Summary of significant accounting policies (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenue Recognition
Estimated service period over which revenues are recognized, low end of range (in months)	5
Estimated service period over which revenues are recognized, high end of range (in years)	1
Recognition of breakage revenues from the sale of packaged software that have not been activated	$ 0	$ 14	$ 5
Recognition of breakage revenues from subscriptions, licensing and other revenues that have not been activated	0	6	8
Allowance for returns and price protection
Impact of a one percent change in the allowance for returns and price protection on net revenues	3
Advertising Expenses
Advertising expense included in sales and marketing expense	$ 343	$ 332	$ 366