Fair value measurements (Details 2) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010
Fair Value Assets and Fair Value Liabilities Measured on Recurring Basis Unobservable Input Reconciliation Asset and Liability [Line Items]
Level 3 measurement reconciliation, recurring basis, fair value assets beginning balance	$ 23	$ 84
Total gains or (losses) (realized/unrealized) included in investment and other income, net	2	0
Purchases or acquired sales, issuances and settlements		(61)
Level 3 measurement reconciliation, recurring basis, fair value assets ending balance	25	23
Percentage of assets measured on recurring basis at fair value using significant unobservable inputs (in hundredths)	1.00%
ARS
Fair Value Assets and Fair Value Liabilities Measured on Recurring Basis Unobservable Input Reconciliation Asset and Liability [Line Items]
Level 3 measurement reconciliation, recurring basis, fair value assets beginning balance	23	77
Total gains or (losses) (realized/unrealized) included in investment and other income, net	2	7
Purchases or acquired sales, issuances and settlements		(61)
Level 3 measurement reconciliation, recurring basis, fair value assets ending balance	25	23
ARS rights from UBS
Fair Value Assets and Fair Value Liabilities Measured on Recurring Basis Unobservable Input Reconciliation Asset and Liability [Line Items]
Level 3 measurement reconciliation, recurring basis, fair value assets beginning balance		7
Total gains or (losses) (realized/unrealized) included in investment and other income, net		(7)
Purchases or acquired sales, issuances and settlements		0
Level 3 measurement reconciliation, recurring basis, fair value assets ending balance		0
Other financial liabilities
Fair Value Assets and Fair Value Liabilities Measured on Recurring Basis Unobservable Input Reconciliation Asset and Liability [Line Items]
Level 3 measurement reconciliation, recurring basis, fair value assets beginning balance		(23)
Total gains or (losses) (realized/unrealized) included in investment and other income, net		0
Purchases or acquired sales, issuances and settlements		0
Level 3 measurement reconciliation, recurring basis, fair value assets ending balance		$ (23)