Summary of significant accounting policies (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenue Recognition
Estimated service period over which revenues are recognized, low end of range	5
Estimated service period over which revenues are recognized, high end of range	1
Recognition of breakage revenues from the sale of packaged software that have not been activated	$ 0	$ 0	$ 14
Recognition of breakage revenues from subscriptions, licensing and other revenues that have not been activated	0	0	6
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	$ 396	$ 343	$ 332