
U.S. treasuries with original maturities of three months or less		0
U.S. treasuries and government sponsored agency debt securities	0	0
ARS held through Morgan Stanley Smith Barney LLC	0	0
Foreign exchange contract derivatives, assets		1
Total financial assets at fair value	0	1
Fair value measurements using significant unobservable inputs (Level 3)
Financial assets
Money market funds	0	0
U.S. treasuries with original maturities of three months or less		0
U.S. treasuries and government sponsored agency debt securities	0	0
ARS held through Morgan Stanley Smith Barney LLC	25	23
Foreign exchange contract derivatives, assets		0
Total financial assets at fair value	25	23
Fair value
Financial assets
Money market funds	2,214	2,216
U.S. treasuries with original maturities of three months or less		332
U.S. treasuries and government sponsored agency debt securities	596	672
ARS held through Morgan Stanley Smith Barney LLC	25	23
Foreign exchange contract derivatives, assets		1
Total financial assets at fair value	$ 2,835	$ 3,244